Strings in Swift 4

Collections already do get a default SubSequence implementation of Slice<Base: Indexable> that is essentially like just that.

The reason types like Array and String have their own is to customize it with more than the default behavior. For example, ArraySlice provides .withUnsafeBufferPointer method just like an Array does. Substring would need all the features String provides.

Now, once we get conditional conformance, we could use that to maybe increase sharing, for example we could create a protocol for types backed by contiguous memory that provided withUnsafeEtc, and then use conditional conformance to add those features to Slice when the Base has them. This probably won't improve user experience particularly though, just help library authors organize/minimize the code.

···

On Jan 19, 2017, at 19:38, David Sweeris <davesweeris@mac.com> wrote:

Regarding substrings... Instead of having separate `ArraySlice` and `Substring` types, what about having just one type, `Slice<T: Sequence>`, for anything which shares memory? Seems like it'd be easier for users who'd only have to worry about shared storage for one type, and for stdlib authors who'd only have to write it once.

Hi Ben,

I just have one concern about the slice of a string being called Substring. Why not StringSlice? The word substring can mean so many things, specially in cocoa.

Thank you. Great manifesto.

···

On Jan 19, 2017, at 8:18 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

On Jan 19, 2017, at 19:38, David Sweeris <davesweeris@mac.com> wrote:

Regarding substrings... Instead of having separate `ArraySlice` and `Substring` types, what about having just one type, `Slice<T: Sequence>`, for anything which shares memory? Seems like it'd be easier for users who'd only have to worry about shared storage for one type, and for stdlib authors who'd only have to write it once.

Collections already do get a default SubSequence implementation of Slice<Base: Indexable> that is essentially like just that.

The reason types like Array and String have their own is to customize it with more than the default behavior. For example, ArraySlice provides .withUnsafeBufferPointer method just like an Array does. Substring would need all the features String provides.

Now, once we get conditional conformance, we could use that to maybe increase sharing, for example we could create a protocol for types backed by contiguous memory that provided withUnsafeEtc, and then use conditional conformance to add those features to Slice when the Base has them. This probably won't improve user experience particularly though, just help library authors organize/minimize the code.

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Well if I had my way we’d have conversion operators so you could have:

struct Substring {
    implicit conversion() -> String {
        //...
    }
}

But I was also a fan of @conversion so don’t mind me :)

Russ

···

On Jan 19, 2017, at 8:20 PM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org> wrote:

Clearly too big to digest in one take. Some initial thoughts:

* Not sure about the wisdom of the ad-hoc Substring : String compiler magic. It seems that whatever needs overcoming here would be equally relevant for ArraySlice. It would be more design work, but perhaps not terribly more implementation work, to have a magical protocol that allows the compiler to apply a similar magic to conforming types (e.g. a `ImplicitlyConvertibleSlice` protocol with an associated type, to which ArraySlice and String could both conform). Alternatively, perhaps all of this is not truly necessary for sufficient ergonomics.

Sent from my iPhone

Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

An enthusiastic +1

A couple more quick thoughts...

1) Is it just me, or is explicitly putting some of the "higher level" functionality in Foundation instead of stdlib kinda reminiscent of MVC? I guess UIKit/Cocoa would be the "View" part.

Might just be you ;-)

2) I like the idea of making String generic over its encoding... Would we need to nail down the hypothetical type promotion system for that to work, or can it all be handled internally?

If you're implicitly asking what the type of s1 + s2 is when they have different encodings,
I don't think we need a type promotion system to handle that. Unlike with fixed-width integers, there is a family of maximally-expressive encodings that can be used for the result of any operation whose result would otherwise be in doubt without any serious loss of performance. I would probably just go with the currency type, "String" for these cases.

Cheers,

···

Sent from my iPad

On Jan 19, 2017, at 8:13 PM, David Sweeris <davesweeris@mac.com> wrote:

On Jan 19, 2017, at 20:56, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

--
Dave

Regarding substrings... Instead of having separate `ArraySlice` and `Substring` types, what about having just one type, `Slice<T: Sequence>`, for anything which shares memory? Seems like it'd be easier for users who'd only have to worry about shared storage for one type, and for stdlib authors who'd only have to write it once.

Collections already do get a default SubSequence implementation of Slice<Base: Indexable> that is essentially like just that.

The reason types like Array and String have their own is to customize it with more than the default behavior. For example, ArraySlice provides .withUnsafeBufferPointer method just like an Array does. Substring would need all the features String provides.

Now, once we get conditional conformance, we could use that to maybe increase sharing, for example we could create a protocol for types backed by contiguous memory that provided withUnsafeEtc, and then use conditional conformance to add those features to Slice when the Base has them.

We don't need conditional conformance in order to replace ArraySlice with MutableRangeReplaceableRandomAccessSlice<Array>, but to spell that Slice<Array> or generalize withUnsafeXXX to a ContiguouslyStored without adding another in our already-too-large menagerie of slice types, we would.

Substring is different, though: we want to specialize its storage to be able to store code units inline, without using any dynamic memory, so it would need to have a different type from the generic Slice<String>.

···

Sent from my iPad

On Jan 19, 2017, at 8:18 PM, Ben Cohen <ben_cohen@apple.com> wrote:

On Jan 19, 2017, at 19:38, David Sweeris <davesweeris@mac.com> wrote:

--
Dave

The defaults for case-, diacritic-, and width-insensitivity are different for localized operations than for non-localized operations, so for example a localized sort should be case-insensitive by default, and a non-localized sort should be case-sensitive by default.

Sorry, a correction already:

Both localized- and non-localized sorts should be case- and diacritic-sensitive by default. Localized searches should be case- and diacritic-insensitive by default.

-Dave

···

Sent from my iPad

On Jan 19, 2017, at 6:07 PM, Ben Cohen <ben_cohen@apple.com> wrote:

Clearly too big to digest in one take. Some initial thoughts:

* Not sure about the wisdom of the ad-hoc Substring : String compiler magic. It seems that whatever needs overcoming here would be equally relevant for ArraySlice.

We have mixed feelings about it as well, and you make a good point about ArraySlice. I'm not convinced trafficking in slices is going to be as important for Array as it is for String, though.

It would be more design work, but perhaps not terribly more implementation work, to have a magical protocol that allows the compiler to apply a similar magic to conforming types (e.g. a `ImplicitlyConvertibleSlice` protocol with an associated type, to which ArraySlice and String could both conform).

It's not just about slices. There are other subtype relationships we'll want in the language eventually anyway. Int8:Int16, for example. We don't t

Alternatively, perhaps all of this is not truly necessary for sufficient ergonomics.

Personally I would be happy to try the design without the implicit conversion first, but there are legitimate concerns about forcing users to write String(someSubstring) and the manifesto needs to at least offer a solid plan in place for addressing it.

* A requirement to transcode UTF-8 strings to UTF-16 for storage seems...inefficient?

To be clear, nobody's suggesting that you can't store a UTF8String, only that it may be necessary to restrict the encodings that can be stored in the currency type "String."

Why any hesitation at all to expose UTF-8-encoded code units as UInt16? Sure, there are going to be unused bits, but so what?

We're still exploring the design space. That idea is relatively fresh and I haven't convinced myself that it is both efficient and ergonomic. But it's promising.

If I understand it correctly, it's only the concrete type exposed on String for code units that's in play here; the backing representations themselves can use whatever is most efficient.

Yes.

So, why _not_ support UTF-32 and expose all code units as UInt32? Isn't that exactly paralleling the design for the extendedASCII view, where users get ASCII characters back as UInt32 and encoding-specific code units as such as well?

Yes, there's a definite parallel.

* Are the backing representations for String also the same types that can be exposed statically (as in the mentioned `NFCNormalizedUTF16String`)?

Roughly. I think we want at least the following backing representations for String:

1. The two compressed representations used by Cocoa "tagged pointer" strings
2. A third "tagged pointer" representation that stores 63 bits of UTF-16 (so arbitrary UnicodeScalars and most Characters can be stored efficiently)
3. A known Latin-1 backing store that we can fast-path
4. A known UTF-16 backing store
5. A type-erased arbitrary (or nearly-arbitrary, if we have to accept a UTF16 subset restriction) instance of Unicode

It's possible that some of the representations in the range 3...5 can be collapsed into one.

* Why `withCString` with a closure instead of just `cString` returning [CChar]? Particularly if the backing store isn't UTF8, isn't the C string going to have to be a newly allocated buffer anyway?

Not if the backing store is Latin-1, which will be very common.
Also not if the string is short and can be transcoded into stack-based storage, which will also be common.

Personally, I find the current `utf8CString` to be quite convenient :P

All these string types should bridge seamlessly to char*. Isn't that enough for the super-lightweight use case?

···

Sent from my iPad

On Jan 19, 2017, at 8:20 PM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

On Thu, Jan 19, 2017 at 8:56 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:
Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben Cohen](https://github.com/airspeedswift\)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined thus
far, with just this short blurb in the
[list of goals](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html\):

> **String re-evaluation**: String is one of the most important fundamental
> types in the language. The standard library leads have numerous ideas of how
> to improve the programming model for it, without jeopardizing the goals of
> providing a unicode-correct-by-default model. Our goal is to be better at
> string processing than Perl!

For Swift 4 and beyond we want to improve three dimensions of text processing:

  1. Ergonomics
  2. Correctness
  3. Performance

This document is meant to both provide a sense of the long-term vision
(including undecided issues and possible approaches), and to define the scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are mutually-reinforcing. An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success. Conversely, an API that's simply hard to use is also hard to use
correctly. Acheiving optimal performance without compromising ergonomics or
correctness is a greater challenge.

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in the
foundation additions to `String` where patterns and practices found elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily understood
given a signature and a one-line summary. Today, `String` fails that test. As
you can see, the Standard Library and Foundation both contribute significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two parameters.[0] String processing is complex enough; users shouldn't have
to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

  * Restoring `Collection` conformance and dropping the `.characters` view.
  * Providing a more general, composable slicing syntax.
  * Altering `Comparable` so that parameterized
    (e.g. case-insensitive) comparison fits smoothly into the basic syntax.
  * Clearly separating language-dependent operations on text produced
    by and for humans from language-independent
    operations on text produced by and for machine processing.
  * Relocating APIs that fall outside the domain of basic string processing and
    discouraging the proliferation of ad-hoc extensions.

### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs for
basic string processing tasks are still inaccessible until `Foundation` is
imported. While it makes sense that `Foundation` is needed for domain-specific
jobs such as
[linguistic tagging](https://developer.apple.com/reference/foundation/nslinguistictagger\),
one should not need to import anything to, for example, do case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle behind
Swift's `String`. That said, the Unicode standard is an evolving document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs should
be written so their correctness does not depend on precise stability of these
semantics across OS versions or platforms. Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find such
errors, the only sure way to deal with this fact of life is to educate users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly. Although documentation could and should be
improved, the sheer size, complexity, and diversity of these APIs is a major
contributor to the problem, causing novices to tune out, and more experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
   as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
    guidance in the
    [Internationalization and Localization Guide](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html\).

Along with appropriate documentation updates, these changes will make localized
operations more teachable, comprehensible, and approachable, thereby lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

#### The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many operations on
Swift `String` (and `NSString`) are really only appropriate for text that is
intended to be processed for, and consumed by, machines. The semantics of the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
   accessible functions appropriate to that use case.

2. The most general localized operations require a locale parameter not required
   by their un-localized counterparts. This naturally skews complexity towards
   localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of clarifying
the proper default behavior of operations such as comparison, and allows us to
make [significant optimizations](#collation-semantics) that were previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that naïve
code is very likely to be correct if it compiles, and that more sophisticated
issues can be revealed progressively. For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized sort
should be case-sensitive by default. We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly like this:

  x.compared(to: y, case: .sensitive, in: swissGerman)

  x.lowercased(in: .currentLocale)

  x.allMatches(
    somePattern, case: .insensitive, diacritic: .insensitive)

This usage might be supported by code like this:

enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
  static var currentLocale: Locale { ... }
}

extension Unicode {
  // An example of the option language in declaration context,
  // with nil defaults indicating unspecified, so defaults can be
  // driven by the presence/absence of a specific Locale
  func frobnicated(
    case caseSensitivity: StringSensitivity? = nil,
    diacritic diacriticSensitivity: StringSensitivity? = nil,
    width widthSensitivity: StringSensitivity? = nil,
    in locale: Locale? = nil
  ) -> Self { ... }
}

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— turns
out to be quite interesting, once you pick it apart. The full Unicode Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a collation key
3. “Flatten” the key by concatenating the sequence of first elements to the
   sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys

While step 1 can usually
be [done quickly](UAX #15: Unicode Normalization Forms) and
incrementally, step 2 uses a collation table that maps matching *sequences* of
unicode scalars in the normalized string to *sequences* of triples, which get
accumulated into a collation key. Predictably, this is where the real costs
lie.

*However*, there are some bright spots to this story. First, as it turns out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](UTS #10: Unicode Collation Algorithm),
which adds a step 3a: append the string's normalized form to the flattened
collation key. At first blush this just adds work, but consider what it does
for equality: two strings that normalize the same, naturally, will collate the
same. But also, *strings that normalize differently will always collate
differently*. In other words, for equality, it is sufficient to compare the
strings' normalized forms and see if they are the same. We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to hashing: it
is sufficient to hash the string's normalized form, bypassing collation keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level applies
even to localized strings, it means that hashing and equality can be implemented
exactly the same way for localized and non-localized text, and hash tables with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to handle
machine-generated and machine-readable text, the default ordering of `String`s
need no longer use the UCA at all. It is sufficient to order them in any way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting behavior
consistent across platforms. Currently, we sort `String` according to the UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into the
existing syntax. At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
(implemented by changes at the head
of
[this branch](https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
We should adopt a modification of that proposal that uses a method rather than
an operator `<=>`:

enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
 func compared(to: Self) -> SortOrder
 ...
}

This change will give us a syntactic platform on which to implement methods with
additional, defaulted arguments, thereby unifying and regularizing comparison
across the library.

extension String {
 func compared(to: Self) -> SortOrder

}

**Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also possible
that the standard library simply adopts Foundation's `ComparisonResult` as is,
but we believe the community should at least consider alternate naming before
that happens. There will be an opportunity to discuss the choices in detail
when the modified
[Comparison Proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, `elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, `String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes appears to
comport perfectly with Unicode. We think the concatenation problem is tolerable,
because the cases where it occurs all represent partially-formed constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](UAX #29: Unicode Text Segmentation)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”. The other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text editor.

Admitting these cases encourages exploration of grapheme composition and is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial:

  * Collection-like operations encourage experimentation with strings to
    investigate and understand their behavior. This is useful for teaching new
    programmers, but also good for experienced programmers who want to
    understand more about strings/unicode.

  * Extended grapheme clusters form a natural element boundary for Unicode
    strings. For example, searching and matching operations will always produce
    results that line up on grapheme cluster boundaries.

  * Character-by-character processing is a legitimate thing to do in many real
    use-cases, including parsing, pattern matching, and language-specific
    transformations such as transliteration.

  * `Collection` conformance makes a wide variety of powerful operations
    available that are appropriate to `String`'s default role as the vehicle for
    machine processed text.

    The methods `String` would inherit from `Collection`, where similar to
    higher-level string algorithms, have the right semantics. For example,
    grapheme-wise `lexicographicalCompare`, `elementsEqual`, and application of
    `flatMap` with case-conversion, produce the same results one would expect
    from whole-string ordering comparison, equality comparison, and
    case-conversion, respectively. `reverse` operates correctly on graphemes,
    keeping diacritics moored to their base characters and leaving emoji intact.
    Other methods such as `indexOf` and `contains` make obvious sense. A few
    `Collection` methods, like `min` and `max`, may not be particularly useful
    on `String`, but we don't consider that to be a problem worth solving, in
    the same way that we wouldn't try to suppress `min` and `max` on a
    `Set([UInt8])` that was used to store IP addresses.

  * Many of the higher-level operations that we want to provide for `String`s,
    such as parsing and pattern matching, should apply to any `Collection`, and
    many of the benefits we want for `Collections`, such
    as unified slicing, should accrue
    equally to `String`. Making `String` part of the same protocol hierarchy
    allows us to write these operations once and not worry about keeping the
    benefits in sync.

  * Slicing strings into substrings is a crucial part of the vocabulary of
    string processing, and all other sliceable things are `Collection`s.
    Because of its collection-like behavior, users naturally think of `String`
    in collection terms, but run into frustrating limitations where it fails to
    conform and are left to wonder where all the differences lie. Many simply
    “correct” this limitation by declaring a trivial conformance:

    ```swift
  extension String : BidirectionalCollection {}
    ```

    Even if we removed indexing-by-element from `String`, users could still do
    this:

    ```swift
      extension String : BidirectionalCollection {
        subscript(i: Index) -> Character { return characters[i] }
      }
    ```

    It would be much better to legitimize the conformance to `Collection` and
    simply document the oddity of any concatenation corner-cases, than to deny
    users the benefits on the grounds that a few cases are confusing.

Note that the fact that `String` is a collection of graphemes does *not* mean
that string operations will necessarily have to do grapheme boundary
recognition. See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](UAX #29: Unicode Text Segmentation),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII. To fix this, we should:

- Add a `unicodeScalars` view much like `String`'s, so that the sub-structure
   of grapheme clusters is discoverable.
- Add a failable `init` from sequences of scalars (returning nil for sequences
   that contain 0 or 2+ graphemes).
- (Lower priority) expose some operations, such as `func uppercase() ->
   String`, `var isASCII: Bool`, and, to the extent they can be sensibly
   generalized, queries of unicode properties that should also be exposed on
   `UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift `UnicodeScalar`
type. This means it is usable on `String`, but only by going through the unicode
scalar view. To deal with this clash in the short term, `CharacterSet` should be
renamed to `UnicodeScalarSet`. In the longer term, it may be appropriate to
introduce a `CharacterSet` that provides similar functionality for extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling and
their naming:

  * Slices with two explicit endpoints are done with subscript, and support
    in-place mutation:

    ```swift
        s[i..<j].mutate()
    ```

  * Slicing from an index to the end, or from the start to an index, is done
    with a method and does not support in-place mutation:
    ```swift
        s.prefix(upTo: i).readOnly()
    ```

Prefix and suffix operations should be migrated to be subscripting operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this proposal](https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md\).
With generic subscripting in the language, that will allow us to collapse a wide
variety of methods and subscript overloads into a single implementation, and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like `s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when making the substring.
3. Make substrings a different type, with a storage copy on conversion to string.

We think number 3 is the best choice. A walk-through of the tradeoffs follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view into a
subrange of the original `String`'s storage. This is why `String` is 3 words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings into
multiple smaller strings. But it does mean that a stored substring keeps the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming languages,
because applications sometimes extract small strings from large ones and keep
those small strings long-term. That is considered a memory leak and was enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For example:

foo.compare(bar, range: start..<end)

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

foo[start..<end].compare(bar)

Not only does this clarify to which string the range applies, it also brings
this sub-range capability to any API that operates on `String` "for free". So
these other combinations also work equally well:

// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())

In all three cases, an explicit range argument need not appear on the `compare`
method itself. The implementation of `compare` does not need to know anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` type.
The inconvenience of a separate type is mitigated by most operations used on
`Array` from the standard library being generic over `Sequence` or `Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` would apply to `Substring`:

> Important: Long-term storage of `Substring` instances is discouraged. A
> substring holds a reference to the entire storage of a larger string, not
> just to the portion it presents, even after the original string's lifetime
> ends. Long-term storage of a `Substring` may therefore prolong the lifetime
> of large strings that are no longer otherwise accessible, which can appear
> to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be performed, and
at this point the substring buffer is copied and the original string's storage
can be released.

A `String` that was not its own `Substring` could be one word—a single tagged
pointer—without requiring additional allocations. `Substring`s would be a view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and a
length. The small string optimization for `Substring` would take advantage of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having a
`Substring` when you need a `String`, and vice-versa. It is likely this would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant to
existing code that assumes `String` is the currency type. To ease the pain of
type mismatches, `Substring` should be a subtype of `String` in the same way
that `Int` is a subtype of `Optional<Int>`. This would give users an implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship should
make the type difference a non-issue and users will not care which type they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is always
a reasonable default**. A `Substring` passed where `String` is expected will be
implicitly copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this guideline:
if for performance reasons you are tempted to add a `Range` argument to your
method as well as a `String` to avoid unnecessary copies, you should instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a `String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you have is
the `Collection`), we propose the following “empty subscript” operation,

extension Collection {
  subscript() -> SubSequence {
    return self[startIndex..<endIndex]
  }
}

which allows the following usage:

funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring

The `` syntax can be offered as a fixit when needed, similar to `&` for an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be done with
a simple `map` (which could also be offered by a fixit):

takesAnArrayOfSubstring(arrayOfString.map { $0[] })

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are proposing
one such mitigation—implicit conversion—as part of the the "different type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy of
the underlying storage when it detects the string is being "stored" for long
term usage, say when it is assigned to a stored property. The trouble with this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely on
stored properties. For example, should the storing of a substring inside an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array of
substrings. It would also be difficult to distinguish intentional medium-term
storage of substrings, say by a lexer. There does not appear to be an effective
consistent rule that could be applied in the general case for detecting when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", the
optimizer could be enhanced to to reduce the impact of some of those copies.
For example, this code could be optimized to pull the invariant substring out
of the loop:

for _ in 0..<lots {
  someFunc(takingString: bigString[bigRange])
}

It's worth noting that a similar optimization is needed to avoid an equivalent
problem with implicit conversion in the "different type, shared storage" case:

let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }

However, in the case of "same type, copied storage" there are many use cases
that cannot be optimized as easily. Consider the following simple definition of
a recursive `contains` algorithm, which when substring slicing is linear makes
the overall algorithm quadratic:

extension String {
    func containsChar(_ x: Character) -> Bool {
        return !isEmpty && (first == x || dropFirst().containsChar(x))
    }
}

For the optimizer to eliminate this problem is unrealistic, forcing the user to
remember to optimize the code to not use string slicing if they want it to be
efficient (assuming they remember):

extension String {
    // add optional argument tracking progress through the string
    func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> Bool {
        let idx = idx ?? startIndex
        return idx != endIndex
            && (self[idx] == x || containsCharacter(x, atOrAfter: index(after: idx)))
    }
}

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several Objective-C
APIs, and is made especially awkward in Swift by the non-interchangeability of
`Range<String.Index>` and `NSRange`.

s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

s2.find(s2[j..<s2.endIndex])

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be imported
without the `NSRange` argument. The Objective-C importer should be changed to
give these APIs special treatment so that when a `Substring` is passed, instead
of being converted to a `String`, the full `NSString` and range are passed to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, which
solves the impedance problem by eliminating the argument, resulting in more
idiomatic Swift code while retaining the performance benefit. To help users
manually handle any cases that remain, Foundation should be augmented to allow
the following syntax for converting to and from `NSRange`:

let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a protocol.
Since Unicode conformance is a key feature of string processing in swift, we
call that protocol `Unicode`:

**Note:** The following assumes several features that are planned but not yet implemented in
  Swift, and should be considered a sketch rather than a final design.

protocol Unicode
  : Comparable, BidirectionalCollection where Element == Character {

  associatedtype Encoding : UnicodeEncoding
  var encoding: Encoding { get }

  associatedtype CodeUnits
    : RandomAccessCollection where Element == Encoding.CodeUnit
  var codeUnits: CodeUnits { get }

  associatedtype UnicodeScalars
    : BidirectionalCollection  where Element == UnicodeScalar
  var unicodeScalars: UnicodeScalars { get }

  associatedtype ExtendedASCII
    : BidirectionalCollection where Element == UInt32
  var extendedASCII: ExtendedASCII { get }

  var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
  // ... define high-level non-mutating string operations, e.g. search ...

  func compared<Other: Unicode>(
    to rhs: Other,
    case caseSensitivity: StringSensitivity? = nil,
    diacritic diacriticSensitivity: StringSensitivity? = nil,
    width widthSensitivity: StringSensitivity? = nil,
    in locale: Locale? = nil
  ) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
  RangeReplaceableCollection {
    // Satisfy protocol requirement
    mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: C)
      where C.Element == Element

  // ... define high-level mutating string operations, e.g. replace ...
}

The goal is that `Unicode` exposes the underlying encoding and code units in
such a way that for types with a known representation (e.g. a high-performance
`UTF8String`) that information can be known at compile-time and can be used to
generate a single path, while still allowing types like `String` that admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.

### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character. For example,
it should be easy to cleanly express, “if this string starts with `"f"`, process
the rest of the string as follows…” Swift is well-suited to expressing this
common pattern beautifully, but we need to add the APIs. Here are two examples
of the sort of code that might be possible given such APIs:

if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
  somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
   ...
}

The specific spelling and functionality of APIs like this are TBD. The larger
point is to make sure matching-and-consuming jobs are well-supported.

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
  the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33\),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals into
patterns, the following pairs would all invoke the same generic methods:

if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)

Note that, because Swift requires the indices of a slice to match the indices of
the range from which it was sliced, operations like `firstMatch` can return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, and
would fall out of this proposal:

// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol mentioned
above provides a suitable foundation for regular expressions, and types such as
`NSRegularExpression` can easily be retrofitted to conform to it. In the
future, support for regular expression literals in the compiler could allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, and
`utf16`—each with its own opaque index type. The APIs used to translate indices
between views add needless complexity, and the opacity of indices makes them
difficult to serialize.

The index translation problem has two aspects:

  1. `String` views cannot consume one anothers' indices without a cumbersome
    conversion step. An index into a `String`'s `characters` must be translated
    before it can be used as a position in its `unicodeScalars`. Although these
    translations are rarely needed, they add conceptual and API complexity.
  2. Many APIs in the core libraries and other frameworks still expose `String`
    positions as `Int`s and regions as `NSRange`s, which can only reference a
    `utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets into
underlying code unit storage makes a good underlying storage type, provided
`String`'s underlying storage supports random-access. We think random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and constructible
solves the serialization problem:

clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a `String`
between grapheme cluster boundaries are TBD—it can either trap or be forgiving).
Having a common index allows easy traversal into the interior of graphemes,
something that is often needed, without making it likely that someone will do it
by accident.

- `String.index(after:)` should advance to the next grapheme, even when the
   index points partway through a grapheme.

- `String.index(before:)` should move to the start of the grapheme before
   the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`. Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

extension Collection {
  func index(offset: IndexDistance) -> Index {
    return index(startIndex, offsetBy: offset)
  }
  func offset(of i: Index) -> IndexDistance {
    return distance(from: startIndex, to: i)
  }
}

Then integers can easily be translated into offsets into a `String`'s `utf16`
view for consumption by Cocoa:

let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture. This
section details some of the existing issues and thinking that may guide future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format string with
textual placeholders for substitution, and an arbitrary list of other arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, the
spelling of these placeholders must match up to the types of the arguments, in
the right order, or the behavior is undefined. Some limited support for
compile-time checking of this correspondence could be implemented, but only for
the cases where the format string is a literal. Second, there's no reasonable
way to extend the formatting vocabulary to cover the needs of new types: you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, offering
both formatting and parsing services. When used for formatting, though, the
design pattern demands more from users than it should:

  * Matching the type of data being formatted to a formatter type
  * Creating an instance of that type
  * Setting stateful options (`currency`, `dateStyle`) on the type. Note: the
    need for this step prevents the instance from being used and discarded in
    the same expression where it is created.
  * Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to printf's
domain-specific language (just write ordinary swift code!) and its type safety
problems (put the data right where it belongs!) but the following issues prevent
it from being useful for localized formatting (among other jobs):

  * [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to restrict
    types used in string interpolation.
  * [SR-1260](https://bugs.swift.org/browse/SR-1260\) String interpolation can't
    distinguish (fragments of) the base string from the string substitutions.

In the long run, we should improve Swift string interpolation to the point where
it can participate in most any formatting job. Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs to be
both lightweight (because it all happens in-situ) and discoverable. One
approach would be to standardize on `format` methods, e.g.:

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered and
incoherent, with 6 ways to transform a C string into a `String` and four ways to
do the inverse. These APIs should be replaced with the following

extension String {
  /// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
  ///
  /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded
  ///   bytes ending just before the first zero byte (NUL character).
  init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

  /// Constructs a `String` having the same contents as `nulTerminatedCodeUnits`.
  ///
  /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
  ///   the given `encoding`, ending just before the first zero code unit.
  /// - Parameter encoding: describes the encoding in which the code units
  ///   should be interpreted.
  init<Encoding: UnicodeEncoding>(
    cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
    encoding: Encoding)

  /// Invokes the given closure on the contents of the string, represented as a
  /// pointer to a null-terminated sequence of UTF-8 code units.
  func withCString<Result>(
    _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}

In both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification. This covers the common case. The
replacement is done *physically* in the underlying storage and the validity of
the result is recorded in the `String`'s `encoding` such that future accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`. String types that retain their
physical encoding even in the presence of errors and are repaired on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which changes
the process of properly identifying `Character` boundaries. We need to update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some come to
us already in some other encoding, such as UTF-8, that would be costly to
translate. Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by many
different representations.

That said, the highest performance code always requires static knowledge of the
data structures on which it operates, and for this code, dynamic selection of
representation comes at too high a cost. Heavy-duty text processing demands a
way to opt out of dynamism and directly use known encodings. Having this
ability can also make it easy to cleanly specialize code that handles dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that encode
representation information into the type, such as `NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies entirely
within the ASCII subset (JSON, YAML, many XML formats). These formats are often
processed most efficiently by recognizing ASCII structural elements as ASCII,
and capturing the arbitrary sections between them in more-general strings. The
current String API offers no way to efficiently recognize ASCII and skip past
everything else without the overhead of full decoding into unicode scalars.

For these purposes, strings should supply an `extendedASCII` view that is a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is specific
to the underlying encoding of the string.

## Language Support

This proposal depends on two new features in the Swift language:

1. **Generic subscripts**, to
   enable unified slicing syntax.

2. **A subtype relationship** between
   `Substring` and `String`, enabling framework APIs to traffic solely in
   `String` while still making it possible to avoid copies by handling
   `Substring`s where necessary.

Additionally, **the ability to nest types and protocols inside
protocols** could significantly shrink the footprint of this proposal
on the top-level Swift namespace.

## Open Questions

### Must `String` be limited to storing UTF-16 subset encodings?

- The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is not in
  question here; this is about what encodings must be storable, without
  transcoding, in the common currency type called “`String`”.
- ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets. UTF-8 is not.
- If we have a way to get at a `String`'s code units, we need a concrete type in
  which to express them in the API of `String`, which is a concrete type
- If String needs to be able to represent UTF-32, presumably the code units need
  to be `UInt32`.
- Not supporting UTF-32-encoded text seems like one reasonable design choice.
- Maybe we can allow UTF-8 storage in `String` and expose its code units as
  `UInt16`, just as we would for Latin-1.
- Supporting only UTF-16-subset encodings would imply that `String` indices can
  be serialized without recording the `String`'s underlying encoding.

### Do we need a type-erasable base protocol for UnicodeEncoding?

UnicodeEncoding has an associated type, but it may be important to be able to
traffic in completely dynamic encoding values, e.g. for “tell me the most
efficient encoding for this string.”

### Should there be a string “facade?”

One possible design alternative makes `Unicode` a vehicle for expressing
the storage and encoding of code units, but does not attempt to give it an API
appropriate for `String`. Instead, string APIs would be provided by a generic
wrapper around an instance of `Unicode`:

struct StringFacade<U: Unicode> : BidirectionalCollection {

  // ...APIs for high-level string processing here...

  var unicode: U // access to lower-level unicode details
}

typealias String = StringFacade<StringStorage>
typealias Substring = StringFacade<StringStorage.SubSequence>

This design would allow us to de-emphasize lower-level `String` APIs such as
access to the specific encoding, by putting them behind a `.unicode` property.
A similar effect in a facade-less design would require a new top-level
`StringProtocol` playing the role of the facade with an an `associatedtype
Storage : Unicode`.

An interesting variation on this design is possible if defaulted generic
parameters are introduced to the language:

struct String<U: Unicode = StringStorage>
  : BidirectionalCollection {

  // ...APIs for high-level string processing here...

  var unicode: U // access to lower-level unicode details
}

typealias Substring = String<StringStorage.SubSequence>

One advantage of such a design is that naïve users will always extend “the right
type” (`String`) without thinking, and the new APIs will show up on `Substring`,
`MyUTF8String`, etc. That said, it also has downsides that should not be
overlooked, not least of which is the confusability of the meaning of the word
“string.” Is it referring to the generic or the concrete type?

### `TextOutputStream` and `TextOutputStreamable`

`TextOutputStreamable` is intended to provide a vehicle for
efficiently transporting formatted representations to an output stream
without forcing the allocation of storage. Its use of `String`, a
type with multiple representations, at the lowest-level unit of
communication, conflicts with this goal. It might be sufficient to
change `TextOutputStream` and `TextOutputStreamable` to traffic in an
associated type conforming to `Unicode`, but that is not yet clear.
This area will require some design work.

### `description` and `debugDescription`

* Should these be creating localized or non-localized representations?
* Is returning a `String` efficient enough?
* Is `debugDescription` pulling the weight of the API surface area it adds?

### `StaticString`

`StaticString` was added as a byproduct of standard library developed and kept
around because it seemed useful, but it was never truly *designed* for client
programmers. We need to decide what happens with it. Presumably *something*
should fill its role, and that should conform to `Unicode`.

## Footnotes

<b id="f0">0</b> The integers rewrite currently underway is expected to
    substantially reduce the scope of `Int`'s API by using more
    generics. [:leftwards_arrow_with_hook:](#a0)

<b id="f1">1</b> In practice, these semantics will usually be tied to the
version of the installed [ICU](http://icu-project.org) library, which
programmatically encodes the most complex rules of the Unicode Standard and its
de-facto extension, CLDR.[:leftwards_arrow_with_hook:](#a1)

<b id="f2">2</b>
See
[UAX #29: Unicode Text Segmentation](UAX #29: Unicode Text Segmentation). Note
that inserting Unicode scalar values to prevent merging of grapheme clusters would
also constitute a kind of misbehavior (one of the clusters at the boundary would
not be found in the result), so would be relatively costly to implement, with
little benefit. [:leftwards_arrow_with_hook:](#a2)

<b id="f4">4</b> The use of non-UCA-compliant ordering is fully sanctioned by
  the Unicode standard for this purpose. In fact there's
  a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf\)
  dedicated to it. In particular, §5.17 says:

  > When comparing text that is visible to end users, a correct linguistic sort
  > should be used, as described in _Section 5.16, Sorting and
  > Searching_. However, in many circumstances the only requirement is for a
  > fast, well-defined ordering. In such cases, a binary ordering can be used.

  [:leftwards_arrow_with_hook:](#a4)

<b id="f5">5</b> The queries supported by `NSCharacterSet` map directly onto
properties in a table that's indexed by unicode scalar value. This table is
part of the Unicode standard. Some of these queries (e.g., “is this an
uppercase character?”) may have fairly obvious generalizations to grapheme
clusters, but exactly how to do it is a research topic and *ideally* we'd either
establish the existing practice that the Unicode committee would standardize, or
the Unicode committee would do the research and we'd implement their
result.[:leftwards_arrow_with_hook:](#a5)

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Yes, that is a bit jargony.

We mean it as in a “common currency” that everyone uses for exchange. That is, what everyone should use to pass strings around, between API boundaries etc. The goal is for people not to have to think “hmm, what kind of string type should I use here”. If you are not sure, String is always a reasonable default.

···

On Jan 20, 2017, at 8:04 AM, Ole Begemann <ole@oleb.net> wrote:

The downside of having two types is the inconvenience of sometimes having
a
`Substring` when you need a `String`, and vice-versa. It is likely this
would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant
to
existing code that assumes `String` is the currency type.

I'm not familiar with the term "currency type" that appears several
times in the document. Could you clarify what it means? Googling it
proved difficult because all results are about the "money" meaning of
"currency".

Thanks for all the hard work!

Still digesting, but I definitely support the goal of string processing even better than Perl. Some random thoughts:

• I also like the suggestion of implicit conversion from substring slices to strings based on a subtype relationship, since I keep running into that issue when trying to use array slices.

Interesting. Could you offer some examples?

It would be nice to be able to specify that conversion behavior with other types that have a similar subtype relationship.

Indeed.

• One thing that stood out was the interpolation format syntax, which seemed a bit convoluted and difficult to parse:

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

Have you considered treating the interpolation parenthesis more like the function call syntax? It should be a familiar pattern and easily parseable to someone versed in other areas of swift:

  “Something with leading zeroes: \(x, fill: .zero, width: 8)"

Yes, we've considered it

1. "\(f(expr1, label2: expr2, label3: expr3))"

    String(describing: f(expr1, label2: expr2, label3: expr3))

2. "\(expr0 + expr1(label2: expr2, label3: expr3))"

    String(describing: expr0 + expr1(label2: expr2, label3: expr3)

3. "\((expr1, label2: expr2, label3: expr3))"

    String(describing: (expr1, label2: expr2, label3: expr3))

4. "\(expr1, label2: expr2, label3: expr3)"

    String(describing: expr1, label2: expr2, label3: expr3)

I think I'm primarily concerned with the differences among cases 1, 3,
and 4, which are extremely minor. 3 and 4 differ by just a set of
parentheses, though that might be mitigated by the ${...} suggestion someone else posted. The point of using string interpolation is to improve
readability, and I fear these cases make too many things look alike that
have very different meanings. Using a common term like "format" calls
out what is being done.

It's possible to produce terser versions of the syntax that don't suffer
from this problem by using a dedicated operator:

"Column 1: \(n⛄(radix:16, width:8)) *** \(message)"
"Something with leading zeroes: \(x⛄(fill: zero, width:8))"

or even

"Column 1: \(n⛄radix:16⛄width:8) *** \(message)"
"Something with leading zeroes: \(x⛄fill:zero⛄width:8)"

I think that should work for the common cases (e.g. padding, truncating, and alignment), with string-returning methods on the type (or even formatting objects ala NSNumberFormatter) being used for more exotic formatting needs (e.g. outputting a number as Hex instead of Decimal)

• Have you considered having an explicit .machine locale which means that the function should treat the string as machine readable? (as opposed to the lack of a locale)

No, we hadn't. What would be the goal of such a design?

• I almost feel like the machine readableness vs human readableness of a string is information that should travel with the string itself. It would be nice to have an extremely terse way to specify that a string is localizable (strawman syntax below), and that might also classify the string as human readable.

  let myLocalizedStr = $”This is localizable” //This gets used as the comment in the localization file

Yes, there are also arguments for encoding "human readable" in the type system. But as noted in https://github.com/apple/swift/blob/master/docs/StringManifesto.md#future-directions those ideas are scoped out of Swift 4.

···

Sent from my iPad
Sent from my iPad

On Jan 20, 2017, at 5:48 AM, Jonathan Hull <jhull@gbis.com> wrote:

• Looking forward to RegEx literals!
Thanks,
Jon

On Jan 19, 2017, at 6:56 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben Cohen](https://github.com/airspeedswift\)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined thus
far, with just this short blurb in the
[list of goals](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html\):

**String re-evaluation**: String is one of the most important fundamental
types in the language. The standard library leads have numerous ideas of how
to improve the programming model for it, without jeopardizing the goals of
providing a unicode-correct-by-default model. Our goal is to be better at
string processing than Perl!

For Swift 4 and beyond we want to improve three dimensions of text processing:

1. Ergonomics
2. Correctness
3. Performance

This document is meant to both provide a sense of the long-term vision
(including undecided issues and possible approaches), and to define the scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are mutually-reinforcing. An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success. Conversely, an API that's simply hard to use is also hard to use
correctly. Acheiving optimal performance without compromising ergonomics or
correctness is a greater challenge.

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in the
foundation additions to `String` where patterns and practices found elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily understood
given a signature and a one-line summary. Today, `String` fails that test. As
you can see, the Standard Library and Foundation both contribute significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two parameters.[0] String processing is complex enough; users shouldn't have
to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

* Restoring `Collection` conformance and dropping the `.characters` view.
* Providing a more general, composable slicing syntax.
* Altering `Comparable` so that parameterized
(e.g. case-insensitive) comparison fits smoothly into the basic syntax.
* Clearly separating language-dependent operations on text produced
by and for humans from language-independent
operations on text produced by and for machine processing.
* Relocating APIs that fall outside the domain of basic string processing and
discouraging the proliferation of ad-hoc extensions.

### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs for
basic string processing tasks are still inaccessible until `Foundation` is
imported. While it makes sense that `Foundation` is needed for domain-specific
jobs such as
[linguistic tagging](https://developer.apple.com/reference/foundation/nslinguistictagger\),
one should not need to import anything to, for example, do case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle behind
Swift's `String`. That said, the Unicode standard is an evolving document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs should
be written so their correctness does not depend on precise stability of these
semantics across OS versions or platforms. Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find such
errors, the only sure way to deal with this fact of life is to educate users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly. Although documentation could and should be
improved, the sheer size, complexity, and diversity of these APIs is a major
contributor to the problem, causing novices to tune out, and more experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
guidance in the
[Internationalization and Localization Guide](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html\).

Along with appropriate documentation updates, these changes will make localized
operations more teachable, comprehensible, and approachable, thereby lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

#### The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many operations on
Swift `String` (and `NSString`) are really only appropriate for text that is
intended to be processed for, and consumed by, machines. The semantics of the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
accessible functions appropriate to that use case.

2. The most general localized operations require a locale parameter not required
by their un-localized counterparts. This naturally skews complexity towards
localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of clarifying
the proper default behavior of operations such as comparison, and allows us to
make [significant optimizations](#collation-semantics) that were previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that naïve
code is very likely to be correct if it compiles, and that more sophisticated
issues can be revealed progressively. For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized sort
should be case-sensitive by default. We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly like this:

x.compared(to: y, case: .sensitive, in: swissGerman)

x.lowercased(in: .currentLocale)

x.allMatches(
 somePattern, case: .insensitive, diacritic: .insensitive)

This usage might be supported by code like this:

enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
static var currentLocale: Locale { ... }
}

extension Unicode {
// An example of the option language in declaration context,
// with nil defaults indicating unspecified, so defaults can be
// driven by the presence/absence of a specific Locale
func frobnicated(
 case caseSensitivity: StringSensitivity? = nil,
 diacritic diacriticSensitivity: StringSensitivity? = nil,
 width widthSensitivity: StringSensitivity? = nil,
 in locale: Locale? = nil
) -> Self { ... }
}

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— turns
out to be quite interesting, once you pick it apart. The full Unicode Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a collation key
3. “Flatten” the key by concatenating the sequence of first elements to the
sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys

While step 1 can usually
be [done quickly](UAX #15: Unicode Normalization Forms) and
incrementally, step 2 uses a collation table that maps matching *sequences* of
unicode scalars in the normalized string to *sequences* of triples, which get
accumulated into a collation key. Predictably, this is where the real costs
lie.

*However*, there are some bright spots to this story. First, as it turns out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](UTS #10: Unicode Collation Algorithm),
which adds a step 3a: append the string's normalized form to the flattened
collation key. At first blush this just adds work, but consider what it does
for equality: two strings that normalize the same, naturally, will collate the
same. But also, *strings that normalize differently will always collate
differently*. In other words, for equality, it is sufficient to compare the
strings' normalized forms and see if they are the same. We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to hashing: it
is sufficient to hash the string's normalized form, bypassing collation keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level applies
even to localized strings, it means that hashing and equality can be implemented
exactly the same way for localized and non-localized text, and hash tables with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to handle
machine-generated and machine-readable text, the default ordering of `String`s
need no longer use the UCA at all. It is sufficient to order them in any way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting behavior
consistent across platforms. Currently, we sort `String` according to the UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into the
existing syntax. At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
(implemented by changes at the head
of
[this branch](https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
We should adopt a modification of that proposal that uses a method rather than
an operator `<=>`:

enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
func compared(to: Self) -> SortOrder
...
}

This change will give us a syntactic platform on which to implement methods with
additional, defaulted arguments, thereby unifying and regularizing comparison
across the library.

extension String {
func compared(to: Self) -> SortOrder

}

**Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also possible
that the standard library simply adopts Foundation's `ComparisonResult` as is,
but we believe the community should at least consider alternate naming before
that happens. There will be an opportunity to discuss the choices in detail
when the modified
[Comparison Proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, `elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, `String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes appears to
comport perfectly with Unicode. We think the concatenation problem is tolerable,
because the cases where it occurs all represent partially-formed constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](UAX #29: Unicode Text Segmentation)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”. The other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text editor.

Admitting these cases encourages exploration of grapheme composition and is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial:

* Collection-like operations encourage experimentation with strings to
investigate and understand their behavior. This is useful for teaching new
programmers, but also good for experienced programmers who want to
understand more about strings/unicode.

* Extended grapheme clusters form a natural element boundary for Unicode
strings. For example, searching and matching operations will always produce
results that line up on grapheme cluster boundaries.

* Character-by-character processing is a legitimate thing to do in many real
use-cases, including parsing, pattern matching, and language-specific
transformations such as transliteration.

* `Collection` conformance makes a wide variety of powerful operations
available that are appropriate to `String`'s default role as the vehicle for
machine processed text.

The methods `String` would inherit from `Collection`, where similar to
higher-level string algorithms, have the right semantics. For example,
grapheme-wise `lexicographicalCompare`, `elementsEqual`, and application of
`flatMap` with case-conversion, produce the same results one would expect
from whole-string ordering comparison, equality comparison, and
case-conversion, respectively. `reverse` operates correctly on graphemes,
keeping diacritics moored to their base characters and leaving emoji intact.
Other methods such as `indexOf` and `contains` make obvious sense. A few
`Collection` methods, like `min` and `max`, may not be particularly useful
on `String`, but we don't consider that to be a problem worth solving, in
the same way that we wouldn't try to suppress `min` and `max` on a
`Set([UInt8])` that was used to store IP addresses.

* Many of the higher-level operations that we want to provide for `String`s,
such as parsing and pattern matching, should apply to any `Collection`, and
many of the benefits we want for `Collections`, such
as unified slicing, should accrue
equally to `String`. Making `String` part of the same protocol hierarchy
allows us to write these operations once and not worry about keeping the
benefits in sync.

* Slicing strings into substrings is a crucial part of the vocabulary of
string processing, and all other sliceable things are `Collection`s.
Because of its collection-like behavior, users naturally think of `String`
in collection terms, but run into frustrating limitations where it fails to
conform and are left to wonder where all the differences lie. Many simply
“correct” this limitation by declaring a trivial conformance:

extension String : BidirectionalCollection {}

Even if we removed indexing-by-element from `String`, users could still do
this:

   extension String : BidirectionalCollection {
     subscript(i: Index) -> Character { return characters[i] }
   }

It would be much better to legitimize the conformance to `Collection` and
simply document the oddity of any concatenation corner-cases, than to deny
users the benefits on the grounds that a few cases are confusing.

Note that the fact that `String` is a collection of graphemes does *not* mean
that string operations will necessarily have to do grapheme boundary
recognition. See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](UAX #29: Unicode Text Segmentation),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII. To fix this, we should:

- Add a `unicodeScalars` view much like `String`'s, so that the sub-structure
of grapheme clusters is discoverable.
- Add a failable `init` from sequences of scalars (returning nil for sequences
that contain 0 or 2+ graphemes).
- (Lower priority) expose some operations, such as `func uppercase() ->
String`, `var isASCII: Bool`, and, to the extent they can be sensibly
generalized, queries of unicode properties that should also be exposed on
`UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift `UnicodeScalar`
type. This means it is usable on `String`, but only by going through the unicode
scalar view. To deal with this clash in the short term, `CharacterSet` should be
renamed to `UnicodeScalarSet`. In the longer term, it may be appropriate to
introduce a `CharacterSet` that provides similar functionality for extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling and
their naming:

* Slices with two explicit endpoints are done with subscript, and support
in-place mutation:

     s[i..<j].mutate()

* Slicing from an index to the end, or from the start to an index, is done
with a method and does not support in-place mutation:

     s.prefix(upTo: i).readOnly()

Prefix and suffix operations should be migrated to be subscripting operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this proposal](https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md\).
With generic subscripting in the language, that will allow us to collapse a wide
variety of methods and subscript overloads into a single implementation, and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like `s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when making the substring.
3. Make substrings a different type, with a storage copy on conversion to string.

We think number 3 is the best choice. A walk-through of the tradeoffs follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view into a
subrange of the original `String`'s storage. This is why `String` is 3 words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings into
multiple smaller strings. But it does mean that a stored substring keeps the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming languages,
because applications sometimes extract small strings from large ones and keep
those small strings long-term. That is considered a memory leak and was enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For example:

foo.compare(bar, range: start..<end)

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

foo[start..<end].compare(bar)

Not only does this clarify to which string the range applies, it also brings
this sub-range capability to any API that operates on `String` "for free". So
these other combinations also work equally well:

// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())

In all three cases, an explicit range argument need not appear on the `compare`
method itself. The implementation of `compare` does not need to know anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` type.
The inconvenience of a separate type is mitigated by most operations used on
`Array` from the standard library being generic over `Sequence` or `Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` would apply to `Substring`:

Important: Long-term storage of `Substring` instances is discouraged. A
substring holds a reference to the entire storage of a larger string, not
just to the portion it presents, even after the original string's lifetime
ends. Long-term storage of a `Substring` may therefore prolong the lifetime
of large strings that are no longer otherwise accessible, which can appear
to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be performed, and
at this point the substring buffer is copied and the original string's storage
can be released.

A `String` that was not its own `Substring` could be one word—a single tagged
pointer—without requiring additional allocations. `Substring`s would be a view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and a
length. The small string optimization for `Substring` would take advantage of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having a
`Substring` when you need a `String`, and vice-versa. It is likely this would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant to
existing code that assumes `String` is the currency type. To ease the pain of
type mismatches, `Substring` should be a subtype of `String` in the same way
that `Int` is a subtype of `Optional<Int>`. This would give users an implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship should
make the type difference a non-issue and users will not care which type they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is always
a reasonable default**. A `Substring` passed where `String` is expected will be
implicitly copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this guideline:
if for performance reasons you are tempted to add a `Range` argument to your
method as well as a `String` to avoid unnecessary copies, you should instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a `String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you have is
the `Collection`), we propose the following “empty subscript” operation,

extension Collection {
subscript() -> SubSequence { 
 return self[startIndex..<endIndex] 
}
}

which allows the following usage:

funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring

The `` syntax can be offered as a fixit when needed, similar to `&` for an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be done with
a simple `map` (which could also be offered by a fixit):

takesAnArrayOfSubstring(arrayOfString.map { $0[] })

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are proposing
one such mitigation—implicit conversion—as part of the the "different type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy of
the underlying storage when it detects the string is being "stored" for long
term usage, say when it is assigned to a stored property. The trouble with this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely on
stored properties. For example, should the storing of a substring inside an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array of
substrings. It would also be difficult to distinguish intentional medium-term
storage of substrings, say by a lexer. There does not appear to be an effective
consistent rule that could be applied in the general case for detecting when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", the
optimizer could be enhanced to to reduce the impact of some of those copies.
For example, this code could be optimized to pull the invariant substring out
of the loop:

for _ in 0..<lots { 
someFunc(takingString: bigString[bigRange]) 
}

It's worth noting that a similar optimization is needed to avoid an equivalent
problem with implicit conversion in the "different type, shared storage" case:

let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }

However, in the case of "same type, copied storage" there are many use cases
that cannot be optimized as easily. Consider the following simple definition of
a recursive `contains` algorithm, which when substring slicing is linear makes
the overall algorithm quadratic:

extension String {
 func containsChar(_ x: Character) -> Bool {
     return !isEmpty && (first == x || dropFirst().containsChar(x))
 }
}

For the optimizer to eliminate this problem is unrealistic, forcing the user to
remember to optimize the code to not use string slicing if they want it to be
efficient (assuming they remember):

extension String {
 // add optional argument tracking progress through the string
 func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> Bool {
     let idx = idx ?? startIndex
     return idx != endIndex
         && (self[idx] == x || containsCharacter(x, atOrAfter: index(after: idx)))
 }
}

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several Objective-C
APIs, and is made especially awkward in Swift by the non-interchangeability of
`Range<String.Index>` and `NSRange`.

s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

s2.find(s2[j..<s2.endIndex])

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be imported
without the `NSRange` argument. The Objective-C importer should be changed to
give these APIs special treatment so that when a `Substring` is passed, instead
of being converted to a `String`, the full `NSString` and range are passed to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, which
solves the impedance problem by eliminating the argument, resulting in more
idiomatic Swift code while retaining the performance benefit. To help users
manually handle any cases that remain, Foundation should be augmented to allow
the following syntax for converting to and from `NSRange`:

let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a protocol.
Since Unicode conformance is a key feature of string processing in swift, we
call that protocol `Unicode`:

**Note:** The following assumes several features that are planned but not yet implemented in
Swift, and should be considered a sketch rather than a final design.

protocol Unicode 
: Comparable, BidirectionalCollection where Element == Character {

associatedtype Encoding : UnicodeEncoding
var encoding: Encoding { get }

associatedtype CodeUnits 
 : RandomAccessCollection where Element == Encoding.CodeUnit
var codeUnits: CodeUnits { get }

associatedtype UnicodeScalars 
 : BidirectionalCollection  where Element == UnicodeScalar
var unicodeScalars: UnicodeScalars { get }

associatedtype ExtendedASCII 
 : BidirectionalCollection where Element == UInt32
var extendedASCII: ExtendedASCII { get }

var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
// ... define high-level non-mutating string operations, e.g. search ...

func compared<Other: Unicode>(
 to rhs: Other,
 case caseSensitivity: StringSensitivity? = nil,
 diacritic diacriticSensitivity: StringSensitivity? = nil,
 width widthSensitivity: StringSensitivity? = nil,
 in locale: Locale? = nil
) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
RangeReplaceableCollection {
 // Satisfy protocol requirement
 mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: C) 
   where C.Element == Element

// ... define high-level mutating string operations, e.g. replace ...
}

The goal is that `Unicode` exposes the underlying encoding and code units in
such a way that for types with a known representation (e.g. a high-performance
`UTF8String`) that information can be known at compile-time and can be used to
generate a single path, while still allowing types like `String` that admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.

### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character. For example,
it should be easy to cleanly express, “if this string starts with `"f"`, process
the rest of the string as follows…” Swift is well-suited to expressing this
common pattern beautifully, but we need to add the APIs. Here are two examples
of the sort of code that might be possible given such APIs:

if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
...
}

The specific spelling and functionality of APIs like this are TBD. The larger
point is to make sure matching-and-consuming jobs are well-supported.

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33\),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals into
patterns, the following pairs would all invoke the same generic methods:

if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)

Note that, because Swift requires the indices of a slice to match the indices of
the range from which it was sliced, operations like `firstMatch` can return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, and
would fall out of this proposal:

// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol mentioned
above provides a suitable foundation for regular expressions, and types such as
`NSRegularExpression` can easily be retrofitted to conform to it. In the
future, support for regular expression literals in the compiler could allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, and
`utf16`—each with its own opaque index type. The APIs used to translate indices
between views add needless complexity, and the opacity of indices makes them
difficult to serialize.

The index translation problem has two aspects:

1. `String` views cannot consume one anothers' indices without a cumbersome
conversion step. An index into a `String`'s `characters` must be translated
before it can be used as a position in its `unicodeScalars`. Although these
translations are rarely needed, they add conceptual and API complexity.
2. Many APIs in the core libraries and other frameworks still expose `String`
positions as `Int`s and regions as `NSRange`s, which can only reference a
`utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets into
underlying code unit storage makes a good underlying storage type, provided
`String`'s underlying storage supports random-access. We think random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and constructible
solves the serialization problem:

clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a `String`
between grapheme cluster boundaries are TBD—it can either trap or be forgiving).
Having a common index allows easy traversal into the interior of graphemes,
something that is often needed, without making it likely that someone will do it
by accident.

- `String.index(after:)` should advance to the next grapheme, even when the
index points partway through a grapheme.

- `String.index(before:)` should move to the start of the grapheme before
the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`. Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

extension Collection {
func index(offset: IndexDistance) -> Index {
 return index(startIndex, offsetBy: offset)
}
func offset(of i: Index) -> IndexDistance {
 return distance(from: startIndex, to: i)
}
}

Then integers can easily be translated into offsets into a `String`'s `utf16`
view for consumption by Cocoa:

let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture. This
section details some of the existing issues and thinking that may guide future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format string with
textual placeholders for substitution, and an arbitrary list of other arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, the
spelling of these placeholders must match up to the types of the arguments, in
the right order, or the behavior is undefined. Some limited support for
compile-time checking of this correspondence could be implemented, but only for
the cases where the format string is a literal. Second, there's no reasonable
way to extend the formatting vocabulary to cover the needs of new types: you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, offering
both formatting and parsing services. When used for formatting, though, the
design pattern demands more from users than it should:

* Matching the type of data being formatted to a formatter type
* Creating an instance of that type
* Setting stateful options (`currency`, `dateStyle`) on the type. Note: the
need for this step prevents the instance from being used and discarded in
the same expression where it is created.
* Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to printf's
domain-specific language (just write ordinary swift code!) and its type safety
problems (put the data right where it belongs!) but the following issues prevent
it from being useful for localized formatting (among other jobs):

* [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to restrict
types used in string interpolation.
* [SR-1260](https://bugs.swift.org/browse/SR-1260\) String interpolation can't
distinguish (fragments of) the base string from the string substitutions.

In the long run, we should improve Swift string interpolation to the point where
it can participate in most any formatting job. Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs to be
both lightweight (because it all happens in-situ) and discoverable. One
approach would be to standardize on `format` methods, e.g.:

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered and
incoherent, with 6 ways to transform a C string into a `String` and four ways to
do the inverse. These APIs should be replaced with the following

extension String {
/// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
///
/// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded 
///   bytes ending just before the first zero byte (NUL character).
init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

/// Constructs a `String` having the same contents as `nulTerminatedCodeUnits`.
///
/// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
///   the given `encoding`, ending just before the first zero code unit.
/// - Parameter encoding: describes the encoding in which the code units
///   should be interpreted.
init<Encoding: UnicodeEncoding>(
 cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
 encoding: Encoding)

/// Invokes the given closure on the contents of the string, represented as a
/// pointer to a null-terminated sequence of UTF-8 code units.
func withCString<Result>(
 _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}

In both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification. This covers the common case. The
replacement is done *physically* in the underlying storage and the validity of
the result is recorded in the `String`'s `encoding` such that future accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`. String types that retain their
physical encoding even in the presence of errors and are repaired on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which changes
the process of properly identifying `Character` boundaries. We need to update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some come to
us already in some other encoding, such as UTF-8, that would be costly to
translate. Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by many
different representations.

That said, the highest performance code always requires static knowledge of the
data structures on which it operates, and for this code, dynamic selection of
representation comes at too high a cost. Heavy-duty text processing demands a
way to opt out of dynamism and directly use known encodings. Having this
ability can also make it easy to cleanly specialize code that handles dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that encode
representation information into the type, such as `NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies entirely
within the ASCII subset (JSON, YAML, many XML formats). These formats are often
processed most efficiently by recognizing ASCII structural elements as ASCII,
and capturing the arbitrary sections between them in more-general strings. The
current String API offers no way to efficiently recognize ASCII and skip past
everything else without the overhead of full decoding into unicode scalars.

For these purposes, strings should supply an `extendedASCII` view that is a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is specific
to the underlying encoding of the string.

## Language Support

This proposal depends on two new features in the Swift language:

1. **Generic subscripts**, to
enable unified slicing syntax.

2. **A subtype relationship** between
`Substring` and `String`, enabling framework APIs to traffic solely in
`String` while still making it possible to avoid copies by handling
`Substring`s where necessary.

Additionally, **the ability to nest types and protocols inside
protocols** could significantly shrink the footprint of this proposal
on the top-level Swift namespace.

## Open Questions

### Must `String` be limited to storing UTF-16 subset encodings?

- The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is not in
question here; this is about what encodings must be storable, without
transcoding, in the common currency type called “`String`”.
- ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets. UTF-8 is not.
- If we have a way to get at a `String`'s code units, we need a concrete type in
which to express them in the API of `String`, which is a concrete type
- If String needs to be able to represent UTF-32, presumably the code units need
to be `UInt32`.
- Not supporting UTF-32-encoded text seems like one reasonable design choice.
- Maybe we can allow UTF-8 storage in `String` and expose its code units as
`UInt16`, just as we would for Latin-1.
- Supporting only UTF-16-subset encodings would imply that `String` indices can
be serialized without recording the `String`'s underlying encoding.

### Do we need a type-erasable base protocol for UnicodeEncoding?

UnicodeEncoding has an associated type, but it may be important to be able to
traffic in completely dynamic encoding values, e.g. for “tell me the most
efficient encoding for this string.”

### Should there be a string “facade?”

One possible design alternative makes `Unicode` a vehicle for expressing
the storage and encoding of code units, but does not attempt to give it an API
appropriate for `String`. Instead, string APIs would be provided by a generic
wrapper around an instance of `Unicode`:

struct StringFacade<U: Unicode> : BidirectionalCollection {

// ...APIs for high-level string processing here...

var unicode: U // access to lower-level unicode details
}

typealias String = StringFacade<StringStorage>
typealias Substring = StringFacade<StringStorage.SubSequence>

This design would allow us to de-emphasize lower-level `String` APIs such as
access to the specific encoding, by putting them behind a `.unicode` property.
A similar effect in a facade-less design would require a new top-level
`StringProtocol` playing the role of the facade with an an `associatedtype
Storage : Unicode`.

An interesting variation on this design is possible if defaulted generic
parameters are introduced to the language:

struct String<U: Unicode = StringStorage> 
: BidirectionalCollection {

// ...APIs for high-level string processing here...

var unicode: U // access to lower-level unicode details
}

typealias Substring = String<StringStorage.SubSequence>

One advantage of such a design is that naïve users will always extend “the right
type” (`String`) without thinking, and the new APIs will show up on `Substring`,
`MyUTF8String`, etc. That said, it also has downsides that should not be
overlooked, not least of which is the confusability of the meaning of the word
“string.” Is it referring to the generic or the concrete type?

### `TextOutputStream` and `TextOutputStreamable`

`TextOutputStreamable` is intended to provide a vehicle for
efficiently transporting formatted representations to an output stream
without forcing the allocation of storage. Its use of `String`, a
type with multiple representations, at the lowest-level unit of
communication, conflicts with this goal. It might be sufficient to
change `TextOutputStream` and `TextOutputStreamable` to traffic in an
associated type conforming to `Unicode`, but that is not yet clear.
This area will require some design work.

### `description` and `debugDescription`

* Should these be creating localized or non-localized representations?
* Is returning a `String` efficient enough?
* Is `debugDescription` pulling the weight of the API surface area it adds?

### `StaticString`

`StaticString` was added as a byproduct of standard library developed and kept
around because it seemed useful, but it was never truly *designed* for client
programmers. We need to decide what happens with it. Presumably *something*
should fill its role, and that should conform to `Unicode`.

## Footnotes

<b id="f0">0</b> The integers rewrite currently underway is expected to
substantially reduce the scope of `Int`'s API by using more
generics. [:leftwards_arrow_with_hook:](#a0)

<b id="f1">1</b> In practice, these semantics will usually be tied to the
version of the installed [ICU](http://icu-project.org) library, which
programmatically encodes the most complex rules of the Unicode Standard and its
de-facto extension, CLDR.[:leftwards_arrow_with_hook:](#a1)

<b id="f2">2</b>
See
[UAX #29: Unicode Text Segmentation](UAX #29: Unicode Text Segmentation). Note
that inserting Unicode scalar values to prevent merging of grapheme clusters would
also constitute a kind of misbehavior (one of the clusters at the boundary would
not be found in the result), so would be relatively costly to implement, with
little benefit. [:leftwards_arrow_with_hook:](#a2)

<b id="f4">4</b> The use of non-UCA-compliant ordering is fully sanctioned by
the Unicode standard for this purpose. In fact there's
a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf\)
dedicated to it. In particular, §5.17 says:

When comparing text that is visible to end users, a correct linguistic sort
should be used, as described in _Section 5.16, Sorting and
Searching_. However, in many circumstances the only requirement is for a
fast, well-defined ordering. In such cases, a binary ordering can be used.

[:leftwards_arrow_with_hook:](#a4)

<b id="f5">5</b> The queries supported by `NSCharacterSet` map directly onto
properties in a table that's indexed by unicode scalar value. This table is
part of the Unicode standard. Some of these queries (e.g., “is this an
uppercase character?”) may have fairly obvious generalizations to grapheme
clusters, but exactly how to do it is a research topic and *ideally* we'd either
establish the existing practice that the Unicode committee would standardize, or
the Unicode committee would do the research and we'd implement their
result.[:leftwards_arrow_with_hook:](#a5)

_______________________________________________
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https://lists.swift.org/mailman/listinfo/swift-evolution

Great document! Pleasure to read and see the excellence design powers that
go into Swift.

One ask - make string interpolation great again?

Taking from examples supplied at

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

Why not use:

"Column 1: ${n.format(radix:16, width:8)} *** $message"

Which for my preference makes the syntax feel more readable, avoids the
"double ))" in terms of string interpolation termination and function
termination points. And if that's not enough brings the "feel" of the
language to be scriptable in nature common in bash, sh, zsh and co..
scripting interpreters and has been adopted as part of ES6 interpolation
syntax[1].

[1]

···

On Fri, Jan 20, 2017 at 9:19 AM Rien via swift-evolution < swift-evolution@swift.org> wrote:

Wow, I fully support the intention (becoming better than Perl) but I
cannot comment on the contents without studying it for a couple of days…

Regards,
Rien

Site: http://balancingrock.nl
Blog: http://swiftrien.blogspot.com
Github: Swiftrien (Rien) · GitHub
Project: http://swiftfire.nl

> On 20 Jan 2017, at 03:56, Ben Cohen via swift-evolution < > swift-evolution@swift.org> wrote:
>
> Hi all,
>
> Below is our take on a design manifesto for Strings in Swift 4 and
beyond.
>
> Probably best read in rendered markdown on GitHub:
> https://github.com/apple/swift/blob/master/docs/StringManifesto.md
>
> We’re eager to hear everyone’s thoughts.
>
> Regards,
> Ben and Dave
>
>
> # String Processing For Swift 4
>
> * Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben Cohen](
https://github.com/airspeedswift\)
>
> The goal of re-evaluating Strings for Swift 4 has been fairly
ill-defined thus
> far, with just this short blurb in the
> [list of goals](
[swift-evolution] Looking back on Swift 3 and ahead to Swift 4
):
>
>> **String re-evaluation**: String is one of the most important
fundamental
>> types in the language. The standard library leads have numerous ideas
of how
>> to improve the programming model for it, without jeopardizing the goals
of
>> providing a unicode-correct-by-default model. Our goal is to be better
at
>> string processing than Perl!
>
> For Swift 4 and beyond we want to improve three dimensions of text
processing:
>
> 1. Ergonomics
> 2. Correctness
> 3. Performance
>
> This document is meant to both provide a sense of the long-term vision
> (including undecided issues and possible approaches), and to define the
scope of
> work that could be done in the Swift 4 timeframe.
>
> ## General Principles
>
> ### Ergonomics
>
> It's worth noting that ergonomics and correctness are
mutually-reinforcing. An
> API that is easy to use—but incorrectly—cannot be considered an ergonomic
> success. Conversely, an API that's simply hard to use is also hard to
use
> correctly. Acheiving optimal performance without compromising
ergonomics or
> correctness is a greater challenge.
>
> Consistency with the Swift language and idioms is also important for
> ergonomics. There are several places both in the standard library and in
the
> foundation additions to `String` where patterns and practices found
elsewhere
> could be applied to improve usability and familiarity.
>
> ### API Surface Area
>
> Primary data types such as `String` should have APIs that are easily
understood
> given a signature and a one-line summary. Today, `String` fails that
test. As
> you can see, the Standard Library and Foundation both contribute
significantly to
> its overall complexity.
>
> **Method Arity** | **Standard Library** | **Foundation**
> ---|:---:|:---:
> 0: `ƒ()` | 5 | 7
> 1: `ƒ(:)` | 19 | 48
> 2: `ƒ(::)` | 13 | 19
> 3: `ƒ(:::)` | 5 | 11
> 4: `ƒ(::::)` | 1 | 7
> 5: `ƒ(:::::)` | - | 2
> 6: `ƒ(::::::)` | - | 1
>
> **API Kind** | **Standard Library** | **Foundation**
> ---|:---:|:---:
> `init` | 41 | 18
> `func` | 42 | 55
> `subscript` | 9 | 0
> `var` | 26 | 14
>
> **Total: 205 APIs**
>
> By contrast, `Int` has 80 APIs, none with more than two parameters.[0]
String processing is complex enough; users shouldn't have
> to press through physical API sprawl just to get started.
>
> Many of the choices detailed below contribute to solving this problem,
> including:
>
> * Restoring `Collection` conformance and dropping the `.characters`
view.
> * Providing a more general, composable slicing syntax.
> * Altering `Comparable` so that parameterized
> (e.g. case-insensitive) comparison fits smoothly into the basic
syntax.
> * Clearly separating language-dependent operations on text produced
> by and for humans from language-independent
> operations on text produced by and for machine processing.
> * Relocating APIs that fall outside the domain of basic string
processing and
> discouraging the proliferation of ad-hoc extensions.
>
>
> ### Batteries Included
>
> While `String` is available to all programs out-of-the-box, crucial APIs
for
> basic string processing tasks are still inaccessible until `Foundation`
is
> imported. While it makes sense that `Foundation` is needed for
domain-specific
> jobs such as
> [linguistic tagging](
https://developer.apple.com/reference/foundation/nslinguistictagger\),
> one should not need to import anything to, for example, do
case-insensitive
> comparison.
>
> ### Unicode Compliance and Platform Support
>
> The Unicode standard provides a crucial objective reference point for
what
> constitutes correct behavior in an extremely complex domain, so
> Unicode-correctness is, and will remain, a fundamental design principle
behind
> Swift's `String`. That said, the Unicode standard is an evolving
document, so
> this objective reference-point is not fixed.[1] While
> many of the most important operations—e.g. string hashing, equality, and
> non-localized comparison—will be stable, the semantics
> of others, such as grapheme breaking and localized comparison and case
> conversion, are expected to change as platforms are updated, so programs
should
> be written so their correctness does not depend on precise stability of
these
> semantics across OS versions or platforms. Although it may be possible
to
> imagine static and/or dynamic analysis tools that will help users find
such
> errors, the only sure way to deal with this fact of life is to educate
users.
>
> ## Design Points
>
> ### Internationalization
>
> There is strong evidence that developers cannot determine how to use
> internationalization APIs correctly. Although documentation could and
should be
> improved, the sheer size, complexity, and diversity of these APIs is a
major
> contributor to the problem, causing novices to tune out, and more
experienced
> programmers to make avoidable mistakes.
>
> The first step in improving this situation is to regularize all localized
> operations as invocations of normal string operations with extra
> parameters. Among other things, this means:
>
> 1. Doing away with `localizedXXX` methods
> 2. Providing a terse way to name the current locale as a parameter
> 3. Automatically adjusting defaults for options such
> as case sensitivity based on whether the operation is localized.
> 4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
> guidance in the
> [Internationalization and Localization Guide](
Internationalizing Your Code
).
>
> Along with appropriate documentation updates, these changes will make
localized
> operations more teachable, comprehensible, and approachable, thereby
lowering a
> barrier that currently leads some developers to ignore localization
issues
> altogether.
>
> #### The Default Behavior of `String`
>
> Although this isn't well-known, the most accessible form of many
operations on
> Swift `String` (and `NSString`) are really only appropriate for text
that is
> intended to be processed for, and consumed by, machines. The semantics
of the
> operations with the simplest spellings are always non-localized and
> language-agnostic.
>
> Two major factors play into this design choice:
>
> 1. Machine processing of text is important, so we should have
first-class,
> accessible functions appropriate to that use case.
>
> 2. The most general localized operations require a locale parameter not
required
> by their un-localized counterparts. This naturally skews complexity
towards
> localized operations.
>
> Reaffirming that `String`'s simplest APIs have
> language-independent/machine-processed semantics has the benefit of
clarifying
> the proper default behavior of operations such as comparison, and allows
us to
> make [significant optimizations](#collation-semantics) that were
previously
> thought to conflict with Unicode.
>
> #### Future Directions
>
> One of the most common internationalization errors is the unintentional
> presentation to users of text that has not been localized, but
regularizing APIs
> and improving documentation can go only so far in preventing this error.
> Combined with the fact that `String` operations are non-localized by
default,
> the environment for processing human-readable text may still be somewhat
> error-prone in Swift 4.
>
> For an audience of mostly non-experts, it is especially important that
naïve
> code is very likely to be correct if it compiles, and that more
sophisticated
> issues can be revealed progressively. For this reason, we intend to
> specifically and separately target localization and internationalization
> problems in the Swift 5 timeframe.
>
> ### Operations With Options
>
> There are three categories of common string operation that commonly need
to be
> tuned in various dimensions:
>
> **Operation**|**Applicable Options**
> ---|---
> sort ordering | locale, case/diacritic/width-insensitivity
> case conversion | locale
> pattern matching | locale, case/diacritic/width-insensitivity
>
> The defaults for case-, diacritic-, and width-insensitivity are
different for
> localized operations than for non-localized operations, so for example a
> localized sort should be case-insensitive by default, and a
non-localized sort
> should be case-sensitive by default. We propose a standard “language” of
> defaulted parameters to be used for these purposes, with usage roughly
like this:
>
> ```swift
> x.compared(to: y, case: .sensitive, in: swissGerman)
>
> x.lowercased(in: .currentLocale)
>
> x.allMatches(
> somePattern, case: .insensitive, diacritic: .insensitive)
> ```
>
> This usage might be supported by code like this:
>
> ```swift
> enum StringSensitivity {
> case sensitive
> case insensitive
> }
>
> extension Locale {
> static var currentLocale: Locale { ... }
> }
>
> extension Unicode {
> // An example of the option language in declaration context,
> // with nil defaults indicating unspecified, so defaults can be
> // driven by the presence/absence of a specific Locale
> func frobnicated(
> case caseSensitivity: StringSensitivity? = nil,
> diacritic diacriticSensitivity: StringSensitivity? = nil,
> width widthSensitivity: StringSensitivity? = nil,
> in locale: Locale? = nil
> ) -> Self { ... }
> }
> ```
>
> ### Comparing and Hashing Strings
>
> #### Collation Semantics
>
> What Unicode says about collation—which is used in `<`, `==`, and
hashing— turns
> out to be quite interesting, once you pick it apart. The full Unicode
Collation
> Algorithm (UCA) works like this:
>
> 1. Fully normalize both strings
> 2. Convert each string to a sequence of numeric triples to form a
collation key
> 3. “Flatten” the key by concatenating the sequence of first elements to
the
> sequence of second elements to the sequence of third elements
> 4. Lexicographically compare the flattened keys
>
> While step 1 can usually
> be [done quickly](UAX #15: Unicode Normalization Forms) and
> incrementally, step 2 uses a collation table that maps matching
*sequences* of
> unicode scalars in the normalized string to *sequences* of triples,
which get
> accumulated into a collation key. Predictably, this is where the real
costs
> lie.
>
> *However*, there are some bright spots to this story. First, as it
turns out,
> string sorting (localized or not) should be done down to what's called
> the
> [“identical” level](
UTS #10: Unicode Collation Algorithm),
> which adds a step 3a: append the string's normalized form to the
flattened
> collation key. At first blush this just adds work, but consider what it
does
> for equality: two strings that normalize the same, naturally, will
collate the
> same. But also, *strings that normalize differently will always collate
> differently*. In other words, for equality, it is sufficient to compare
the
> strings' normalized forms and see if they are the same. We can therefore
> entirely skip the expensive part of collation for equality comparison.
>
> Next, naturally, anything that applies to equality also applies to
hashing: it
> is sufficient to hash the string's normalized form, bypassing collation
keys.
> This should provide significant speedups over the current implementation.
> Perhaps more importantly, since comparison down to the “identical” level
applies
> even to localized strings, it means that hashing and equality can be
implemented
> exactly the same way for localized and non-localized text, and hash
tables with
> localized keys will remain valid across current-locale changes.
>
> Finally, once it is agreed that the *default* role for `String` is to
handle
> machine-generated and machine-readable text, the default ordering of
`String`s
> need no longer use the UCA at all. It is sufficient to order them in
any way
> that's consistent with equality, so `String` ordering can simply be a
> lexicographical comparison of normalized forms,[4]
> (which is equivalent to lexicographically comparing the sequences of
grapheme
> clusters), again bypassing step 2 and offering another speedup.
>
> This leaves us executing the full UCA *only* for localized sorting, and
ICU's
> implementation has apparently been very well optimized.
>
> Following this scheme everywhere would also allow us to make sorting
behavior
> consistent across platforms. Currently, we sort `String` according to
the UCA,
> except that—*only on Apple platforms*—pairs of ASCII characters are
ordered by
> unicode scalar value.
>
> #### Syntax
>
> Because the current `Comparable` protocol expresses all comparisons with
binary
> operators, string comparisons—which may require
> additional [options](#operations-with-options)—do not fit smoothly into
the
> existing syntax. At the same time, we'd like to solve other problems
with
> comparison, as outlined
> in
> [this proposal](
https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
> (implemented by changes at the head
> of
> [this branch](
https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
> We should adopt a modification of that proposal that uses a method
rather than
> an operator `<=>`:
>
> ```swift
> enum SortOrder { case before, same, after }
>
> protocol Comparable : Equatable {
> func compared(to: Self) -> SortOrder
> ...
> }
> ```
>
> This change will give us a syntactic platform on which to implement
methods with
> additional, defaulted arguments, thereby unifying and regularizing
comparison
> across the library.
>
> ```swift
> extension String {
> func compared(to: Self) -> SortOrder
>
> }
> ```
>
> **Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also
possible
> that the standard library simply adopts Foundation's `ComparisonResult`
as is,
> but we believe the community should at least consider alternate naming
before
> that happens. There will be an opportunity to discuss the choices in
detail
> when the modified
> [Comparison Proposal](
https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
> up for review.
>
> ### `String` should be a `Collection` of `Character`s Again
>
> In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
> convinced ourselves that its semantics differed from those of
`Collection` too
> significantly.
>
> It was always well understood that if strings were treated as sequences
of
> `UnicodeScalar`s, algorithms such as `lexicographicalCompare`,
`elementsEqual`,
> and `reversed` would produce nonsense results. Thus, in Swift 1.0,
`String` was
> a collection of `Character` (extended grapheme clusters). During 2.0
> development, though, we realized that correct string concatenation could
> occasionally merge distinct grapheme clusters at the start and end of
combined
> strings.
>
> This quirk aside, every aspect of strings-as-collections-of-graphemes
appears to
> comport perfectly with Unicode. We think the concatenation problem is
tolerable,
> because the cases where it occurs all represent partially-formed
constructs. The
> largest class—isolated combining characters such as ◌́ (U+0301 COMBINING
ACUTE
> ACCENT)—are explicitly called out in the Unicode standard as
> “[degenerate](
UAX #29: Unicode Text Segmentation)” or
> “[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”.
The other
> cases—such as a string ending in a zero-width joiner or half of a
regional
> indicator—appear to be equally transient and unlikely outside of a text
editor.
>
> Admitting these cases encourages exploration of grapheme composition and
is
> consistent with what appears to be an overall Unicode philosophy that “no
> special provisions are made to get marginally better behavior for… cases
that
> never occur in practice.”[2] Furthermore, it seems
> unlikely to disturb the semantics of any plausible algorithms. We can
handle
> these cases by documenting them, explicitly stating that the elements of
a
> `String` are an emergent property based on Unicode rules.
>
> The benefits of restoring `Collection` conformance are substantial:
>
> * Collection-like operations encourage experimentation with strings to
> investigate and understand their behavior. This is useful for
teaching new
> programmers, but also good for experienced programmers who want to
> understand more about strings/unicode.
>
> * Extended grapheme clusters form a natural element boundary for Unicode
> strings. For example, searching and matching operations will always
produce
> results that line up on grapheme cluster boundaries.
>
> * Character-by-character processing is a legitimate thing to do in many
real
> use-cases, including parsing, pattern matching, and language-specific
> transformations such as transliteration.
>
> * `Collection` conformance makes a wide variety of powerful operations
> available that are appropriate to `String`'s default role as the
vehicle for
> machine processed text.
>
> The methods `String` would inherit from `Collection`, where similar to
> higher-level string algorithms, have the right semantics. For
example,
> grapheme-wise `lexicographicalCompare`, `elementsEqual`, and
application of
> `flatMap` with case-conversion, produce the same results one would
expect
> from whole-string ordering comparison, equality comparison, and
> case-conversion, respectively. `reverse` operates correctly on
graphemes,
> keeping diacritics moored to their base characters and leaving emoji
intact.
> Other methods such as `indexOf` and `contains` make obvious sense. A
few
> `Collection` methods, like `min` and `max`, may not be particularly
useful
> on `String`, but we don't consider that to be a problem worth
solving, in
> the same way that we wouldn't try to suppress `min` and `max` on a
> `Set([UInt8])` that was used to store IP addresses.
>
> * Many of the higher-level operations that we want to provide for
`String`s,
> such as parsing and pattern matching, should apply to any
`Collection`, and
> many of the benefits we want for `Collections`, such
> as unified slicing, should accrue
> equally to `String`. Making `String` part of the same protocol
hierarchy
> allows us to write these operations once and not worry about keeping
the
> benefits in sync.
>
> * Slicing strings into substrings is a crucial part of the vocabulary of
> string processing, and all other sliceable things are `Collection`s.
> Because of its collection-like behavior, users naturally think of
`String`
> in collection terms, but run into frustrating limitations where it
fails to
> conform and are left to wonder where all the differences lie. Many
simply
> “correct” this limitation by declaring a trivial conformance:
>
> ```swift
> extension String : BidirectionalCollection {}
> ```
>
> Even if we removed indexing-by-element from `String`, users could
still do
> this:
>
> ```swift
> extension String : BidirectionalCollection {
> subscript(i: Index) -> Character { return characters[i] }
> }
> ```
>
> It would be much better to legitimize the conformance to `Collection`
and
> simply document the oddity of any concatenation corner-cases, than to
deny
> users the benefits on the grounds that a few cases are confusing.
>
> Note that the fact that `String` is a collection of graphemes does *not*
mean
> that string operations will necessarily have to do grapheme boundary
> recognition. See the Unicode protocol section for details.
>
> ### `Character` and `CharacterSet`
>
> `Character`, which represents a
> Unicode
> [extended grapheme cluster](
UAX #29: Unicode Text Segmentation),
> is a bit of a black box, requiring conversion to `String` in order to
> do any introspection, including interoperation with ASCII. To fix this,
we should:
>
> - Add a `unicodeScalars` view much like `String`'s, so that the
sub-structure
> of grapheme clusters is discoverable.
> - Add a failable `init` from sequences of scalars (returning nil for
sequences
> that contain 0 or 2+ graphemes).
> - (Lower priority) expose some operations, such as `func uppercase() ->
> String`, `var isASCII: Bool`, and, to the extent they can be sensibly
> generalized, queries of unicode properties that should also be exposed
on
> `UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .
>
> Despite its name, `CharacterSet` currently operates on the Swift
`UnicodeScalar`
> type. This means it is usable on `String`, but only by going through the
unicode
> scalar view. To deal with this clash in the short term, `CharacterSet`
should be
> renamed to `UnicodeScalarSet`. In the longer term, it may be
appropriate to
> introduce a `CharacterSet` that provides similar functionality for
extended
> grapheme clusters.[5]
>
> ### Unification of Slicing Operations
>
> Creating substrings is a basic part of String processing, but the slicing
> operations that we have in Swift are inconsistent in both their spelling
and
> their naming:
>
> * Slices with two explicit endpoints are done with subscript, and
support
> in-place mutation:
>
> ```swift
> s[i..<j].mutate()
> ```
>
> * Slicing from an index to the end, or from the start to an index, is
done
> with a method and does not support in-place mutation:
> ```swift
> s.prefix(upTo: i).readOnly()
> ```
>
> Prefix and suffix operations should be migrated to be subscripting
operations
> with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`,
as
> in
> [this proposal](
https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md
).
> With generic subscripting in the language, that will allow us to
collapse a wide
> variety of methods and subscript overloads into a single implementation,
and
> give users an easy-to-use and composable way to describe subranges.
>
> Further extending this EDSL to integrate use-cases like
`s.prefix(maxLength: 5)`
> is an ongoing research project that can be considered part of the
potential
> long-term vision of text (and collection) processing.
>
> ### Substrings
>
> When implementing substring slicing, languages are faced with three
options:
>
> 1. Make the substrings the same type as string, and share storage.
> 2. Make the substrings the same type as string, and copy storage when
making the substring.
> 3. Make substrings a different type, with a storage copy on conversion
to string.
>
> We think number 3 is the best choice. A walk-through of the tradeoffs
follows.
>
> #### Same type, shared storage
>
> In Swift 3.0, slicing a `String` produces a new `String` that is a view
into a
> subrange of the original `String`'s storage. This is why `String` is 3
words in
> size (the start, length and buffer owner), unlike the similar `Array`
type
> which is only one.
>
> This is a simple model with big efficiency gains when chopping up
strings into
> multiple smaller strings. But it does mean that a stored substring keeps
the
> entire original string buffer alive even after it would normally have
been
> released.
>
> This arrangement has proven to be problematic in other programming
languages,
> because applications sometimes extract small strings from large ones and
keep
> those small strings long-term. That is considered a memory leak and was
enough
> of a problem in Java that they changed from substrings sharing storage to
> making a copy in 1.7.
>
> #### Same type, copied storage
>
> Copying of substrings is also the choice made in C#, and in the default
> `NSString` implementation. This approach avoids the memory leak issue,
but has
> obvious performance overhead in performing the copies.
>
> This in turn encourages trafficking in string/range pairs instead of in
> substrings, for performance reasons, leading to API challenges. For
example:
>
> ```swift
> foo.compare(bar, range: start..<end)
> ```
>
> Here, it is not clear whether `range` applies to `foo` or `bar`. This
> relationship is better expressed in Swift as a slicing operation:
>
> ```swift
> foo[start..<end].compare(bar)
> ```
>
> Not only does this clarify to which string the range applies, it also
brings
> this sub-range capability to any API that operates on `String` "for
free". So
> these other combinations also work equally well:
>
> ```swift
> // apply range on argument rather than target
> foo.compare(bar[start..<end])
> // apply range on both
> foo[start..<end].compare(bar[start1..<end1])
> // compare two strings ignoring first character
> foo.dropFirst().compare(bar.dropFirst())
> ```
>
> In all three cases, an explicit range argument need not appear on the
`compare`
> method itself. The implementation of `compare` does not need to know
anything
> about ranges. Methods need only take range arguments when that was an
> integral part of their purpose (for example, setting the start and end
of a
> user's current selection in a text box).
>
> #### Different type, shared storage
>
> The desire to share underlying storage while preventing accidental
memory leaks
> occurs with slices of `Array`. For this reason we have an `ArraySlice`
type.
> The inconvenience of a separate type is mitigated by most operations
used on
> `Array` from the standard library being generic over `Sequence` or
`Collection`.
>
> We should apply the same approach for `String` by introducing a distinct
> `SubSequence` type, `Substring`. Similar advice given for `ArraySlice`
would apply to `Substring`:
>
>> Important: Long-term storage of `Substring` instances is discouraged. A
>> substring holds a reference to the entire storage of a larger string,
not
>> just to the portion it presents, even after the original string's
lifetime
>> ends. Long-term storage of a `Substring` may therefore prolong the
lifetime
>> of large strings that are no longer otherwise accessible, which can
appear
>> to be memory leakage.
>
> When assigning a `Substring` to a longer-lived variable (usually a stored
> property) explicitly of type `String`, a type conversion will be
performed, and
> at this point the substring buffer is copied and the original string's
storage
> can be released.
>
> A `String` that was not its own `Substring` could be one word—a single
tagged
> pointer—without requiring additional allocations. `Substring`s would be
a view
> onto a `String`, so are 3 words - pointer to owner, pointer to start,
and a
> length. The small string optimization for `Substring` would take
advantage of
> the larger size, probably with a less compressed encoding for speed.
>
> The downside of having two types is the inconvenience of sometimes
having a
> `Substring` when you need a `String`, and vice-versa. It is likely this
would
> be a significantly bigger problem than with `Array` and `ArraySlice`, as
> slicing of `String` is such a common operation. It is especially
relevant to
> existing code that assumes `String` is the currency type. To ease the
pain of
> type mismatches, `Substring` should be a subtype of `String` in the same
way
> that `Int` is a subtype of `Optional<Int>`. This would give users an
implicit
> conversion from `Substring` to `String`, as well as the usual implicit
> conversions such as `[Substring]` to `[String]` that other subtype
> relationships receive.
>
> In most cases, type inference combined with the subtype relationship
should
> make the type difference a non-issue and users will not care which type
they
> are using. For flexibility and optimizability, most operations from the
> standard library will traffic in generic models of
> [`Unicode`](#the--code-unicode--code--protocol).
>
> ##### Guidance for API Designers
>
> In this model, **if a user is unsure about which type to use, `String`
is always
> a reasonable default**. A `Substring` passed where `String` is expected
will be
> implicitly copied. When compared to the “same type, copied storage”
model, we
> have effectively deferred the cost of copying from the point where a
substring
> is created until it must be converted to `String` for use with an API.
>
> A user who needs to optimize away copies altogether should use this
guideline:
> if for performance reasons you are tempted to add a `Range` argument to
your
> method as well as a `String` to avoid unnecessary copies, you should
instead
> use `Substring`.
>
> ##### The “Empty Subscript”
>
> To make it easy to call such an optimized API when you only have a
`String` (or
> to call any API that takes a `Collection`'s `SubSequence` when all you
have is
> the `Collection`), we propose the following “empty subscript” operation,
>
> ```swift
> extension Collection {
> subscript() -> SubSequence {
> return self[startIndex..<endIndex]
> }
> }
> ```
>
> which allows the following usage:
>
> ```swift
> funcThatIsJustLooking(at: person.name) // pass person.name as
Substring
> ```
>
> The `` syntax can be offered as a fixit when needed, similar to `&`
for an
> `inout` argument. While it doesn't help a user to convert `[String]` to
> `[Substring]`, the need for such conversions is extremely rare, can be
done with
> a simple `map` (which could also be offered by a fixit):
>
> ```swift
> takesAnArrayOfSubstring(arrayOfString.map { $0 })
> ```
>
> #### Other Options Considered
>
> As we have seen, all three options above have downsides, but it's
possible
> these downsides could be eliminated/mitigated by the compiler. We are
proposing
> one such mitigation—implicit conversion—as part of the the "different
type,
> shared storage" option, to help avoid the cognitive load on developers of
> having to deal with a separate `Substring` type.
>
> To avoid the memory leak issues of a "same type, shared storage"
substring
> option, we considered whether the compiler could perform an implicit
copy of
> the underlying storage when it detects the string is being "stored" for
long
> term usage, say when it is assigned to a stored property. The trouble
with this
> approach is it is very difficult for the compiler to distinguish between
> long-term storage versus short-term in the case of abstractions that
rely on
> stored properties. For example, should the storing of a substring inside
an
> `Optional` be considered long-term? Or the storing of multiple substrings
> inside an array? The latter would not work well in the case of a
> `components(separatedBy:)` implementation that intended to return an
array of
> substrings. It would also be difficult to distinguish intentional
medium-term
> storage of substrings, say by a lexer. There does not appear to be an
effective
> consistent rule that could be applied in the general case for detecting
when a
> substring is truly being stored long-term.
>
> To avoid the cost of copying substrings under "same type, copied
storage", the
> optimizer could be enhanced to to reduce the impact of some of those
copies.
> For example, this code could be optimized to pull the invariant
substring out
> of the loop:
>
> ```swift
> for _ in 0..<lots {
> someFunc(takingString: bigString[bigRange])
> }
> ```
>
> It's worth noting that a similar optimization is needed to avoid an
equivalent
> problem with implicit conversion in the "different type, shared storage"
case:
>
> ```swift
> let substring = bigString[bigRange]
> for _ in 0..<lots { someFunc(takingString: substring) }
> ```
>
> However, in the case of "same type, copied storage" there are many use
cases
> that cannot be optimized as easily. Consider the following simple
definition of
> a recursive `contains` algorithm, which when substring slicing is linear
makes
> the overall algorithm quadratic:
>
> ```swift
> extension String {
> func containsChar(_ x: Character) -> Bool {
> return !isEmpty && (first == x || dropFirst().containsChar(x))
> }
> }
> ```
>
> For the optimizer to eliminate this problem is unrealistic, forcing the
user to
> remember to optimize the code to not use string slicing if they want it
to be
> efficient (assuming they remember):
>
> ```swift
> extension String {
> // add optional argument tracking progress through the string
> func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil)
-> Bool {
> let idx = idx ?? startIndex
> return idx != endIndex
> && (self[idx] == x || containsCharacter(x, atOrAfter:
index(after: idx)))
> }
> }
> ```
>
> #### Substrings, Ranges and Objective-C Interop
>
> The pattern of passing a string/range pair is common in several
Objective-C
> APIs, and is made especially awkward in Swift by the
non-interchangeability of
> `Range<String.Index>` and `NSRange`.
>
> ```swift
> s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))
> ```
>
> In general, however, the Swift idiom for operating on a sub-range of a
> `Collection` is to *slice* the collection and operate on that:
>
> ```swift
> s2.find(s2[j..<s2.endIndex])
> ```
>
> Therefore, APIs that operate on an `NSString`/`NSRange` pair should be
imported
> without the `NSRange` argument. The Objective-C importer should be
changed to
> give these APIs special treatment so that when a `Substring` is passed,
instead
> of being converted to a `String`, the full `NSString` and range are
passed to
> the Objective-C method, thereby avoiding a copy.
>
> As a result, you would never need to pass an `NSRange` to these APIs,
which
> solves the impedance problem by eliminating the argument, resulting in
more
> idiomatic Swift code while retaining the performance benefit. To help
users
> manually handle any cases that remain, Foundation should be augmented to
allow
> the following syntax for converting to and from `NSRange`:
>
> ```swift
> let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
> let iToJ = Range(nsr, in: s) // Equivalent to i..<j
> ```
>
> ### The `Unicode` protocol
>
> With `Substring` and `String` being distinct types and sharing almost all
> interface and semantics, and with the highest-performance string
processing
> requiring knowledge of encoding and layout that the currency types can't
> provide, it becomes important to capture the common “string API” in a
protocol.
> Since Unicode conformance is a key feature of string processing in
swift, we
> call that protocol `Unicode`:
>
> **Note:** The following assumes several features that are planned but
not yet implemented in
> Swift, and should be considered a sketch rather than a final design.
>
> ```swift
> protocol Unicode
> : Comparable, BidirectionalCollection where Element == Character {
>
> associatedtype Encoding : UnicodeEncoding
> var encoding: Encoding { get }
>
> associatedtype CodeUnits
> : RandomAccessCollection where Element == Encoding.CodeUnit
> var codeUnits: CodeUnits { get }
>
> associatedtype UnicodeScalars
> : BidirectionalCollection where Element == UnicodeScalar
> var unicodeScalars: UnicodeScalars { get }
>
> associatedtype ExtendedASCII
> : BidirectionalCollection where Element == UInt32
> var extendedASCII: ExtendedASCII { get }
>
> var unicodeScalars: UnicodeScalars { get }
> }
>
> extension Unicode {
> // ... define high-level non-mutating string operations, e.g. search ...
>
> func compared<Other: Unicode>(
> to rhs: Other,
> case caseSensitivity: StringSensitivity? = nil,
> diacritic diacriticSensitivity: StringSensitivity? = nil,
> width widthSensitivity: StringSensitivity? = nil,
> in locale: Locale? = nil
> ) -> SortOrder { ... }
> }
>
> extension Unicode : RangeReplaceableCollection where CodeUnits :
> RangeReplaceableCollection {
> // Satisfy protocol requirement
> mutating func replaceSubrange<C : Collection>(_: Range<Index>, with:
C)
> where C.Element == Element
>
> // ... define high-level mutating string operations, e.g. replace ...
> }
>
> ```
>
> The goal is that `Unicode` exposes the underlying encoding and code
units in
> such a way that for types with a known representation (e.g. a
high-performance
> `UTF8String`) that information can be known at compile-time and can be
used to
> generate a single path, while still allowing types like `String` that
admit
> multiple representations to use runtime queries and branches to fast path
> specializations.
>
> **Note:** `Unicode` would make a fantastic namespace for much of
> what's in this proposal if we could get the ability to nest types and
> protocols in protocols.
>
>
> ### Scanning, Matching, and Tokenization
>
> #### Low-Level Textual Analysis
>
> We should provide convenient APIs processing strings by character. For
example,
> it should be easy to cleanly express, “if this string starts with `"f"`,
process
> the rest of the string as follows…” Swift is well-suited to expressing
this
> common pattern beautifully, but we need to add the APIs. Here are two
examples
> of the sort of code that might be possible given such APIs:
>
> ```swift
> if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
> somethingWith(input) // process the rest of input
> }
>
> if let (number, restOfInput) = input.parsingPrefix(Int.self) {
> ...
> }
> ```
>
> The specific spelling and functionality of APIs like this are TBD. The
larger
> point is to make sure matching-and-consuming jobs are well-supported.
>
> #### Unified Pattern Matcher Protocol
>
> Many of the current methods that do matching are overloaded to do the
same
> logical operations in different ways, with the following axes:
>
> - Logical Operation: `find`, `split`, `replace`, match at start
> - Kind of pattern: `CharacterSet`, `String`, a regex, a closure
> - Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
> the method name, and sometimes an argument
> - Whole string or subrange.
>
> We should represent these aspects as orthogonal, composable components,
> abstracting pattern matchers into a protocol like
> [this one](
https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33
),
> that can allow us to define logical operations once, without introducing
> overloads, and massively reducing API surface area.
>
> For example, using the strawman prefix `%` syntax to turn string
literals into
> patterns, the following pairs would all invoke the same generic methods:
>
> ```swift
> if let found = s.firstMatch(%"searchString") { ... }
> if let found = s.firstMatch(someRegex) { ... }
>
> for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
> for m in s.allMatches(someRegex) { ... }
>
> let items = s.split(separatedBy: ", ")
> let tokens = s.split(separatedBy: CharacterSet.whitespace)
> ```
>
> Note that, because Swift requires the indices of a slice to match the
indices of
> the range from which it was sliced, operations like `firstMatch` can
return a
> `Substring?` in lieu of a `Range<String.Index>?`: the indices of the
match in
> the string being searched, if needed, can easily be recovered as the
> `startIndex` and `endIndex` of the `Substring`.
>
> Note also that matching operations are useful for collections in
general, and
> would fall out of this proposal:
>
> ```
> // replace subsequences of contiguous NaNs with zero
> forces.replace(oneOrMore([Float.nan]), [0.0])
> ```
>
> #### Regular Expressions
>
> Addressing regular expressions is out of scope for this proposal.
> That said, it is important that to note the pattern matching protocol
mentioned
> above provides a suitable foundation for regular expressions, and types
such as
> `NSRegularExpression` can easily be retrofitted to conform to it. In the
> future, support for regular expression literals in the compiler could
allow for
> compile-time syntax checking and optimization.
>
> ### String Indices
>
> `String` currently has four views—`characters`, `unicodeScalars`,
`utf8`, and
> `utf16`—each with its own opaque index type. The APIs used to translate
indices
> between views add needless complexity, and the opacity of indices makes
them
> difficult to serialize.
>
> The index translation problem has two aspects:
>
> 1. `String` views cannot consume one anothers' indices without a
cumbersome
> conversion step. An index into a `String`'s `characters` must be
translated
> before it can be used as a position in its `unicodeScalars`.
Although these
> translations are rarely needed, they add conceptual and API
complexity.
> 2. Many APIs in the core libraries and other frameworks still expose
`String`
> positions as `Int`s and regions as `NSRange`s, which can only
reference a
> `utf16` view and interoperate poorly with `String` itself.
>
> #### Index Interchange Among Views
>
> String's need for flexible backing storage and reasonably-efficient
indexing
> (i.e. without dynamically allocating and reference-counting the indices
> themselves) means indices need an efficient underlying storage type.
Although
> we do not wish to expose `String`'s indices *as* integers, `Int` offsets
into
> underlying code unit storage makes a good underlying storage type,
provided
> `String`'s underlying storage supports random-access. We think
random-access
> *code-unit storage* is a reasonable requirement to impose on all `String`
> instances.
>
> Making these `Int` code unit offsets conveniently accessible and
constructible
> solves the serialization problem:
>
> ```swift
> clipboard.write(s.endIndex.codeUnitOffset)
> let offset = clipboard.read(Int.self)
> let i = String.Index(codeUnitOffset: offset)
> ```
>
> Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
> and [`extendedASCII`](#parsing-ascii-structure) views can be made
entirely
> seamless by having them share an index type (semantics of indexing a
`String`
> between grapheme cluster boundaries are TBD—it can either trap or be
forgiving).
> Having a common index allows easy traversal into the interior of
graphemes,
> something that is often needed, without making it likely that someone
will do it
> by accident.
>
> - `String.index(after:)` should advance to the next grapheme, even when
the
> index points partway through a grapheme.
>
> - `String.index(before:)` should move to the start of the grapheme before
> the current position.
>
> Seamless index interchange between `String` and its UTF-8 or UTF-16
views is not
> crucial, as the specifics of encoding should not be a concern for most
use
> cases, and would impose needless costs on the indices of other views.
That
> said, we can make translation much more straightforward by exposing
simple
> bidirectional converting `init`s on both index types:
>
> ```swift
> let u8Position = String.UTF8.Index(someStringIndex)
> let originalPosition = String.Index(u8Position)
> ```
>
> #### Index Interchange with Cocoa
>
> We intend to address `NSRange`s that denote substrings in Cocoa APIs as
> described [later in this
document](#substrings--ranges-and-objective-c-interop).
> That leaves the interchange of bare indices with Cocoa APIs trafficking
in
> `Int`. Hopefully such APIs will be rare, but when needed, the following
> extension, which would be useful for all `Collections`, can help:
>
> ```swift
> extension Collection {
> func index(offset: IndexDistance) -> Index {
> return index(startIndex, offsetBy: offset)
> }
> func offset(of i: Index) -> IndexDistance {
> return distance(from: startIndex, to: i)
> }
> }
> ```
>
> Then integers can easily be translated into offsets into a `String`'s
`utf16`
> view for consumption by Cocoa:
>
> ```swift
> let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
> let swiftIndex = s.utf16.index(offset: cocoaIndex)
> ```
>
> ### Formatting
>
> A full treatment of formatting is out of scope of this proposal, but
> we believe it's crucial for completing the text processing picture. This
> section details some of the existing issues and thinking that may guide
future
> development.
>
> #### Printf-Style Formatting
>
> `String.format` is designed on the `printf` model: it takes a format
string with
> textual placeholders for substitution, and an arbitrary list of other
arguments.
> The syntax and meaning of these placeholders has a long history in
> C, but for anyone who doesn't use them regularly they are cryptic and
complex,
> as the `printf (3)` man page attests.
>
> Aside from complexity, this style of API has two major problems: First,
the
> spelling of these placeholders must match up to the types of the
arguments, in
> the right order, or the behavior is undefined. Some limited support for
> compile-time checking of this correspondence could be implemented, but
only for
> the cases where the format string is a literal. Second, there's no
reasonable
> way to extend the formatting vocabulary to cover the needs of new types:
you are
> stuck with what's in the box.
>
> #### Foundation Formatters
>
> The formatters supplied by Foundation are highly capable and versatile,
offering
> both formatting and parsing services. When used for formatting, though,
the
> design pattern demands more from users than it should:
>
> * Matching the type of data being formatted to a formatter type
> * Creating an instance of that type
> * Setting stateful options (`currency`, `dateStyle`) on the type.
Note: the
> need for this step prevents the instance from being used and
discarded in
> the same expression where it is created.
> * Overall, introduction of needless verbosity into source
>
> These may seem like small issues, but the experience of Apple
localization
> experts is that the total drag of these factors on programmers is such
that they
> tend to reach for `String.format` instead.
>
> #### String Interpolation
>
> Swift string interpolation provides a user-friendly alternative to
printf's
> domain-specific language (just write ordinary swift code!) and its type
safety
> problems (put the data right where it belongs!) but the following issues
prevent
> it from being useful for localized formatting (among other jobs):
>
> * [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to
restrict
> types used in string interpolation.
> * [SR-1260](https://bugs.swift.org/browse/SR-1260\) String
interpolation can't
> distinguish (fragments of) the base string from the string
substitutions.
>
> In the long run, we should improve Swift string interpolation to the
point where
> it can participate in most any formatting job. Mostly this centers
around
> fixing the interpolation protocols per the previous item, and supporting
> localization.
>
> To be able to use formatting effectively inside interpolations, it needs
to be
> both lightweight (because it all happens in-situ) and discoverable. One
> approach would be to standardize on `format` methods, e.g.:
>
> ```swift
> "Column 1: \(n.format(radix:16, width:8)) *** \(message)"
>
> "Something with leading zeroes: \(x.format(fill: zero, width:8))"
> ```
>
> ### C String Interop
>
> Our support for interoperation with nul-terminated C strings is
scattered and
> incoherent, with 6 ways to transform a C string into a `String` and four
ways to
> do the inverse. These APIs should be replaced with the following
>
> ```swift
> extension String {
> /// Constructs a `String` having the same contents as
`nulTerminatedUTF8`.
> ///
> /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8
encoded
> /// bytes ending just before the first zero byte (NUL character).
> init(cString nulTerminatedUTF8: UnsafePointer<CChar>)
>
> /// Constructs a `String` having the same contents as
`nulTerminatedCodeUnits`.
> ///
> /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code
units in
> /// the given `encoding`, ending just before the first zero code unit.
> /// - Parameter encoding: describes the encoding in which the code units
> /// should be interpreted.
> init<Encoding: UnicodeEncoding>(
> cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
> encoding: Encoding)
>
> /// Invokes the given closure on the contents of the string,
represented as a
> /// pointer to a null-terminated sequence of UTF-8 code units.
> func withCString<Result>(
> _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
> }
> ```
>
> In both of the construction APIs, any invalid encoding sequence detected
will
> have its longest valid prefix replaced by U+FFFD, the Unicode replacement
> character, per Unicode specification. This covers the common case. The
> replacement is done *physically* in the underlying storage and the
validity of
> the result is recorded in the `String`'s `encoding` such that future
accesses
> need not be slowed down by possible error repair separately.
>
> Construction that is aborted when encoding errors are detected can be
> accomplished using APIs on the `encoding`. String types that retain
their
> physical encoding even in the presence of errors and are repaired
on-the-fly can
> be built as different instances of the `Unicode` protocol.
>
> ### Unicode 9 Conformance
>
> Unicode 9 (and MacOS 10.11) brought us support for family emoji, which
changes
> the process of properly identifying `Character` boundaries. We need to
update
> `String` to account for this change.
>
> ### High-Performance String Processing
>
> Many strings are short enough to store in 64 bits, many can be stored
using only
> 8 bits per unicode scalar, others are best encoded in UTF-16, and some
come to
> us already in some other encoding, such as UTF-8, that would be costly to
> translate. Supporting these formats while maintaining usability for
> general-purpose APIs demands that a single `String` type can be backed
by many
> different representations.
>
> That said, the highest performance code always requires static knowledge
of the
> data structures on which it operates, and for this code, dynamic
selection of
> representation comes at too high a cost. Heavy-duty text processing
demands a
> way to opt out of dynamism and directly use known encodings. Having this
> ability can also make it easy to cleanly specialize code that handles
dynamic
> cases for maximal efficiency on the most common representations.
>
> To address this need, we can build models of the `Unicode` protocol that
encode
> representation information into the type, such as
`NFCNormalizedUTF16String`.
>
> ### Parsing ASCII Structure
>
> Although many machine-readable formats support the inclusion of arbitrary
> Unicode text, it is also common that their fundamental structure lies
entirely
> within the ASCII subset (JSON, YAML, many XML formats). These formats
are often
> processed most efficiently by recognizing ASCII structural elements as
ASCII,
> and capturing the arbitrary sections between them in more-general
strings. The
> current String API offers no way to efficiently recognize ASCII and skip
past
> everything else without the overhead of full decoding into unicode
scalars.
>
> For these purposes, strings should supply an `extendedASCII` view that
is a
> collection of `UInt32`, where values less than `0x80` represent the
> corresponding ASCII character, and other values represent data that is
specific
> to the underlying encoding of the string.
>
> ## Language Support
>
> This proposal depends on two new features in the Swift language:
>
> 1. **Generic subscripts**, to
> enable unified slicing syntax.
>
> 2. **A subtype relationship** between
> `Substring` and `String`, enabling framework APIs to traffic solely in
> `String` while still making it possible to avoid copies by handling
> `Substring`s where necessary.
>
> Additionally, **the ability to nest types and protocols inside
> protocols** could significantly shrink the footprint of this proposal
> on the top-level Swift namespace.
>
>
> ## Open Questions
>
> ### Must `String` be limited to storing UTF-16 subset encodings?
>
> - The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is
not in
> question here; this is about what encodings must be storable, without
> transcoding, in the common currency type called “`String`”.
> - ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets. UTF-8 is not.
> - If we have a way to get at a `String`'s code units, we need a concrete
type in
> which to express them in the API of `String`, which is a concrete type
> - If String needs to be able to represent UTF-32, presumably the code
units need
> to be `UInt32`.
> - Not supporting UTF-32-encoded text seems like one reasonable design
choice.
> - Maybe we can allow UTF-8 storage in `String` and expose its code units
as
> `UInt16`, just as we would for Latin-1.
> - Supporting only UTF-16-subset encodings would imply that `String`
indices can
> be serialized without recording the `String`'s underlying encoding.
>
> ### Do we need a type-erasable base protocol for UnicodeEncoding?
>
> UnicodeEncoding has an associated type, but it may be important to be
able to
> traffic in completely dynamic encoding values, e.g. for “tell me the most
> efficient encoding for this string.”
>
> ### Should there be a string “facade?”
>
> One possible design alternative makes `Unicode` a vehicle for expressing
> the storage and encoding of code units, but does not attempt to give it
an API
> appropriate for `String`. Instead, string APIs would be provided by a
generic
> wrapper around an instance of `Unicode`:
>
> ```swift
> struct StringFacade<U: Unicode> : BidirectionalCollection {
>
> // ...APIs for high-level string processing here...
>
> var unicode: U // access to lower-level unicode details
> }
>
> typealias String = StringFacade<StringStorage>
> typealias Substring = StringFacade<StringStorage.SubSequence>
> ```
>
> This design would allow us to de-emphasize lower-level `String` APIs
such as
> access to the specific encoding, by putting them behind a `.unicode`
property.
> A similar effect in a facade-less design would require a new top-level
> `StringProtocol` playing the role of the facade with an an
`associatedtype
> Storage : Unicode`.
>
> An interesting variation on this design is possible if defaulted generic
> parameters are introduced to the language:
>
> ```swift
> struct String<U: Unicode = StringStorage>
> : BidirectionalCollection {
>
> // ...APIs for high-level string processing here...
>
> var unicode: U // access to lower-level unicode details
> }
>
> typealias Substring = String<StringStorage.SubSequence>
> ```
>
> One advantage of such a design is that naïve users will always extend
“the right
> type” (`String`) without thinking, and the new APIs will show up on
`Substring`,
> `MyUTF8String`, etc. That said, it also has downsides that should not be
> overlooked, not least of which is the confusability of the meaning of
the word
> “string.” Is it referring to the generic or the concrete type?
>
> ### `TextOutputStream` and `TextOutputStreamable`
>
> `TextOutputStreamable` is intended to provide a vehicle for
> efficiently transporting formatted representations to an output stream
> without forcing the allocation of storage. Its use of `String`, a
> type with multiple representations, at the lowest-level unit of
> communication, conflicts with this goal. It might be sufficient to
> change `TextOutputStream` and `TextOutputStreamable` to traffic in an
> associated type conforming to `Unicode`, but that is not yet clear.
> This area will require some design work.
>
> ### `description` and `debugDescription`
>
> * Should these be creating localized or non-localized representations?
> * Is returning a `String` efficient enough?
> * Is `debugDescription` pulling the weight of the API surface area it
adds?
>
> ### `StaticString`
>
> `StaticString` was added as a byproduct of standard library developed
and kept
> around because it seemed useful, but it was never truly *designed* for
client
> programmers. We need to decide what happens with it. Presumably
*something*
> should fill its role, and that should conform to `Unicode`.
>
> ## Footnotes
>
> <b id="f0">0</b> The integers rewrite currently underway is expected to
> substantially reduce the scope of `Int`'s API by using more
> generics. [:leftwards_arrow_with_hook:](#a0)
>
> <b id="f1">1</b> In practice, these semantics will usually be tied to the
> version of the installed [ICU](http://icu-project.org) library, which
> programmatically encodes the most complex rules of the Unicode Standard
and its
> de-facto extension, CLDR.[:leftwards_arrow_with_hook:](#a1)
>
> <b id="f2">2</b>
> See
> [
UAX #29: Unicode Text Segmentation](UAX #29: Unicode Text Segmentation).
Note
> that inserting Unicode scalar values to prevent merging of grapheme
clusters would
> also constitute a kind of misbehavior (one of the clusters at the
boundary would
> not be found in the result), so would be relatively costly to implement,
with
> little benefit. [:leftwards_arrow_with_hook:](#a2)
>
> <b id="f4">4</b> The use of non-UCA-compliant ordering is fully
sanctioned by
> the Unicode standard for this purpose. In fact there's
> a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf
)
> dedicated to it. In particular, §5.17 says:
>
>> When comparing text that is visible to end users, a correct linguistic
sort
>> should be used, as described in _Section 5.16, Sorting and
>> Searching_. However, in many circumstances the only requirement is for a
>> fast, well-defined ordering. In such cases, a binary ordering can be
used.
>
> [:leftwards_arrow_with_hook:](#a4)
>
>
> <b id="f5">5</b> The queries supported by `NSCharacterSet` map directly
onto
> properties in a table that's indexed by unicode scalar value. This
table is
> part of the Unicode standard. Some of these queries (e.g., “is this an
> uppercase character?”) may have fairly obvious generalizations to
grapheme
> clusters, but exactly how to do it is a research topic and *ideally*
we'd either
> establish the existing practice that the Unicode committee would
standardize, or
> the Unicode committee would do the research and we'd implement their
> result.[:leftwards_arrow_with_hook:](#a5)
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Looks pretty good in general from my quick glance–at least, it’s much
better than the current situation. I do have a couple of comments and
questions, which I’ve inlined below.

Saagar Jha

Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben

Cohen](https://github.com/airspeedswift\)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined thus
far, with just this short blurb in the
[list of

goals](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html\):

**String re-evaluation**: String is one of the most important fundamental
types in the language. The standard library leads have numerous ideas of how
to improve the programming model for it, without jeopardizing the goals of
providing a unicode-correct-by-default model. Our goal is to be better at
string processing than Perl!

For Swift 4 and beyond we want to improve three dimensions of text processing:

1. Ergonomics
2. Correctness
3. Performance

This document is meant to both provide a sense of the long-term vision
(including undecided issues and possible approaches), and to define the scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are mutually-reinforcing. An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success. Conversely, an API that's simply hard to use is also hard to use
correctly. Acheiving optimal performance without compromising ergonomics or
correctness is a greater challenge.

Minor typo: acheiving->achieving

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in the
foundation additions to `String` where patterns and practices found elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily understood
given a signature and a one-line summary. Today, `String` fails that test. As
you can see, the Standard Library and Foundation both contribute significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two

parameters.[0] String processing is complex enough; users shouldn't
have

to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

* Restoring `Collection` conformance and dropping the `.characters` view.
* Providing a more general, composable slicing syntax.
* Altering `Comparable` so that parameterized
   (e.g. case-insensitive) comparison fits smoothly into the basic syntax.
* Clearly separating language-dependent operations on text produced
   by and for humans from language-independent
   operations on text produced by and for machine processing.
* Relocating APIs that fall outside the domain of basic string processing and
   discouraging the proliferation of ad-hoc extensions.

### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs for
basic string processing tasks are still inaccessible until `Foundation` is
imported. While it makes sense that `Foundation` is needed for domain-specific
jobs such as
[linguistic tagging](https://developer.apple.com/reference/foundation/nslinguistictagger\),
one should not need to import anything to, for example, do case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle behind
Swift's `String`. That said, the Unicode standard is an evolving document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs should
be written so their correctness does not depend on precise stability of these
semantics across OS versions or platforms. Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find such
errors, the only sure way to deal with this fact of life is to educate users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly. Although documentation could and should be
improved, the sheer size, complexity, and diversity of these APIs is a major
contributor to the problem, causing novices to tune out, and more experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
  as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
   guidance in the
[Internationalization and Localization

Guide](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html\).

Along with appropriate documentation updates, these changes will make localized
operations more teachable, comprehensible, and approachable, thereby lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

#### The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many operations on
Swift `String` (and `NSString`) are really only appropriate for text that is
intended to be processed for, and consumed by, machines. The semantics of the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
  accessible functions appropriate to that use case.

2. The most general localized operations require a locale parameter not required
  by their un-localized counterparts. This naturally skews complexity towards
  localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of clarifying
the proper default behavior of operations such as comparison, and allows us to
make [significant optimizations](#collation-semantics) that were previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that naïve
code is very likely to be correct if it compiles, and that more sophisticated
issues can be revealed progressively. For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized sort
should be case-sensitive by default. We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly like this:

 x.compared(to: y, case: .sensitive, in: swissGerman)

 x.lowercased(in: .currentLocale)

 x.allMatches(
   somePattern, case: .insensitive, diacritic: .insensitive)

This usage might be supported by code like this:

enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
 static var currentLocale: Locale { ... }
}

extension Unicode {
 // An example of the option language in declaration context,
 // with nil defaults indicating unspecified, so defaults can be
 // driven by the presence/absence of a specific Locale
 func frobnicated(
   case caseSensitivity: StringSensitivity? = nil,
   diacritic diacriticSensitivity: StringSensitivity? = nil,
   width widthSensitivity: StringSensitivity? = nil,
   in locale: Locale? = nil
 ) -> Self { ... }
}

Any reason why Locale is defaulted to nil, instead of currentLocale?
It seems more useful to me.

We're establishing a repeating pattern: string (and Unicode) operations
are locale-insensitive by default, meaning the string is treated as
machine-readable rather than human-readable text.

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— turns
out to be quite interesting, once you pick it apart. The full Unicode Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a collation key
3. “Flatten” the key by concatenating the sequence of first elements to the
  sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys

While step 1 can usually
be [done quickly](UAX #15: Unicode Normalization Forms) and
incrementally, step 2 uses a collation table that maps matching *sequences* of
unicode scalars in the normalized string to *sequences* of triples, which get
accumulated into a collation key. Predictably, this is where the real costs
lie.

*However*, there are some bright spots to this story. First, as it turns out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](UTS #10: Unicode Collation Algorithm),
which adds a step 3a: append the string's normalized form to the flattened
collation key. At first blush this just adds work, but consider what it does
for equality: two strings that normalize the same, naturally, will collate the
same. But also, *strings that normalize differently will always collate
differently*. In other words, for equality, it is sufficient to compare the
strings' normalized forms and see if they are the same. We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to hashing: it
is sufficient to hash the string's normalized form, bypassing collation keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level applies
even to localized strings, it means that hashing and equality can be implemented
exactly the same way for localized and non-localized text, and hash tables with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to handle
machine-generated and machine-readable text, the default ordering of `String`s
need no longer use the UCA at all. It is sufficient to order them in any way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting behavior
consistent across platforms. Currently, we sort `String` according to the UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into the
existing syntax. At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
(implemented by changes at the head
of
[this branch](https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
We should adopt a modification of that proposal that uses a method rather than
an operator `<=>`:

Why not both? Have the “UFO” operator, with the methods as support for
more complicated use cases where the sugar doesn’t hold up.

Two reasons:

1. It's more API surface area for very little benefit

2. <,<=,==,>=, and > offer more than enough sugar. We don't see many
  circumstances where <=> would actually get used, and those few cases
  can live with the weight of x.compared(to:y).

enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
func compared(to: Self) -> SortOrder
...
}

This change will give us a syntactic platform on which to implement methods with
additional, defaulted arguments, thereby unifying and regularizing comparison
across the library.

extension String {
func compared(to: Self) -> SortOrder

}

**Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also possible
that the standard library simply adopts Foundation's `ComparisonResult` as is,
but we believe the community should at least consider alternate naming before
that happens. There will be an opportunity to discuss the choices in detail
when the modified
[Comparison Proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, `elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, `String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes appears to
comport perfectly with Unicode. We think the concatenation problem is tolerable,
because the cases where it occurs all represent partially-formed constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](UAX #29: Unicode Text Segmentation)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”. The other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text editor.

Admitting these cases encourages exploration of grapheme composition and is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial:

* Collection-like operations encourage experimentation with strings to
   investigate and understand their behavior. This is useful for teaching new
   programmers, but also good for experienced programmers who want to
   understand more about strings/unicode.

* Extended grapheme clusters form a natural element boundary for Unicode
   strings. For example, searching and matching operations will always produce
   results that line up on grapheme cluster boundaries.

* Character-by-character processing is a legitimate thing to do in many real
   use-cases, including parsing, pattern matching, and language-specific
   transformations such as transliteration.

* `Collection` conformance makes a wide variety of powerful operations
   available that are appropriate to `String`'s default role as the vehicle for
   machine processed text.

   The methods `String` would inherit from `Collection`, where similar to
   higher-level string algorithms, have the right semantics. For example,
   grapheme-wise `lexicographicalCompare`, `elementsEqual`, and application of
   `flatMap` with case-conversion, produce the same results one would expect
   from whole-string ordering comparison, equality comparison, and
   case-conversion, respectively. `reverse` operates correctly on graphemes,
   keeping diacritics moored to their base characters and leaving emoji intact.
   Other methods such as `indexOf` and `contains` make obvious sense. A few
   `Collection` methods, like `min` and `max`, may not be particularly useful
   on `String`, but we don't consider that to be a problem worth solving, in
   the same way that we wouldn't try to suppress `min` and `max` on a
   `Set([UInt8])` that was used to store IP addresses.

* Many of the higher-level operations that we want to provide for `String`s,
   such as parsing and pattern matching, should apply to any `Collection`, and
   many of the benefits we want for `Collections`, such
   as unified slicing, should accrue
   equally to `String`. Making `String` part of the same protocol hierarchy
   allows us to write these operations once and not worry about keeping the
   benefits in sync.

* Slicing strings into substrings is a crucial part of the vocabulary of
   string processing, and all other sliceable things are `Collection`s.
   Because of its collection-like behavior, users naturally think of `String`
   in collection terms, but run into frustrating limitations where it fails to
   conform and are left to wonder where all the differences lie. Many simply
   “correct” this limitation by declaring a trivial conformance:

 extension String : BidirectionalCollection {}

   Even if we removed indexing-by-element from `String`, users could still do
   this:

     extension String : BidirectionalCollection {
       subscript(i: Index) -> Character { return characters[i] }
     }

   It would be much better to legitimize the conformance to `Collection` and
   simply document the oddity of any concatenation corner-cases, than to deny
   users the benefits on the grounds that a few cases are confusing.

Will String also conform to SequenceType?

You mean Sequence, I presume (SequenceType is the old name). Every
Collection is-a Sequence, so yes.

I’ve seen many users (coming from other languages) confused that they
can’t “just” loop over a String’s characters.

Note that the fact that `String` is a collection of graphemes does *not* mean
that string operations will necessarily have to do grapheme boundary
recognition. See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](UAX #29: Unicode Text Segmentation),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII. To fix this, we should:

- Add a `unicodeScalars` view much like `String`'s, so that the sub-structure
  of grapheme clusters is discoverable.
- Add a failable `init` from sequences of scalars (returning nil for sequences
  that contain 0 or 2+ graphemes).
- (Lower priority) expose some operations, such as `func uppercase() ->
  String`, `var isASCII: Bool`, and, to the extent they can be sensibly
  generalized, queries of unicode properties that should also be exposed on
  `UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift `UnicodeScalar`
type. This means it is usable on `String`, but only by going through the unicode
scalar view. To deal with this clash in the short term, `CharacterSet` should be
renamed to `UnicodeScalarSet`. In the longer term, it may be appropriate to
introduce a `CharacterSet` that provides similar functionality for extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling and
their naming:

* Slices with two explicit endpoints are done with subscript, and support
   in-place mutation:

       s[i..<j].mutate()

* Slicing from an index to the end, or from the start to an index, is done
   with a method and does not support in-place mutation:

       s.prefix(upTo: i).readOnly()

Prefix and suffix operations should be migrated to be subscripting operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this

proposal](https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md\).

With generic subscripting in the language, that will allow us to collapse a wide
variety of methods and subscript overloads into a single implementation, and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like `s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when making the substring.
3. Make substrings a different type, with a storage copy on conversion to string.

We think number 3 is the best choice. A walk-through of the tradeoffs follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view into a
subrange of the original `String`'s storage. This is why `String` is 3 words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings into
multiple smaller strings. But it does mean that a stored substring keeps the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming languages,
because applications sometimes extract small strings from large ones and keep
those small strings long-term. That is considered a memory leak and was enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For example:

foo.compare(bar, range: start..<end)

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

foo[start..<end].compare(bar)

Not only does this clarify to which string the range applies, it also brings
this sub-range capability to any API that operates on `String` "for free". So
these other combinations also work equally well:

// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())

In all three cases, an explicit range argument need not appear on the `compare`
method itself. The implementation of `compare` does not need to know anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` type.
The inconvenience of a separate type is mitigated by most operations used on
`Array` from the standard library being generic over `Sequence` or `Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` would apply to `Substring`:

Important: Long-term storage of `Substring` instances is discouraged. A
substring holds a reference to the entire storage of a larger string, not
just to the portion it presents, even after the original string's lifetime
ends. Long-term storage of a `Substring` may therefore prolong the lifetime
of large strings that are no longer otherwise accessible, which can appear
to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be performed, and
at this point the substring buffer is copied and the original string's storage
can be released.

A `String` that was not its own `Substring` could be one word—a single tagged
pointer—without requiring additional allocations. `Substring`s would be a view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and a
length. The small string optimization for `Substring` would take advantage of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having a
`Substring` when you need a `String`, and vice-versa. It is likely this would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant to
existing code that assumes `String` is the currency type. To ease the pain of
type mismatches, `Substring` should be a subtype of `String` in the same way
that `Int` is a subtype of `Optional<Int>`. This would give users an implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship should
make the type difference a non-issue and users will not care which type they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is always
a reasonable default**. A `Substring` passed where `String` is expected will be
implicitly copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this guideline:
if for performance reasons you are tempted to add a `Range` argument to your
method as well as a `String` to avoid unnecessary copies, you should instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a `String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you have is
the `Collection`), we propose the following “empty subscript” operation,

extension Collection {
 subscript() -> SubSequence { 
   return self[startIndex..<endIndex] 
 }
}

which allows the following usage:

funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring

The `` syntax can be offered as a fixit when needed, similar to `&` for an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be done with
a simple `map` (which could also be offered by a fixit):

takesAnArrayOfSubstring(arrayOfString.map { $0[] })

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are proposing
one such mitigation—implicit conversion—as part of the the "different type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy of
the underlying storage when it detects the string is being "stored" for long
term usage, say when it is assigned to a stored property. The trouble with this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely on
stored properties. For example, should the storing of a substring inside an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array of
substrings. It would also be difficult to distinguish intentional medium-term
storage of substrings, say by a lexer. There does not appear to be an effective
consistent rule that could be applied in the general case for detecting when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", the
optimizer could be enhanced to to reduce the impact of some of those copies.
For example, this code could be optimized to pull the invariant substring out
of the loop:

for _ in 0..<lots { 
 someFunc(takingString: bigString[bigRange]) 
}

It's worth noting that a similar optimization is needed to avoid an equivalent
problem with implicit conversion in the "different type, shared storage" case:

let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }

However, in the case of "same type, copied storage" there are many use cases
that cannot be optimized as easily. Consider the following simple definition of
a recursive `contains` algorithm, which when substring slicing is linear makes
the overall algorithm quadratic:

extension String {
   func containsChar(_ x: Character) -> Bool {
       return !isEmpty && (first == x || dropFirst().containsChar(x))
   }
}

For the optimizer to eliminate this problem is unrealistic, forcing the user to
remember to optimize the code to not use string slicing if they want it to be
efficient (assuming they remember):

extension String {
   // add optional argument tracking progress through the string
   func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> Bool {
       let idx = idx ?? startIndex
       return idx != endIndex
           && (self[idx] == x || containsCharacter(x, atOrAfter: index(after: idx)))
   }
}

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several Objective-C
APIs, and is made especially awkward in Swift by the non-interchangeability of
`Range<String.Index>` and `NSRange`.

s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

s2.find(s2[j..<s2.endIndex])

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be imported
without the `NSRange` argument. The Objective-C importer should be changed to
give these APIs special treatment so that when a `Substring` is passed, instead
of being converted to a `String`, the full `NSString` and range are passed to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, which
solves the impedance problem by eliminating the argument, resulting in more
idiomatic Swift code while retaining the performance benefit. To help users
manually handle any cases that remain, Foundation should be augmented to allow
the following syntax for converting to and from `NSRange`:

let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a protocol.
Since Unicode conformance is a key feature of string processing in swift, we
call that protocol `Unicode`:

Another minor typo: capitalize “Swift"

**Note:** The following assumes several features that are planned but not yet implemented in
Swift, and should be considered a sketch rather than a final design.

protocol Unicode 
 : Comparable, BidirectionalCollection where Element == Character {

 associatedtype Encoding : UnicodeEncoding
 var encoding: Encoding { get }

 associatedtype CodeUnits 
   : RandomAccessCollection where Element == Encoding.CodeUnit
 var codeUnits: CodeUnits { get }

 associatedtype UnicodeScalars 
   : BidirectionalCollection  where Element == UnicodeScalar
 var unicodeScalars: UnicodeScalars { get }

 associatedtype ExtendedASCII 
   : BidirectionalCollection where Element == UInt32
 var extendedASCII: ExtendedASCII { get }

 var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
 // ... define high-level non-mutating string operations, e.g. search ...

 func compared<Other: Unicode>(
   to rhs: Other,
   case caseSensitivity: StringSensitivity? = nil,
   diacritic diacriticSensitivity: StringSensitivity? = nil,
   width widthSensitivity: StringSensitivity? = nil,
   in locale: Locale? = nil
 ) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
 RangeReplaceableCollection {
   // Satisfy protocol requirement
   mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: C) 
     where C.Element == Element

 // ... define high-level mutating string operations, e.g. replace ...
}

The goal is that `Unicode` exposes the underlying encoding and code units in
such a way that for types with a known representation (e.g. a high-performance
`UTF8String`) that information can be known at compile-time and can be used to
generate a single path, while still allowing types like `String` that admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.

### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character. For example,
it should be easy to cleanly express, “if this string starts with `"f"`, process
the rest of the string as follows…” Swift is well-suited to expressing this
common pattern beautifully, but we need to add the APIs. Here are two examples
of the sort of code that might be possible given such APIs:

if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
 somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
  ...
}

The specific spelling and functionality of APIs like this are TBD. The larger
point is to make sure matching-and-consuming jobs are well-supported.

+100, this kind of work is currently quite painful in Swift. Looking forward to seeing this
implemented!

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33\),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals into
patterns, the following pairs would all invoke the same generic methods:

if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)

Note that, because Swift requires the indices of a slice to match the indices of
the range from which it was sliced, operations like `firstMatch` can return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, and
would fall out of this proposal:

// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol mentioned
above provides a suitable foundation for regular expressions, and types such as
`NSRegularExpression` can easily be retrofitted to conform to it. In the
future, support for regular expression literals in the compiler could allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, and
`utf16`—each with its own opaque index type. The APIs used to translate indices
between views add needless complexity, and the opacity of indices makes them
difficult to serialize.

The index translation problem has two aspects:

1. `String` views cannot consume one anothers' indices without a cumbersome
   conversion step. An index into a `String`'s `characters` must be translated
   before it can be used as a position in its `unicodeScalars`. Although these
   translations are rarely needed, they add conceptual and API complexity.
2. Many APIs in the core libraries and other frameworks still expose `String`
   positions as `Int`s and regions as `NSRange`s, which can only reference a
   `utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets into
underlying code unit storage makes a good underlying storage type, provided
`String`'s underlying storage supports random-access. We think random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and constructible
solves the serialization problem:

clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a `String`
between grapheme cluster boundaries are TBD—it can either trap or be forgiving).
Having a common index allows easy traversal into the interior of graphemes,
something that is often needed, without making it likely that someone will do it
by accident.

- `String.index(after:)` should advance to the next grapheme, even when the
  index points partway through a grapheme.

- `String.index(before:)` should move to the start of the grapheme before
  the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`. Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

extension Collection {
 func index(offset: IndexDistance) -> Index {
   return index(startIndex, offsetBy: offset)
 }
 func offset(of i: Index) -> IndexDistance {
   return distance(from: startIndex, to: i)
 }
}

Then integers can easily be translated into offsets into a `String`'s `utf16`
view for consumption by Cocoa:

let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture. This
section details some of the existing issues and thinking that may guide future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format string with
textual placeholders for substitution, and an arbitrary list of other arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, the
spelling of these placeholders must match up to the types of the arguments, in
the right order, or the behavior is undefined. Some limited support for
compile-time checking of this correspondence could be implemented, but only for
the cases where the format string is a literal. Second, there's no reasonable
way to extend the formatting vocabulary to cover the needs of new types: you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, offering
both formatting and parsing services. When used for formatting, though, the
design pattern demands more from users than it should:

* Matching the type of data being formatted to a formatter type
* Creating an instance of that type
* Setting stateful options (`currency`, `dateStyle`) on the type. Note: the
   need for this step prevents the instance from being used and discarded in
   the same expression where it is created.
* Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to printf's
domain-specific language (just write ordinary swift code!) and its type safety
problems (put the data right where it belongs!) but the following issues prevent
it from being useful for localized formatting (among other jobs):

* [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to restrict
   types used in string interpolation.
* [SR-1260](https://bugs.swift.org/browse/SR-1260\) String interpolation can't
   distinguish (fragments of) the base string from the string substitutions.

In the long run, we should improve Swift string interpolation to the point where
it can participate in most any formatting job. Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs to be
both lightweight (because it all happens in-situ) and discoverable. One
approach would be to standardize on `format` methods, e.g.:

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

Another thing that might limit adoption is the verbosity of this
format. It works fine if I need to print one or two things, but it
gets unwieldy very quickly.

I'd like to see examples of the sorts of uses you're concerned about.

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered and
incoherent, with 6 ways to transform a C string into a `String` and four ways to
do the inverse. These APIs should be replaced with the following

extension String {
 /// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
 ///
 /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded 
 ///   bytes ending just before the first zero byte (NUL character).
 init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

 /// Constructs a `String` having the same contents as `nulTerminatedCodeUnits`.
 ///
 /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
 ///   the given `encoding`, ending just before the first zero code unit.
 /// - Parameter encoding: describes the encoding in which the code units
 ///   should be interpreted.
 init<Encoding: UnicodeEncoding>(
   cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
   encoding: Encoding)

 /// Invokes the given closure on the contents of the string, represented as a
 /// pointer to a null-terminated sequence of UTF-8 code units.
 func withCString<Result>(
   _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}

In both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification. This covers the common case. The
replacement is done *physically* in the underlying storage and the validity of
the result is recorded in the `String`'s `encoding` such that future accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`. String types that retain their
physical encoding even in the presence of errors and are repaired on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which changes
the process of properly identifying `Character` boundaries. We need to update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some come to
us already in some other encoding, such as UTF-8, that would be costly to
translate. Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by many
different representations.

That said, the highest performance code always requires static knowledge of the
data structures on which it operates, and for this code, dynamic selection of
representation comes at too high a cost. Heavy-duty text processing demands a
way to opt out of dynamism and directly use known encodings. Having this
ability can also make it easy to cleanly specialize code that handles dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that encode
representation information into the type, such as `NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies entirely
within the ASCII subset (JSON, YAML, many XML formats). These formats are often
processed most efficiently by recognizing ASCII structural elements as ASCII,
and capturing the arbitrary sections between them in more-general strings. The
current String API offers no way to efficiently recognize ASCII and skip past
everything else without the overhead of full decoding into unicode scalars.

For these purposes, strings should supply an `extendedASCII` view that is a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is specific
to the underlying encoding of the string.

There are some things that are know to lie entirely with ASCII–are
there any plans to add a way to work with them in a simple manner
(subscripting, looping, etc.), possibly through the use of a
Array<ASCIIChar>? property or whatever?

Maybe I'm misunderstanding what you have in mind but it sounds like
that's exactly what extendedASCII is designed for.

···

on Thu Jan 19 2017, Saagar Jha <swift-evolution@swift.org> wrote:

On Jan 19, 2017, at 6:56 PM, Ben Cohen via swift-evolution > <swift-evolution@swift.org> wrote:

--
-Dave

Don’t know the rational here, but it may prevent a lots of bugs.
Having a call that behave differently depending the system locale is a pain. I can’t recall how many time I had ti fix tests cases because they where implicitly relying on the default locale.
It also caused many issue with scanning and formatting non localized values as you have to explicitly specify a neutral local which is usually not properly done by the developers.

···

Le 20 janv. 2017 à 05:07, Saagar Jha via swift-evolution <swift-evolution@swift.org> a écrit :

Looks pretty good in general from my quick glance–at least, it’s much better than the current situation. I do have a couple of comments and questions, which I’ve inlined below.

Saagar Jha

Any reason why Locale is defaulted to nil, instead of currentLocale? It seems more useful to me.

Good point, Jordan.

···

on Fri Jan 20 2017, Joe Groff <swift-evolution@swift.org> wrote:

Jordan points out that the generalized slicing syntax stomps on '...x'
and 'x...', which would be somewhat obvious candidates for variadic
splatting if that ever becomes a thing. Now, variadics are a much more
esoteric feature and slicing is much more important to day-to-day
programming, so this isn't the end of the world IMO, but it is
something we'd be giving up.

--
-Dave

Very nice improvements overall!

To ease the pain of type mismatches, Substring should be a subtype
of String in the same way that Int is a subtype of
Optional<Int>. This would give users an implicit conversion from
Substring to String, as well as the usual implicit conversions such
as [Substring] to [String] that other subtype relationships receive.

As others have said, it would be nice for this to be more
general. Perhaps we can have a special type or protocol, something
like RecursiveSlice?

A general feature for subtyping is out-of-scope for the String
redesign, so I ask that you bring it up in a separate discussion.

A Substring passed where String is expected will be implicitly
copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a
substring is created until it must be converted to Stringfor use
with an API.

Could noescape parameters/new memory model with borrowing make this
more general?

It makes everything more general. That said, we don't have a design
yet, and one of the important premises is that users who don't want to
think about borrowing won't have to. So because we have to design
Strings for the language we have today, and because they have to work
in a user-model without borrowing, we're going to focus on traditional
copyable value semantics.

Again it seems very useful for all kinds of Collections.

The “Empty Subscript”

Empty subscript seems weird. IMO, it’s because of the asymmetry
between subscripts and computed properties. I would favour a model
which unifies computed properties and subscripts (e.g. computed
properties could return “addressors” for in-place mutation).
Maybe this could be an “entireCollection”/“entireSlice" computed
property?

It could, but x.entireSlice is syntactically heavyweight compared to
x, and x lives on a continuum with x[a...], x[..<b], and x[a..<b]

The goal is that Unicode exposes the underlying encoding and code
units in such a way that for types with a known representation
(e.g. a high-performance UTF8String) that information can be known
at compile-time and can be used to generate a single path, while
still allowing types like String that admit multiple representations
to use runtime queries and branches to fast path specializations.

Typo: “unicodeScalars" is in the protocol twice.

Nice catch!

If I understand it, CodeUnits is the thing which should always be
defined by conformers to Unicode, and UnicodeScalars and ExtendedASCII
could have default implementations (for example,
UTF8String/UTF16String/3rd party conformers will use those), and
String might decide to return its native buffer (e.g. if
Encoding.CodeUnit == UnicodeScalar).

Yes. I'm not certain that associated types are needed for anything but
the CodeUnits, FWIW.

I’m just wondering how difficult it would be for a 3rd-party type to
conform to Unicode. If you’re developing a text editor, for example,
it’s possible that you may need to implement your own String-like type
with some optimised storage model and it would be nice to be able to
use generic algorithms with them. I’m thinking that you will have some
kind of backing buffer, and you will want to expose regions of that to
clients as Strings so that they can render them for UI or search
through them, etc, without introducing a copy just for the semantic
understanding that this data region contains some text content.

I’ll need to examine the generic String idea more, but it’s certainly
very interesting...

Indexes

One thing which I think it critical is the ability to advance an index
by a given number of codeUnits. I was writing some code which
interfaced with the Cocoa NSTextStorage class, tagging parts of a
string that a user was editing. If this was an Array, when the user
inserts some elements before your stored indexes, those indexes become
invalid but you can easily advance by the difference to efficiently
have your indexes pointing to the same characters.

  a = Index(codeUnitOffset: a.codeUnitOffset + 5)

Currently, that’s impossible with String.

No, it's just super-cumbersome. You have to go through the utf16 view.
But I agree we need to make it easier.

If the user inserts a string at a given index, your old indexes may
not even point to the start of a grapheme cluster any more, and
advancing the index is needlessly costly. For example:

var characters = "This is a test".characters
assert(characters.count == 14)

// Store an index to something.
let endBeforePrepending = characters.endIndex

// Insert some characters somewhere.
let insertedCharacters = "[PREPENDED]".characters
assert(insertedCharacters.count == 11)
characters.replaceSubrange(characters.startIndex..<characters.startIndex, with: insertedCharacters)

// This isn’t really correct.
let endAfterPrepending = characters.index(endBeforePrepending, offsetBy: insertedCharacters.count)
assert(endAfterPrepending == characters.endIndex) // Fails Anyway. 24 != 25

The manifesto is correct to emphasise machine processing of Strings,
but it should also ensure that machine processing of mutable Strings
is efficient. That way we can tag backing-Strings inside
user-interface components and maintain those indices in a unicode-safe
way.

The way to solve this would be that, when replacing or removing a
portion of a String, you learn how many CodeUnits in the receiver’s
encoding were inserted/removed so you can shift your indexes
accordingly.

replaceRange should always return the new range of the replaced
elements, for all RangeReplaceableCollections. We just need to
implement that.

···

on Fri Jan 20 2017, Karl Wagner <swift-evolution@swift.org> wrote:

--
-Dave

...

• One thing that stood out was the interpolation format syntax, which seemed a bit convoluted and difficult to parse:

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

Have you considered treating the interpolation parenthesis more like the function call syntax? It should be a familiar pattern and easily parseable to someone versed in other areas of swift:

  “Something with leading zeroes: \(x, fill: .zero, width: 8)"

+1 - really like this. If we could make this customizable, it would be great. E.g. if x has a method called "format" that takes some arguments and returns String...

struct Foo {
  func format(withLocale locale: Locale?) -> String { ... }
}

let foo = Foo()
"\(foo, locale: Locale.current)"

···

On Jan 20, 2017, at 2:48 PM, Jonathan Hull via swift-evolution <swift-evolution@swift.org> wrote:

I think that should work for the common cases (e.g. padding, truncating, and alignment), with string-returning methods on the type (or even formatting objects ala NSNumberFormatter) being used for more exotic formatting needs (e.g. outputting a number as Hex instead of Decimal)

• Have you considered having an explicit .machine locale which means that the function should treat the string as machine readable? (as opposed to the lack of a locale)

• I almost feel like the machine readableness vs human readableness of a string is information that should travel with the string itself. It would be nice to have an extremely terse way to specify that a string is localizable (strawman syntax below), and that might also classify the string as human readable.

  let myLocalizedStr = $”This is localizable” //This gets used as the comment in the localization file

• Looking forward to RegEx literals!

Thanks,
Jon

On Jan 19, 2017, at 6:56 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben Cohen](https://github.com/airspeedswift\)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined thus
far, with just this short blurb in the
[list of goals](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html\):

**String re-evaluation**: String is one of the most important fundamental
types in the language. The standard library leads have numerous ideas of how
to improve the programming model for it, without jeopardizing the goals of
providing a unicode-correct-by-default model. Our goal is to be better at
string processing than Perl!

For Swift 4 and beyond we want to improve three dimensions of text processing:

1. Ergonomics
2. Correctness
3. Performance

This document is meant to both provide a sense of the long-term vision
(including undecided issues and possible approaches), and to define the scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are mutually-reinforcing. An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success. Conversely, an API that's simply hard to use is also hard to use
correctly. Acheiving optimal performance without compromising ergonomics or
correctness is a greater challenge.

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in the
foundation additions to `String` where patterns and practices found elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily understood
given a signature and a one-line summary. Today, `String` fails that test. As
you can see, the Standard Library and Foundation both contribute significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two parameters.[0] String processing is complex enough; users shouldn't have
to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

* Restoring `Collection` conformance and dropping the `.characters` view.
* Providing a more general, composable slicing syntax.
* Altering `Comparable` so that parameterized
  (e.g. case-insensitive) comparison fits smoothly into the basic syntax.
* Clearly separating language-dependent operations on text produced
  by and for humans from language-independent
  operations on text produced by and for machine processing.
* Relocating APIs that fall outside the domain of basic string processing and
  discouraging the proliferation of ad-hoc extensions.

### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs for
basic string processing tasks are still inaccessible until `Foundation` is
imported. While it makes sense that `Foundation` is needed for domain-specific
jobs such as
[linguistic tagging](https://developer.apple.com/reference/foundation/nslinguistictagger\),
one should not need to import anything to, for example, do case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle behind
Swift's `String`. That said, the Unicode standard is an evolving document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs should
be written so their correctness does not depend on precise stability of these
semantics across OS versions or platforms. Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find such
errors, the only sure way to deal with this fact of life is to educate users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly. Although documentation could and should be
improved, the sheer size, complexity, and diversity of these APIs is a major
contributor to the problem, causing novices to tune out, and more experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
  guidance in the
  [Internationalization and Localization Guide](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html\).

Along with appropriate documentation updates, these changes will make localized
operations more teachable, comprehensible, and approachable, thereby lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

#### The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many operations on
Swift `String` (and `NSString`) are really only appropriate for text that is
intended to be processed for, and consumed by, machines. The semantics of the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
accessible functions appropriate to that use case.

2. The most general localized operations require a locale parameter not required
by their un-localized counterparts. This naturally skews complexity towards
localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of clarifying
the proper default behavior of operations such as comparison, and allows us to
make [significant optimizations](#collation-semantics) that were previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that naïve
code is very likely to be correct if it compiles, and that more sophisticated
issues can be revealed progressively. For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized sort
should be case-sensitive by default. We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly like this:

x.compared(to: y, case: .sensitive, in: swissGerman)

x.lowercased(in: .currentLocale)

x.allMatches(
  somePattern, case: .insensitive, diacritic: .insensitive)

This usage might be supported by code like this:

enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
static var currentLocale: Locale { ... }
}

extension Unicode {
// An example of the option language in declaration context,
// with nil defaults indicating unspecified, so defaults can be
// driven by the presence/absence of a specific Locale
func frobnicated(
  case caseSensitivity: StringSensitivity? = nil,
  diacritic diacriticSensitivity: StringSensitivity? = nil,
  width widthSensitivity: StringSensitivity? = nil,
  in locale: Locale? = nil
) -> Self { ... }
}

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— turns
out to be quite interesting, once you pick it apart. The full Unicode Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a collation key
3. “Flatten” the key by concatenating the sequence of first elements to the
sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys

While step 1 can usually
be [done quickly](UAX #15: Unicode Normalization Forms) and
incrementally, step 2 uses a collation table that maps matching *sequences* of
unicode scalars in the normalized string to *sequences* of triples, which get
accumulated into a collation key. Predictably, this is where the real costs
lie.

*However*, there are some bright spots to this story. First, as it turns out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](UTS #10: Unicode Collation Algorithm),
which adds a step 3a: append the string's normalized form to the flattened
collation key. At first blush this just adds work, but consider what it does
for equality: two strings that normalize the same, naturally, will collate the
same. But also, *strings that normalize differently will always collate
differently*. In other words, for equality, it is sufficient to compare the
strings' normalized forms and see if they are the same. We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to hashing: it
is sufficient to hash the string's normalized form, bypassing collation keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level applies
even to localized strings, it means that hashing and equality can be implemented
exactly the same way for localized and non-localized text, and hash tables with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to handle
machine-generated and machine-readable text, the default ordering of `String`s
need no longer use the UCA at all. It is sufficient to order them in any way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting behavior
consistent across platforms. Currently, we sort `String` according to the UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into the
existing syntax. At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
(implemented by changes at the head
of
[this branch](https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
We should adopt a modification of that proposal that uses a method rather than
an operator `<=>`:

enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
func compared(to: Self) -> SortOrder
...
}

This change will give us a syntactic platform on which to implement methods with
additional, defaulted arguments, thereby unifying and regularizing comparison
across the library.

extension String {
func compared(to: Self) -> SortOrder

}

**Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also possible
that the standard library simply adopts Foundation's `ComparisonResult` as is,
but we believe the community should at least consider alternate naming before
that happens. There will be an opportunity to discuss the choices in detail
when the modified
[Comparison Proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, `elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, `String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes appears to
comport perfectly with Unicode. We think the concatenation problem is tolerable,
because the cases where it occurs all represent partially-formed constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](UAX #29: Unicode Text Segmentation)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”. The other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text editor.

Admitting these cases encourages exploration of grapheme composition and is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial:

* Collection-like operations encourage experimentation with strings to
  investigate and understand their behavior. This is useful for teaching new
  programmers, but also good for experienced programmers who want to
  understand more about strings/unicode.

* Extended grapheme clusters form a natural element boundary for Unicode
  strings. For example, searching and matching operations will always produce
  results that line up on grapheme cluster boundaries.

* Character-by-character processing is a legitimate thing to do in many real
  use-cases, including parsing, pattern matching, and language-specific
  transformations such as transliteration.

* `Collection` conformance makes a wide variety of powerful operations
  available that are appropriate to `String`'s default role as the vehicle for
  machine processed text.

  The methods `String` would inherit from `Collection`, where similar to
  higher-level string algorithms, have the right semantics. For example,
  grapheme-wise `lexicographicalCompare`, `elementsEqual`, and application of
  `flatMap` with case-conversion, produce the same results one would expect
  from whole-string ordering comparison, equality comparison, and
  case-conversion, respectively. `reverse` operates correctly on graphemes,
  keeping diacritics moored to their base characters and leaving emoji intact.
  Other methods such as `indexOf` and `contains` make obvious sense. A few
  `Collection` methods, like `min` and `max`, may not be particularly useful
  on `String`, but we don't consider that to be a problem worth solving, in
  the same way that we wouldn't try to suppress `min` and `max` on a
  `Set([UInt8])` that was used to store IP addresses.

* Many of the higher-level operations that we want to provide for `String`s,
  such as parsing and pattern matching, should apply to any `Collection`, and
  many of the benefits we want for `Collections`, such
  as unified slicing, should accrue
  equally to `String`. Making `String` part of the same protocol hierarchy
  allows us to write these operations once and not worry about keeping the
  benefits in sync.

* Slicing strings into substrings is a crucial part of the vocabulary of
  string processing, and all other sliceable things are `Collection`s.
  Because of its collection-like behavior, users naturally think of `String`
  in collection terms, but run into frustrating limitations where it fails to
  conform and are left to wonder where all the differences lie. Many simply
  “correct” this limitation by declaring a trivial conformance:

extension String : BidirectionalCollection {}

  Even if we removed indexing-by-element from `String`, users could still do
  this:

    extension String : BidirectionalCollection {
      subscript(i: Index) -> Character { return characters[i] }
    }

  It would be much better to legitimize the conformance to `Collection` and
  simply document the oddity of any concatenation corner-cases, than to deny
  users the benefits on the grounds that a few cases are confusing.

Note that the fact that `String` is a collection of graphemes does *not* mean
that string operations will necessarily have to do grapheme boundary
recognition. See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](UAX #29: Unicode Text Segmentation),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII. To fix this, we should:

- Add a `unicodeScalars` view much like `String`'s, so that the sub-structure
of grapheme clusters is discoverable.
- Add a failable `init` from sequences of scalars (returning nil for sequences
that contain 0 or 2+ graphemes).
- (Lower priority) expose some operations, such as `func uppercase() ->
String`, `var isASCII: Bool`, and, to the extent they can be sensibly
generalized, queries of unicode properties that should also be exposed on
`UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift `UnicodeScalar`
type. This means it is usable on `String`, but only by going through the unicode
scalar view. To deal with this clash in the short term, `CharacterSet` should be
renamed to `UnicodeScalarSet`. In the longer term, it may be appropriate to
introduce a `CharacterSet` that provides similar functionality for extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling and
their naming:

* Slices with two explicit endpoints are done with subscript, and support
  in-place mutation:

      s[i..<j].mutate()

* Slicing from an index to the end, or from the start to an index, is done
  with a method and does not support in-place mutation:

      s.prefix(upTo: i).readOnly()

Prefix and suffix operations should be migrated to be subscripting operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this proposal](https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md\).
With generic subscripting in the language, that will allow us to collapse a wide
variety of methods and subscript overloads into a single implementation, and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like `s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when making the substring.
3. Make substrings a different type, with a storage copy on conversion to string.

We think number 3 is the best choice. A walk-through of the tradeoffs follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view into a
subrange of the original `String`'s storage. This is why `String` is 3 words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings into
multiple smaller strings. But it does mean that a stored substring keeps the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming languages,
because applications sometimes extract small strings from large ones and keep
those small strings long-term. That is considered a memory leak and was enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For example:

foo.compare(bar, range: start..<end)

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

foo[start..<end].compare(bar)

Not only does this clarify to which string the range applies, it also brings
this sub-range capability to any API that operates on `String` "for free". So
these other combinations also work equally well:

// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())

In all three cases, an explicit range argument need not appear on the `compare`
method itself. The implementation of `compare` does not need to know anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` type.
The inconvenience of a separate type is mitigated by most operations used on
`Array` from the standard library being generic over `Sequence` or `Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` would apply to `Substring`:

Important: Long-term storage of `Substring` instances is discouraged. A
substring holds a reference to the entire storage of a larger string, not
just to the portion it presents, even after the original string's lifetime
ends. Long-term storage of a `Substring` may therefore prolong the lifetime
of large strings that are no longer otherwise accessible, which can appear
to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be performed, and
at this point the substring buffer is copied and the original string's storage
can be released.

A `String` that was not its own `Substring` could be one word—a single tagged
pointer—without requiring additional allocations. `Substring`s would be a view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and a
length. The small string optimization for `Substring` would take advantage of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having a
`Substring` when you need a `String`, and vice-versa. It is likely this would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant to
existing code that assumes `String` is the currency type. To ease the pain of
type mismatches, `Substring` should be a subtype of `String` in the same way
that `Int` is a subtype of `Optional<Int>`. This would give users an implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship should
make the type difference a non-issue and users will not care which type they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is always
a reasonable default**. A `Substring` passed where `String` is expected will be
implicitly copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this guideline:
if for performance reasons you are tempted to add a `Range` argument to your
method as well as a `String` to avoid unnecessary copies, you should instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a `String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you have is
the `Collection`), we propose the following “empty subscript” operation,

extension Collection {
subscript() -> SubSequence { 
  return self[startIndex..<endIndex] 
}
}

which allows the following usage:

funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring

The `` syntax can be offered as a fixit when needed, similar to `&` for an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be done with
a simple `map` (which could also be offered by a fixit):

takesAnArrayOfSubstring(arrayOfString.map { $0[] })

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are proposing
one such mitigation—implicit conversion—as part of the the "different type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy of
the underlying storage when it detects the string is being "stored" for long
term usage, say when it is assigned to a stored property. The trouble with this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely on
stored properties. For example, should the storing of a substring inside an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array of
substrings. It would also be difficult to distinguish intentional medium-term
storage of substrings, say by a lexer. There does not appear to be an effective
consistent rule that could be applied in the general case for detecting when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", the
optimizer could be enhanced to to reduce the impact of some of those copies.
For example, this code could be optimized to pull the invariant substring out
of the loop:

for _ in 0..<lots { 
someFunc(takingString: bigString[bigRange]) 
}

It's worth noting that a similar optimization is needed to avoid an equivalent
problem with implicit conversion in the "different type, shared storage" case:

let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }

However, in the case of "same type, copied storage" there are many use cases
that cannot be optimized as easily. Consider the following simple definition of
a recursive `contains` algorithm, which when substring slicing is linear makes
the overall algorithm quadratic:

extension String {
  func containsChar(_ x: Character) -> Bool {
      return !isEmpty && (first == x || dropFirst().containsChar(x))
  }
}

For the optimizer to eliminate this problem is unrealistic, forcing the user to
remember to optimize the code to not use string slicing if they want it to be
efficient (assuming they remember):

extension String {
  // add optional argument tracking progress through the string
  func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> Bool {
      let idx = idx ?? startIndex
      return idx != endIndex
          && (self[idx] == x || containsCharacter(x, atOrAfter: index(after: idx)))
  }
}

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several Objective-C
APIs, and is made especially awkward in Swift by the non-interchangeability of
`Range<String.Index>` and `NSRange`.

s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

s2.find(s2[j..<s2.endIndex])

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be imported
without the `NSRange` argument. The Objective-C importer should be changed to
give these APIs special treatment so that when a `Substring` is passed, instead
of being converted to a `String`, the full `NSString` and range are passed to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, which
solves the impedance problem by eliminating the argument, resulting in more
idiomatic Swift code while retaining the performance benefit. To help users
manually handle any cases that remain, Foundation should be augmented to allow
the following syntax for converting to and from `NSRange`:

let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a protocol.
Since Unicode conformance is a key feature of string processing in swift, we
call that protocol `Unicode`:

**Note:** The following assumes several features that are planned but not yet implemented in
Swift, and should be considered a sketch rather than a final design.

protocol Unicode 
: Comparable, BidirectionalCollection where Element == Character {

associatedtype Encoding : UnicodeEncoding
var encoding: Encoding { get }

associatedtype CodeUnits 
  : RandomAccessCollection where Element == Encoding.CodeUnit
var codeUnits: CodeUnits { get }

associatedtype UnicodeScalars 
  : BidirectionalCollection  where Element == UnicodeScalar
var unicodeScalars: UnicodeScalars { get }

associatedtype ExtendedASCII 
  : BidirectionalCollection where Element == UInt32
var extendedASCII: ExtendedASCII { get }

var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
// ... define high-level non-mutating string operations, e.g. search ...

func compared<Other: Unicode>(
  to rhs: Other,
  case caseSensitivity: StringSensitivity? = nil,
  diacritic diacriticSensitivity: StringSensitivity? = nil,
  width widthSensitivity: StringSensitivity? = nil,
  in locale: Locale? = nil
) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
RangeReplaceableCollection {
  // Satisfy protocol requirement
  mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: C) 
    where C.Element == Element

// ... define high-level mutating string operations, e.g. replace ...
}

The goal is that `Unicode` exposes the underlying encoding and code units in
such a way that for types with a known representation (e.g. a high-performance
`UTF8String`) that information can be known at compile-time and can be used to
generate a single path, while still allowing types like `String` that admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.

### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character. For example,
it should be easy to cleanly express, “if this string starts with `"f"`, process
the rest of the string as follows…” Swift is well-suited to expressing this
common pattern beautifully, but we need to add the APIs. Here are two examples
of the sort of code that might be possible given such APIs:

if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
 ...
}

The specific spelling and functionality of APIs like this are TBD. The larger
point is to make sure matching-and-consuming jobs are well-supported.

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33\),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals into
patterns, the following pairs would all invoke the same generic methods:

if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)

Note that, because Swift requires the indices of a slice to match the indices of
the range from which it was sliced, operations like `firstMatch` can return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, and
would fall out of this proposal:

// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol mentioned
above provides a suitable foundation for regular expressions, and types such as
`NSRegularExpression` can easily be retrofitted to conform to it. In the
future, support for regular expression literals in the compiler could allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, and
`utf16`—each with its own opaque index type. The APIs used to translate indices
between views add needless complexity, and the opacity of indices makes them
difficult to serialize.

The index translation problem has two aspects:

1. `String` views cannot consume one anothers' indices without a cumbersome
  conversion step. An index into a `String`'s `characters` must be translated
  before it can be used as a position in its `unicodeScalars`. Although these
  translations are rarely needed, they add conceptual and API complexity.
2. Many APIs in the core libraries and other frameworks still expose `String`
  positions as `Int`s and regions as `NSRange`s, which can only reference a
  `utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets into
underlying code unit storage makes a good underlying storage type, provided
`String`'s underlying storage supports random-access. We think random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and constructible
solves the serialization problem:

clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a `String`
between grapheme cluster boundaries are TBD—it can either trap or be forgiving).
Having a common index allows easy traversal into the interior of graphemes,
something that is often needed, without making it likely that someone will do it
by accident.

- `String.index(after:)` should advance to the next grapheme, even when the
index points partway through a grapheme.

- `String.index(before:)` should move to the start of the grapheme before
the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`. Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

extension Collection {
func index(offset: IndexDistance) -> Index {
  return index(startIndex, offsetBy: offset)
}
func offset(of i: Index) -> IndexDistance {
  return distance(from: startIndex, to: i)
}
}

Then integers can easily be translated into offsets into a `String`'s `utf16`
view for consumption by Cocoa:

let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture. This
section details some of the existing issues and thinking that may guide future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format string with
textual placeholders for substitution, and an arbitrary list of other arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, the
spelling of these placeholders must match up to the types of the arguments, in
the right order, or the behavior is undefined. Some limited support for
compile-time checking of this correspondence could be implemented, but only for
the cases where the format string is a literal. Second, there's no reasonable
way to extend the formatting vocabulary to cover the needs of new types: you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, offering
both formatting and parsing services. When used for formatting, though, the
design pattern demands more from users than it should:

* Matching the type of data being formatted to a formatter type
* Creating an instance of that type
* Setting stateful options (`currency`, `dateStyle`) on the type. Note: the
  need for this step prevents the instance from being used and discarded in
  the same expression where it is created.
* Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to printf's
domain-specific language (just write ordinary swift code!) and its type safety
problems (put the data right where it belongs!) but the following issues prevent
it from being useful for localized formatting (among other jobs):

* [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to restrict
  types used in string interpolation.
* [SR-1260](https://bugs.swift.org/browse/SR-1260\) String interpolation can't
  distinguish (fragments of) the base string from the string substitutions.

In the long run, we should improve Swift string interpolation to the point where
it can participate in most any formatting job. Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs to be
both lightweight (because it all happens in-situ) and discoverable. One
approach would be to standardize on `format` methods, e.g.:

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered and
incoherent, with 6 ways to transform a C string into a `String` and four ways to
do the inverse. These APIs should be replaced with the following

extension String {
/// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
///
/// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded 
///   bytes ending just before the first zero byte (NUL character).
init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

/// Constructs a `String` having the same contents as `nulTerminatedCodeUnits`.
///
/// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
///   the given `encoding`, ending just before the first zero code unit.
/// - Parameter encoding: describes the encoding in which the code units
///   should be interpreted.
init<Encoding: UnicodeEncoding>(
  cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
  encoding: Encoding)

/// Invokes the given closure on the contents of the string, represented as a
/// pointer to a null-terminated sequence of UTF-8 code units.
func withCString<Result>(
  _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}

In both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification. This covers the common case. The
replacement is done *physically* in the underlying storage and the validity of
the result is recorded in the `String`'s `encoding` such that future accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`. String types that retain their
physical encoding even in the presence of errors and are repaired on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which changes
the process of properly identifying `Character` boundaries. We need to update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some come to
us already in some other encoding, such as UTF-8, that would be costly to
translate. Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by many
different representations.

That said, the highest performance code always requires static knowledge of the
data structures on which it operates, and for this code, dynamic selection of
representation comes at too high a cost. Heavy-duty text processing demands a
way to opt out of dynamism and directly use known encodings. Having this
ability can also make it easy to cleanly specialize code that handles dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that encode
representation information into the type, such as `NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies entirely
within the ASCII subset (JSON, YAML, many XML formats). These formats are often
processed most efficiently by recognizing ASCII structural elements as ASCII,
and capturing the arbitrary sections between them in more-general strings. The
current String API offers no way to efficiently recognize ASCII and skip past
everything else without the overhead of full decoding into unicode scalars.

For these purposes, strings should supply an `extendedASCII` view that is a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is specific
to the underlying encoding of the string.

## Language Support

This proposal depends on two new features in the Swift language:

1. **Generic subscripts**, to
enable unified slicing syntax.

2. **A subtype relationship** between
`Substring` and `String`, enabling framework APIs to traffic solely in
`String` while still making it possible to avoid copies by handling
`Substring`s where necessary.

Additionally, **the ability to nest types and protocols inside
protocols** could significantly shrink the footprint of this proposal
on the top-level Swift namespace.

## Open Questions

### Must `String` be limited to storing UTF-16 subset encodings?

- The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is not in
question here; this is about what encodings must be storable, without
transcoding, in the common currency type called “`String`”.
- ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets. UTF-8 is not.
- If we have a way to get at a `String`'s code units, we need a concrete type in
which to express them in the API of `String`, which is a concrete type
- If String needs to be able to represent UTF-32, presumably the code units need
to be `UInt32`.
- Not supporting UTF-32-encoded text seems like one reasonable design choice.
- Maybe we can allow UTF-8 storage in `String` and expose its code units as
`UInt16`, just as we would for Latin-1.
- Supporting only UTF-16-subset encodings would imply that `String` indices can
be serialized without recording the `String`'s underlying encoding.

### Do we need a type-erasable base protocol for UnicodeEncoding?

UnicodeEncoding has an associated type, but it may be important to be able to
traffic in completely dynamic encoding values, e.g. for “tell me the most
efficient encoding for this string.”

### Should there be a string “facade?”

One possible design alternative makes `Unicode` a vehicle for expressing
the storage and encoding of code units, but does not attempt to give it an API
appropriate for `String`. Instead, string APIs would be provided by a generic
wrapper around an instance of `Unicode`:

struct StringFacade<U: Unicode> : BidirectionalCollection {

// ...APIs for high-level string processing here...

var unicode: U // access to lower-level unicode details
}

typealias String = StringFacade<StringStorage>
typealias Substring = StringFacade<StringStorage.SubSequence>

This design would allow us to de-emphasize lower-level `String` APIs such as
access to the specific encoding, by putting them behind a `.unicode` property.
A similar effect in a facade-less design would require a new top-level
`StringProtocol` playing the role of the facade with an an `associatedtype
Storage : Unicode`.

An interesting variation on this design is possible if defaulted generic
parameters are introduced to the language:

struct String<U: Unicode = StringStorage> 
: BidirectionalCollection {

// ...APIs for high-level string processing here...

var unicode: U // access to lower-level unicode details
}

typealias Substring = String<StringStorage.SubSequence>

One advantage of such a design is that naïve users will always extend “the right
type” (`String`) without thinking, and the new APIs will show up on `Substring`,
`MyUTF8String`, etc. That said, it also has downsides that should not be
overlooked, not least of which is the confusability of the meaning of the word
“string.” Is it referring to the generic or the concrete type?

### `TextOutputStream` and `TextOutputStreamable`

`TextOutputStreamable` is intended to provide a vehicle for
efficiently transporting formatted representations to an output stream
without forcing the allocation of storage. Its use of `String`, a
type with multiple representations, at the lowest-level unit of
communication, conflicts with this goal. It might be sufficient to
change `TextOutputStream` and `TextOutputStreamable` to traffic in an
associated type conforming to `Unicode`, but that is not yet clear.
This area will require some design work.

### `description` and `debugDescription`

* Should these be creating localized or non-localized representations?
* Is returning a `String` efficient enough?
* Is `debugDescription` pulling the weight of the API surface area it adds?

### `StaticString`

`StaticString` was added as a byproduct of standard library developed and kept
around because it seemed useful, but it was never truly *designed* for client
programmers. We need to decide what happens with it. Presumably *something*
should fill its role, and that should conform to `Unicode`.

## Footnotes

<b id="f0">0</b> The integers rewrite currently underway is expected to
  substantially reduce the scope of `Int`'s API by using more
  generics. [:leftwards_arrow_with_hook:](#a0)

<b id="f1">1</b> In practice, these semantics will usually be tied to the
version of the installed [ICU](http://icu-project.org) library, which
programmatically encodes the most complex rules of the Unicode Standard and its
de-facto extension, CLDR.[:leftwards_arrow_with_hook:](#a1)

<b id="f2">2</b>
See
[UAX #29: Unicode Text Segmentation](UAX #29: Unicode Text Segmentation). Note
that inserting Unicode scalar values to prevent merging of grapheme clusters would
also constitute a kind of misbehavior (one of the clusters at the boundary would
not be found in the result), so would be relatively costly to implement, with
little benefit. [:leftwards_arrow_with_hook:](#a2)

<b id="f4">4</b> The use of non-UCA-compliant ordering is fully sanctioned by
the Unicode standard for this purpose. In fact there's
a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf\)
dedicated to it. In particular, §5.17 says:

When comparing text that is visible to end users, a correct linguistic sort
should be used, as described in _Section 5.16, Sorting and
Searching_. However, in many circumstances the only requirement is for a
fast, well-defined ordering. In such cases, a binary ordering can be used.

[:leftwards_arrow_with_hook:](#a4)

<b id="f5">5</b> The queries supported by `NSCharacterSet` map directly onto
properties in a table that's indexed by unicode scalar value. This table is
part of the Unicode standard. Some of these queries (e.g., “is this an
uppercase character?”) may have fairly obvious generalizations to grapheme
clusters, but exactly how to do it is a research topic and *ideally* we'd either
establish the existing practice that the Unicode committee would standardize, or
the Unicode committee would do the research and we'd implement their
result.[:leftwards_arrow_with_hook:](#a5)

_______________________________________________
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swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

extension Unicode {
// An example of the option language in declaration context,
// with nil defaults indicating unspecified, so defaults can be
// driven by the presence/absence of a specific Locale
func frobnicated(
   case caseSensitivity: StringSensitivity? = nil,
   diacritic diacriticSensitivity: StringSensitivity? = nil,
   width widthSensitivity: StringSensitivity? = nil,
   in locale: Locale? = nil
) -> Self { ... }
}
```

Any reason why Locale is defaulted to nil, instead of currentLocale? It
seems more useful to me.

The reason is given in the text above the code sample: "The defaults for case-, diacritic-, and width-insensitivity are different for localized operations than for non-localized operations, so for example a localized sort should be case-insensitive by default, and a non-localized sort should be case-sensitive by default."

So the API would need to distinguish between "current locale" and "no locale" (i.e. use the non-localized algorithm).

Will String also conform to SequenceType? I’ve seen many users (coming
from other languages) confused that they can’t “just” loop over a
String’s characters.

Collection refines Sequence, so all collections are sequences. This would also be true for String. So yes, you could do `for char in string` again.

Wouldn’t `x[…]` be more consistent with these other syntaxes?

···

On Jan 20, 2017, at 2:57 PM, Dave Abrahams via swift-evolution <swift-evolution@swift.org> wrote:

The “Empty Subscript”

Empty subscript seems weird. IMO, it’s because of the asymmetry
between subscripts and computed properties. I would favour a model
which unifies computed properties and subscripts (e.g. computed
properties could return “addressors” for in-place mutation).
Maybe this could be an “entireCollection”/“entireSlice" computed
property?

It could, but x.entireSlice is syntactically heavyweight compared to
x, and x lives on a continuum with x[a...], x[..<b], and x[a..<b]

This looks really great to me. I am not an expert in this area so I don’t have a lot of detailed comments. That said, it looks like it will significantly improve the string handling experience of app developers, including better bridging to the APIs we work with every day.

I did notice one particularly interesting thing in the sketch of the Unicode protocol. This section specifically calls out that it relies on features that are “planned but not yet implemented”. I was surprised to see this:

extension Unicode : RangeReplaceableCollection where CodeUnits :
  RangeReplaceableCollection

Conformances via protocol extensions is listed as “unlikely” in the generics manifesto. Has something changed such that this is now a “planned” feature (or at least less “unlikely”)?

Dang! Oops, no. This is an argument for the "facade" design variant, I guess.

···

Sent from my moss-covered three-handled family gradunza

On Jan 20, 2017, at 2:36 PM, Matthew Johnson <matthew@anandabits.com> wrote:

On Jan 19, 2017, at 8:56 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

Hi all,

Below is our take on a design manifesto for Strings in Swift 4 and beyond.

Probably best read in rendered markdown on GitHub:
https://github.com/apple/swift/blob/master/docs/StringManifesto.md

We’re eager to hear everyone’s thoughts.

Regards,
Ben and Dave

# String Processing For Swift 4

* Authors: [Dave Abrahams](https://github.com/dabrahams\), [Ben Cohen](https://github.com/airspeedswift\)

The goal of re-evaluating Strings for Swift 4 has been fairly ill-defined thus
far, with just this short blurb in the
[list of goals](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025676.html\):

**String re-evaluation**: String is one of the most important fundamental
types in the language. The standard library leads have numerous ideas of how
to improve the programming model for it, without jeopardizing the goals of
providing a unicode-correct-by-default model. Our goal is to be better at
string processing than Perl!

For Swift 4 and beyond we want to improve three dimensions of text processing:

1. Ergonomics
2. Correctness
3. Performance

This document is meant to both provide a sense of the long-term vision
(including undecided issues and possible approaches), and to define the scope of
work that could be done in the Swift 4 timeframe.

## General Principles

### Ergonomics

It's worth noting that ergonomics and correctness are mutually-reinforcing. An
API that is easy to use—but incorrectly—cannot be considered an ergonomic
success. Conversely, an API that's simply hard to use is also hard to use
correctly. Acheiving optimal performance without compromising ergonomics or
correctness is a greater challenge.

Consistency with the Swift language and idioms is also important for
ergonomics. There are several places both in the standard library and in the
foundation additions to `String` where patterns and practices found elsewhere
could be applied to improve usability and familiarity.

### API Surface Area

Primary data types such as `String` should have APIs that are easily understood
given a signature and a one-line summary. Today, `String` fails that test. As
you can see, the Standard Library and Foundation both contribute significantly to
its overall complexity.

**Method Arity** | **Standard Library** | **Foundation**
---|:---:|:---:
0: `ƒ()` | 5 | 7
1: `ƒ(:)` | 19 | 48
2: `ƒ(::)` | 13 | 19
3: `ƒ(:::)` | 5 | 11
4: `ƒ(::::)` | 1 | 7
5: `ƒ(:::::)` | - | 2
6: `ƒ(::::::)` | - | 1

**API Kind** | **Standard Library** | **Foundation**
---|:---:|:---:
`init` | 41 | 18
`func` | 42 | 55
`subscript` | 9 | 0
`var` | 26 | 14

**Total: 205 APIs**

By contrast, `Int` has 80 APIs, none with more than two parameters.[0] String processing is complex enough; users shouldn't have
to press through physical API sprawl just to get started.

Many of the choices detailed below contribute to solving this problem,
including:

* Restoring `Collection` conformance and dropping the `.characters` view.
* Providing a more general, composable slicing syntax.
* Altering `Comparable` so that parameterized
   (e.g. case-insensitive) comparison fits smoothly into the basic syntax.
* Clearly separating language-dependent operations on text produced
   by and for humans from language-independent
   operations on text produced by and for machine processing.
* Relocating APIs that fall outside the domain of basic string processing and
   discouraging the proliferation of ad-hoc extensions.

### Batteries Included

While `String` is available to all programs out-of-the-box, crucial APIs for
basic string processing tasks are still inaccessible until `Foundation` is
imported. While it makes sense that `Foundation` is needed for domain-specific
jobs such as
[linguistic tagging](https://developer.apple.com/reference/foundation/nslinguistictagger\),
one should not need to import anything to, for example, do case-insensitive
comparison.

### Unicode Compliance and Platform Support

The Unicode standard provides a crucial objective reference point for what
constitutes correct behavior in an extremely complex domain, so
Unicode-correctness is, and will remain, a fundamental design principle behind
Swift's `String`. That said, the Unicode standard is an evolving document, so
this objective reference-point is not fixed.[1] While
many of the most important operations—e.g. string hashing, equality, and
non-localized comparison—will be stable, the semantics
of others, such as grapheme breaking and localized comparison and case
conversion, are expected to change as platforms are updated, so programs should
be written so their correctness does not depend on precise stability of these
semantics across OS versions or platforms. Although it may be possible to
imagine static and/or dynamic analysis tools that will help users find such
errors, the only sure way to deal with this fact of life is to educate users.

## Design Points

### Internationalization

There is strong evidence that developers cannot determine how to use
internationalization APIs correctly. Although documentation could and should be
improved, the sheer size, complexity, and diversity of these APIs is a major
contributor to the problem, causing novices to tune out, and more experienced
programmers to make avoidable mistakes.

The first step in improving this situation is to regularize all localized
operations as invocations of normal string operations with extra
parameters. Among other things, this means:

1. Doing away with `localizedXXX` methods
2. Providing a terse way to name the current locale as a parameter
3. Automatically adjusting defaults for options such
  as case sensitivity based on whether the operation is localized.
4. Removing correctness traps like `localizedCaseInsensitiveCompare` (see
   guidance in the
   [Internationalization and Localization Guide](https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/InternationalizingYourCode/InternationalizingYourCode.html\).

Along with appropriate documentation updates, these changes will make localized
operations more teachable, comprehensible, and approachable, thereby lowering a
barrier that currently leads some developers to ignore localization issues
altogether.

#### The Default Behavior of `String`

Although this isn't well-known, the most accessible form of many operations on
Swift `String` (and `NSString`) are really only appropriate for text that is
intended to be processed for, and consumed by, machines. The semantics of the
operations with the simplest spellings are always non-localized and
language-agnostic.

Two major factors play into this design choice:

1. Machine processing of text is important, so we should have first-class,
  accessible functions appropriate to that use case.

2. The most general localized operations require a locale parameter not required
  by their un-localized counterparts. This naturally skews complexity towards
  localized operations.

Reaffirming that `String`'s simplest APIs have
language-independent/machine-processed semantics has the benefit of clarifying
the proper default behavior of operations such as comparison, and allows us to
make [significant optimizations](#collation-semantics) that were previously
thought to conflict with Unicode.

#### Future Directions

One of the most common internationalization errors is the unintentional
presentation to users of text that has not been localized, but regularizing APIs
and improving documentation can go only so far in preventing this error.
Combined with the fact that `String` operations are non-localized by default,
the environment for processing human-readable text may still be somewhat
error-prone in Swift 4.

For an audience of mostly non-experts, it is especially important that naïve
code is very likely to be correct if it compiles, and that more sophisticated
issues can be revealed progressively. For this reason, we intend to
specifically and separately target localization and internationalization
problems in the Swift 5 timeframe.

### Operations With Options

There are three categories of common string operation that commonly need to be
tuned in various dimensions:

**Operation**|**Applicable Options**
---|---
sort ordering | locale, case/diacritic/width-insensitivity
case conversion | locale
pattern matching | locale, case/diacritic/width-insensitivity

The defaults for case-, diacritic-, and width-insensitivity are different for
localized operations than for non-localized operations, so for example a
localized sort should be case-insensitive by default, and a non-localized sort
should be case-sensitive by default. We propose a standard “language” of
defaulted parameters to be used for these purposes, with usage roughly like this:

 x.compared(to: y, case: .sensitive, in: swissGerman)

 x.lowercased(in: .currentLocale)

 x.allMatches(
   somePattern, case: .insensitive, diacritic: .insensitive)

This usage might be supported by code like this:

enum StringSensitivity {
case sensitive
case insensitive
}

extension Locale {
 static var currentLocale: Locale { ... }
}

extension Unicode {
 // An example of the option language in declaration context,
 // with nil defaults indicating unspecified, so defaults can be
 // driven by the presence/absence of a specific Locale
 func frobnicated(
   case caseSensitivity: StringSensitivity? = nil,
   diacritic diacriticSensitivity: StringSensitivity? = nil,
   width widthSensitivity: StringSensitivity? = nil,
   in locale: Locale? = nil
 ) -> Self { ... }
}

### Comparing and Hashing Strings

#### Collation Semantics

What Unicode says about collation—which is used in `<`, `==`, and hashing— turns
out to be quite interesting, once you pick it apart. The full Unicode Collation
Algorithm (UCA) works like this:

1. Fully normalize both strings
2. Convert each string to a sequence of numeric triples to form a collation key
3. “Flatten” the key by concatenating the sequence of first elements to the
  sequence of second elements to the sequence of third elements
4. Lexicographically compare the flattened keys

While step 1 can usually
be [done quickly](UAX #15: Unicode Normalization Forms) and
incrementally, step 2 uses a collation table that maps matching *sequences* of
unicode scalars in the normalized string to *sequences* of triples, which get
accumulated into a collation key. Predictably, this is where the real costs
lie.

*However*, there are some bright spots to this story. First, as it turns out,
string sorting (localized or not) should be done down to what's called
the
[“identical” level](UTS #10: Unicode Collation Algorithm),
which adds a step 3a: append the string's normalized form to the flattened
collation key. At first blush this just adds work, but consider what it does
for equality: two strings that normalize the same, naturally, will collate the
same. But also, *strings that normalize differently will always collate
differently*. In other words, for equality, it is sufficient to compare the
strings' normalized forms and see if they are the same. We can therefore
entirely skip the expensive part of collation for equality comparison.

Next, naturally, anything that applies to equality also applies to hashing: it
is sufficient to hash the string's normalized form, bypassing collation keys.
This should provide significant speedups over the current implementation.
Perhaps more importantly, since comparison down to the “identical” level applies
even to localized strings, it means that hashing and equality can be implemented
exactly the same way for localized and non-localized text, and hash tables with
localized keys will remain valid across current-locale changes.

Finally, once it is agreed that the *default* role for `String` is to handle
machine-generated and machine-readable text, the default ordering of `String`s
need no longer use the UCA at all. It is sufficient to order them in any way
that's consistent with equality, so `String` ordering can simply be a
lexicographical comparison of normalized forms,[4]
(which is equivalent to lexicographically comparing the sequences of grapheme
clusters), again bypassing step 2 and offering another speedup.

This leaves us executing the full UCA *only* for localized sorting, and ICU's
implementation has apparently been very well optimized.

Following this scheme everywhere would also allow us to make sorting behavior
consistent across platforms. Currently, we sort `String` according to the UCA,
except that—*only on Apple platforms*—pairs of ASCII characters are ordered by
unicode scalar value.

#### Syntax

Because the current `Comparable` protocol expresses all comparisons with binary
operators, string comparisons—which may require
additional [options](#operations-with-options)—do not fit smoothly into the
existing syntax. At the same time, we'd like to solve other problems with
comparison, as outlined
in
[this proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\)
(implemented by changes at the head
of
[this branch](https://github.com/CodaFi/swift/commits/space-the-final-frontier\)).
We should adopt a modification of that proposal that uses a method rather than
an operator `<=>`:

enum SortOrder { case before, same, after }

protocol Comparable : Equatable {
func compared(to: Self) -> SortOrder
...
}

This change will give us a syntactic platform on which to implement methods with
additional, defaulted arguments, thereby unifying and regularizing comparison
across the library.

extension String {
func compared(to: Self) -> SortOrder

}

**Note:** `SortOrder` should bridge to `NSComparisonResult`. It's also possible
that the standard library simply adopts Foundation's `ComparisonResult` as is,
but we believe the community should at least consider alternate naming before
that happens. There will be an opportunity to discuss the choices in detail
when the modified
[Comparison Proposal](https://gist.github.com/CodaFi/f0347bd37f1c407bf7ea0c429ead380e\) comes
up for review.

### `String` should be a `Collection` of `Character`s Again

In Swift 2.0, `String`'s `Collection` conformance was dropped, because we
convinced ourselves that its semantics differed from those of `Collection` too
significantly.

It was always well understood that if strings were treated as sequences of
`UnicodeScalar`s, algorithms such as `lexicographicalCompare`, `elementsEqual`,
and `reversed` would produce nonsense results. Thus, in Swift 1.0, `String` was
a collection of `Character` (extended grapheme clusters). During 2.0
development, though, we realized that correct string concatenation could
occasionally merge distinct grapheme clusters at the start and end of combined
strings.

This quirk aside, every aspect of strings-as-collections-of-graphemes appears to
comport perfectly with Unicode. We think the concatenation problem is tolerable,
because the cases where it occurs all represent partially-formed constructs. The
largest class—isolated combining characters such as ◌́ (U+0301 COMBINING ACUTE
ACCENT)—are explicitly called out in the Unicode standard as
“[degenerate](UAX #29: Unicode Text Segmentation)” or
“[defective](http://www.unicode.org/versions/Unicode9.0.0/ch03.pdf\)”. The other
cases—such as a string ending in a zero-width joiner or half of a regional
indicator—appear to be equally transient and unlikely outside of a text editor.

Admitting these cases encourages exploration of grapheme composition and is
consistent with what appears to be an overall Unicode philosophy that “no
special provisions are made to get marginally better behavior for… cases that
never occur in practice.”[2] Furthermore, it seems
unlikely to disturb the semantics of any plausible algorithms. We can handle
these cases by documenting them, explicitly stating that the elements of a
`String` are an emergent property based on Unicode rules.

The benefits of restoring `Collection` conformance are substantial:

* Collection-like operations encourage experimentation with strings to
   investigate and understand their behavior. This is useful for teaching new
   programmers, but also good for experienced programmers who want to
   understand more about strings/unicode.

* Extended grapheme clusters form a natural element boundary for Unicode
   strings. For example, searching and matching operations will always produce
   results that line up on grapheme cluster boundaries.

* Character-by-character processing is a legitimate thing to do in many real
   use-cases, including parsing, pattern matching, and language-specific
   transformations such as transliteration.

* `Collection` conformance makes a wide variety of powerful operations
   available that are appropriate to `String`'s default role as the vehicle for
   machine processed text.

   The methods `String` would inherit from `Collection`, where similar to
   higher-level string algorithms, have the right semantics. For example,
   grapheme-wise `lexicographicalCompare`, `elementsEqual`, and application of
   `flatMap` with case-conversion, produce the same results one would expect
   from whole-string ordering comparison, equality comparison, and
   case-conversion, respectively. `reverse` operates correctly on graphemes,
   keeping diacritics moored to their base characters and leaving emoji intact.
   Other methods such as `indexOf` and `contains` make obvious sense. A few
   `Collection` methods, like `min` and `max`, may not be particularly useful
   on `String`, but we don't consider that to be a problem worth solving, in
   the same way that we wouldn't try to suppress `min` and `max` on a
   `Set([UInt8])` that was used to store IP addresses.

* Many of the higher-level operations that we want to provide for `String`s,
   such as parsing and pattern matching, should apply to any `Collection`, and
   many of the benefits we want for `Collections`, such
   as unified slicing, should accrue
   equally to `String`. Making `String` part of the same protocol hierarchy
   allows us to write these operations once and not worry about keeping the
   benefits in sync.

* Slicing strings into substrings is a crucial part of the vocabulary of
   string processing, and all other sliceable things are `Collection`s.
   Because of its collection-like behavior, users naturally think of `String`
   in collection terms, but run into frustrating limitations where it fails to
   conform and are left to wonder where all the differences lie. Many simply
   “correct” this limitation by declaring a trivial conformance:

 extension String : BidirectionalCollection {}

   Even if we removed indexing-by-element from `String`, users could still do
   this:

     extension String : BidirectionalCollection {
       subscript(i: Index) -> Character { return characters[i] }
     }

   It would be much better to legitimize the conformance to `Collection` and
   simply document the oddity of any concatenation corner-cases, than to deny
   users the benefits on the grounds that a few cases are confusing.

Note that the fact that `String` is a collection of graphemes does *not* mean
that string operations will necessarily have to do grapheme boundary
recognition. See the Unicode protocol section for details.

### `Character` and `CharacterSet`

`Character`, which represents a
Unicode
[extended grapheme cluster](UAX #29: Unicode Text Segmentation),
is a bit of a black box, requiring conversion to `String` in order to
do any introspection, including interoperation with ASCII. To fix this, we should:

- Add a `unicodeScalars` view much like `String`'s, so that the sub-structure
  of grapheme clusters is discoverable.
- Add a failable `init` from sequences of scalars (returning nil for sequences
  that contain 0 or 2+ graphemes).
- (Lower priority) expose some operations, such as `func uppercase() ->
  String`, `var isASCII: Bool`, and, to the extent they can be sensibly
  generalized, queries of unicode properties that should also be exposed on
  `UnicodeScalar` such as `isAlphabetic` and `isGraphemeBase` .

Despite its name, `CharacterSet` currently operates on the Swift `UnicodeScalar`
type. This means it is usable on `String`, but only by going through the unicode
scalar view. To deal with this clash in the short term, `CharacterSet` should be
renamed to `UnicodeScalarSet`. In the longer term, it may be appropriate to
introduce a `CharacterSet` that provides similar functionality for extended
grapheme clusters.[5]

### Unification of Slicing Operations

Creating substrings is a basic part of String processing, but the slicing
operations that we have in Swift are inconsistent in both their spelling and
their naming:

* Slices with two explicit endpoints are done with subscript, and support
   in-place mutation:

       s[i..<j].mutate()

* Slicing from an index to the end, or from the start to an index, is done
   with a method and does not support in-place mutation:

       s.prefix(upTo: i).readOnly()

Prefix and suffix operations should be migrated to be subscripting operations
with one-sided ranges i.e. `s.prefix(upTo: i)` should become `s[..<i]`, as
in
[this proposal](https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0132-sequence-end-ops.md\).
With generic subscripting in the language, that will allow us to collapse a wide
variety of methods and subscript overloads into a single implementation, and
give users an easy-to-use and composable way to describe subranges.

Further extending this EDSL to integrate use-cases like `s.prefix(maxLength: 5)`
is an ongoing research project that can be considered part of the potential
long-term vision of text (and collection) processing.

### Substrings

When implementing substring slicing, languages are faced with three options:

1. Make the substrings the same type as string, and share storage.
2. Make the substrings the same type as string, and copy storage when making the substring.
3. Make substrings a different type, with a storage copy on conversion to string.

We think number 3 is the best choice. A walk-through of the tradeoffs follows.

#### Same type, shared storage

In Swift 3.0, slicing a `String` produces a new `String` that is a view into a
subrange of the original `String`'s storage. This is why `String` is 3 words in
size (the start, length and buffer owner), unlike the similar `Array` type
which is only one.

This is a simple model with big efficiency gains when chopping up strings into
multiple smaller strings. But it does mean that a stored substring keeps the
entire original string buffer alive even after it would normally have been
released.

This arrangement has proven to be problematic in other programming languages,
because applications sometimes extract small strings from large ones and keep
those small strings long-term. That is considered a memory leak and was enough
of a problem in Java that they changed from substrings sharing storage to
making a copy in 1.7.

#### Same type, copied storage

Copying of substrings is also the choice made in C#, and in the default
`NSString` implementation. This approach avoids the memory leak issue, but has
obvious performance overhead in performing the copies.

This in turn encourages trafficking in string/range pairs instead of in
substrings, for performance reasons, leading to API challenges. For example:

foo.compare(bar, range: start..<end)

Here, it is not clear whether `range` applies to `foo` or `bar`. This
relationship is better expressed in Swift as a slicing operation:

foo[start..<end].compare(bar)

Not only does this clarify to which string the range applies, it also brings
this sub-range capability to any API that operates on `String` "for free". So
these other combinations also work equally well:

// apply range on argument rather than target
foo.compare(bar[start..<end])
// apply range on both
foo[start..<end].compare(bar[start1..<end1])
// compare two strings ignoring first character
foo.dropFirst().compare(bar.dropFirst())

In all three cases, an explicit range argument need not appear on the `compare`
method itself. The implementation of `compare` does not need to know anything
about ranges. Methods need only take range arguments when that was an
integral part of their purpose (for example, setting the start and end of a
user's current selection in a text box).

#### Different type, shared storage

The desire to share underlying storage while preventing accidental memory leaks
occurs with slices of `Array`. For this reason we have an `ArraySlice` type.
The inconvenience of a separate type is mitigated by most operations used on
`Array` from the standard library being generic over `Sequence` or `Collection`.

We should apply the same approach for `String` by introducing a distinct
`SubSequence` type, `Substring`. Similar advice given for `ArraySlice` would apply to `Substring`:

Important: Long-term storage of `Substring` instances is discouraged. A
substring holds a reference to the entire storage of a larger string, not
just to the portion it presents, even after the original string's lifetime
ends. Long-term storage of a `Substring` may therefore prolong the lifetime
of large strings that are no longer otherwise accessible, which can appear
to be memory leakage.

When assigning a `Substring` to a longer-lived variable (usually a stored
property) explicitly of type `String`, a type conversion will be performed, and
at this point the substring buffer is copied and the original string's storage
can be released.

A `String` that was not its own `Substring` could be one word—a single tagged
pointer—without requiring additional allocations. `Substring`s would be a view
onto a `String`, so are 3 words - pointer to owner, pointer to start, and a
length. The small string optimization for `Substring` would take advantage of
the larger size, probably with a less compressed encoding for speed.

The downside of having two types is the inconvenience of sometimes having a
`Substring` when you need a `String`, and vice-versa. It is likely this would
be a significantly bigger problem than with `Array` and `ArraySlice`, as
slicing of `String` is such a common operation. It is especially relevant to
existing code that assumes `String` is the currency type. To ease the pain of
type mismatches, `Substring` should be a subtype of `String` in the same way
that `Int` is a subtype of `Optional<Int>`. This would give users an implicit
conversion from `Substring` to `String`, as well as the usual implicit
conversions such as `[Substring]` to `[String]` that other subtype
relationships receive.

In most cases, type inference combined with the subtype relationship should
make the type difference a non-issue and users will not care which type they
are using. For flexibility and optimizability, most operations from the
standard library will traffic in generic models of
[`Unicode`](#the--code-unicode--code--protocol).

##### Guidance for API Designers

In this model, **if a user is unsure about which type to use, `String` is always
a reasonable default**. A `Substring` passed where `String` is expected will be
implicitly copied. When compared to the “same type, copied storage” model, we
have effectively deferred the cost of copying from the point where a substring
is created until it must be converted to `String` for use with an API.

A user who needs to optimize away copies altogether should use this guideline:
if for performance reasons you are tempted to add a `Range` argument to your
method as well as a `String` to avoid unnecessary copies, you should instead
use `Substring`.

##### The “Empty Subscript”

To make it easy to call such an optimized API when you only have a `String` (or
to call any API that takes a `Collection`'s `SubSequence` when all you have is
the `Collection`), we propose the following “empty subscript” operation,

extension Collection {
 subscript() -> SubSequence { 
   return self[startIndex..<endIndex] 
 }
}

which allows the following usage:

funcThatIsJustLooking(at: person.name[]) // pass person.name as Substring

The `` syntax can be offered as a fixit when needed, similar to `&` for an
`inout` argument. While it doesn't help a user to convert `[String]` to
`[Substring]`, the need for such conversions is extremely rare, can be done with
a simple `map` (which could also be offered by a fixit):

takesAnArrayOfSubstring(arrayOfString.map { $0[] })

#### Other Options Considered

As we have seen, all three options above have downsides, but it's possible
these downsides could be eliminated/mitigated by the compiler. We are proposing
one such mitigation—implicit conversion—as part of the the "different type,
shared storage" option, to help avoid the cognitive load on developers of
having to deal with a separate `Substring` type.

To avoid the memory leak issues of a "same type, shared storage" substring
option, we considered whether the compiler could perform an implicit copy of
the underlying storage when it detects the string is being "stored" for long
term usage, say when it is assigned to a stored property. The trouble with this
approach is it is very difficult for the compiler to distinguish between
long-term storage versus short-term in the case of abstractions that rely on
stored properties. For example, should the storing of a substring inside an
`Optional` be considered long-term? Or the storing of multiple substrings
inside an array? The latter would not work well in the case of a
`components(separatedBy:)` implementation that intended to return an array of
substrings. It would also be difficult to distinguish intentional medium-term
storage of substrings, say by a lexer. There does not appear to be an effective
consistent rule that could be applied in the general case for detecting when a
substring is truly being stored long-term.

To avoid the cost of copying substrings under "same type, copied storage", the
optimizer could be enhanced to to reduce the impact of some of those copies.
For example, this code could be optimized to pull the invariant substring out
of the loop:

for _ in 0..<lots { 
 someFunc(takingString: bigString[bigRange]) 
}

It's worth noting that a similar optimization is needed to avoid an equivalent
problem with implicit conversion in the "different type, shared storage" case:

let substring = bigString[bigRange]
for _ in 0..<lots { someFunc(takingString: substring) }

However, in the case of "same type, copied storage" there are many use cases
that cannot be optimized as easily. Consider the following simple definition of
a recursive `contains` algorithm, which when substring slicing is linear makes
the overall algorithm quadratic:

extension String {
   func containsChar(_ x: Character) -> Bool {
       return !isEmpty && (first == x || dropFirst().containsChar(x))
   }
}

For the optimizer to eliminate this problem is unrealistic, forcing the user to
remember to optimize the code to not use string slicing if they want it to be
efficient (assuming they remember):

extension String {
   // add optional argument tracking progress through the string
   func containsCharacter(_ x: Character, atOrAfter idx: Index? = nil) -> Bool {
       let idx = idx ?? startIndex
       return idx != endIndex
           && (self[idx] == x || containsCharacter(x, atOrAfter: index(after: idx)))
   }
}

#### Substrings, Ranges and Objective-C Interop

The pattern of passing a string/range pair is common in several Objective-C
APIs, and is made especially awkward in Swift by the non-interchangeability of
`Range<String.Index>` and `NSRange`.

s2.find(s2, sourceRange: NSRange(j..<s2.endIndex, in: s2))

In general, however, the Swift idiom for operating on a sub-range of a
`Collection` is to *slice* the collection and operate on that:

s2.find(s2[j..<s2.endIndex])

Therefore, APIs that operate on an `NSString`/`NSRange` pair should be imported
without the `NSRange` argument. The Objective-C importer should be changed to
give these APIs special treatment so that when a `Substring` is passed, instead
of being converted to a `String`, the full `NSString` and range are passed to
the Objective-C method, thereby avoiding a copy.

As a result, you would never need to pass an `NSRange` to these APIs, which
solves the impedance problem by eliminating the argument, resulting in more
idiomatic Swift code while retaining the performance benefit. To help users
manually handle any cases that remain, Foundation should be augmented to allow
the following syntax for converting to and from `NSRange`:

let nsr = NSRange(i..<j, in: s) // An NSRange corresponding to s[i..<j]
let iToJ = Range(nsr, in: s)    // Equivalent to i..<j

### The `Unicode` protocol

With `Substring` and `String` being distinct types and sharing almost all
interface and semantics, and with the highest-performance string processing
requiring knowledge of encoding and layout that the currency types can't
provide, it becomes important to capture the common “string API” in a protocol.
Since Unicode conformance is a key feature of string processing in swift, we
call that protocol `Unicode`:

**Note:** The following assumes several features that are planned but not yet implemented in
Swift, and should be considered a sketch rather than a final design.

protocol Unicode 
 : Comparable, BidirectionalCollection where Element == Character {

 associatedtype Encoding : UnicodeEncoding
 var encoding: Encoding { get }

 associatedtype CodeUnits 
   : RandomAccessCollection where Element == Encoding.CodeUnit
 var codeUnits: CodeUnits { get }

 associatedtype UnicodeScalars 
   : BidirectionalCollection  where Element == UnicodeScalar
 var unicodeScalars: UnicodeScalars { get }

 associatedtype ExtendedASCII 
   : BidirectionalCollection where Element == UInt32
 var extendedASCII: ExtendedASCII { get }

 var unicodeScalars: UnicodeScalars { get }
}

extension Unicode {
 // ... define high-level non-mutating string operations, e.g. search ...

 func compared<Other: Unicode>(
   to rhs: Other,
   case caseSensitivity: StringSensitivity? = nil,
   diacritic diacriticSensitivity: StringSensitivity? = nil,
   width widthSensitivity: StringSensitivity? = nil,
   in locale: Locale? = nil
 ) -> SortOrder { ... }
}

extension Unicode : RangeReplaceableCollection where CodeUnits :
 RangeReplaceableCollection {
   // Satisfy protocol requirement
   mutating func replaceSubrange<C : Collection>(_: Range<Index>, with: C) 
     where C.Element == Element

 // ... define high-level mutating string operations, e.g. replace ...
}

The goal is that `Unicode` exposes the underlying encoding and code units in
such a way that for types with a known representation (e.g. a high-performance
`UTF8String`) that information can be known at compile-time and can be used to
generate a single path, while still allowing types like `String` that admit
multiple representations to use runtime queries and branches to fast path
specializations.

**Note:** `Unicode` would make a fantastic namespace for much of
what's in this proposal if we could get the ability to nest types and
protocols in protocols.

### Scanning, Matching, and Tokenization

#### Low-Level Textual Analysis

We should provide convenient APIs processing strings by character. For example,
it should be easy to cleanly express, “if this string starts with `"f"`, process
the rest of the string as follows…” Swift is well-suited to expressing this
common pattern beautifully, but we need to add the APIs. Here are two examples
of the sort of code that might be possible given such APIs:

if let firstLetter = input.droppingPrefix(alphabeticCharacter) {
 somethingWith(input) // process the rest of input
}

if let (number, restOfInput) = input.parsingPrefix(Int.self) {
  ...
}

The specific spelling and functionality of APIs like this are TBD. The larger
point is to make sure matching-and-consuming jobs are well-supported.

#### Unified Pattern Matcher Protocol

Many of the current methods that do matching are overloaded to do the same
logical operations in different ways, with the following axes:

- Logical Operation: `find`, `split`, `replace`, match at start
- Kind of pattern: `CharacterSet`, `String`, a regex, a closure
- Options, e.g. case/diacritic sensitivity, locale. Sometimes a part of
the method name, and sometimes an argument
- Whole string or subrange.

We should represent these aspects as orthogonal, composable components,
abstracting pattern matchers into a protocol like
[this one](https://github.com/apple/swift/blob/master/test/Prototypes/PatternMatching.swift#L33\),
that can allow us to define logical operations once, without introducing
overloads, and massively reducing API surface area.

For example, using the strawman prefix `%` syntax to turn string literals into
patterns, the following pairs would all invoke the same generic methods:

if let found = s.firstMatch(%"searchString") { ... }
if let found = s.firstMatch(someRegex) { ... }

for m in s.allMatches((%"searchString"), case: .insensitive) { ... }
for m in s.allMatches(someRegex) { ... }

let items = s.split(separatedBy: ", ")
let tokens = s.split(separatedBy: CharacterSet.whitespace)

Note that, because Swift requires the indices of a slice to match the indices of
the range from which it was sliced, operations like `firstMatch` can return a
`Substring?` in lieu of a `Range<String.Index>?`: the indices of the match in
the string being searched, if needed, can easily be recovered as the
`startIndex` and `endIndex` of the `Substring`.

Note also that matching operations are useful for collections in general, and
would fall out of this proposal:

// replace subsequences of contiguous NaNs with zero
forces.replace(oneOrMore([Float.nan]), [0.0])

#### Regular Expressions

Addressing regular expressions is out of scope for this proposal.
That said, it is important that to note the pattern matching protocol mentioned
above provides a suitable foundation for regular expressions, and types such as
`NSRegularExpression` can easily be retrofitted to conform to it. In the
future, support for regular expression literals in the compiler could allow for
compile-time syntax checking and optimization.

### String Indices

`String` currently has four views—`characters`, `unicodeScalars`, `utf8`, and
`utf16`—each with its own opaque index type. The APIs used to translate indices
between views add needless complexity, and the opacity of indices makes them
difficult to serialize.

The index translation problem has two aspects:

1. `String` views cannot consume one anothers' indices without a cumbersome
   conversion step. An index into a `String`'s `characters` must be translated
   before it can be used as a position in its `unicodeScalars`. Although these
   translations are rarely needed, they add conceptual and API complexity.
2. Many APIs in the core libraries and other frameworks still expose `String`
   positions as `Int`s and regions as `NSRange`s, which can only reference a
   `utf16` view and interoperate poorly with `String` itself.

#### Index Interchange Among Views

String's need for flexible backing storage and reasonably-efficient indexing
(i.e. without dynamically allocating and reference-counting the indices
themselves) means indices need an efficient underlying storage type. Although
we do not wish to expose `String`'s indices *as* integers, `Int` offsets into
underlying code unit storage makes a good underlying storage type, provided
`String`'s underlying storage supports random-access. We think random-access
*code-unit storage* is a reasonable requirement to impose on all `String`
instances.

Making these `Int` code unit offsets conveniently accessible and constructible
solves the serialization problem:

clipboard.write(s.endIndex.codeUnitOffset)
let offset = clipboard.read(Int.self)
let i = String.Index(codeUnitOffset: offset)

Index interchange between `String` and its `unicodeScalars`, `codeUnits`,
and [`extendedASCII`](#parsing-ascii-structure) views can be made entirely
seamless by having them share an index type (semantics of indexing a `String`
between grapheme cluster boundaries are TBD—it can either trap or be forgiving).
Having a common index allows easy traversal into the interior of graphemes,
something that is often needed, without making it likely that someone will do it
by accident.

- `String.index(after:)` should advance to the next grapheme, even when the
  index points partway through a grapheme.

- `String.index(before:)` should move to the start of the grapheme before
  the current position.

Seamless index interchange between `String` and its UTF-8 or UTF-16 views is not
crucial, as the specifics of encoding should not be a concern for most use
cases, and would impose needless costs on the indices of other views. That
said, we can make translation much more straightforward by exposing simple
bidirectional converting `init`s on both index types:

let u8Position = String.UTF8.Index(someStringIndex)
let originalPosition = String.Index(u8Position)

#### Index Interchange with Cocoa

We intend to address `NSRange`s that denote substrings in Cocoa APIs as
described [later in this document](#substrings--ranges-and-objective-c-interop).
That leaves the interchange of bare indices with Cocoa APIs trafficking in
`Int`. Hopefully such APIs will be rare, but when needed, the following
extension, which would be useful for all `Collections`, can help:

extension Collection {
 func index(offset: IndexDistance) -> Index {
   return index(startIndex, offsetBy: offset)
 }
 func offset(of i: Index) -> IndexDistance {
   return distance(from: startIndex, to: i)
 }
}

Then integers can easily be translated into offsets into a `String`'s `utf16`
view for consumption by Cocoa:

let cocoaIndex = s.utf16.offset(of: String.UTF16Index(i))
let swiftIndex = s.utf16.index(offset: cocoaIndex)

### Formatting

A full treatment of formatting is out of scope of this proposal, but
we believe it's crucial for completing the text processing picture. This
section details some of the existing issues and thinking that may guide future
development.

#### Printf-Style Formatting

`String.format` is designed on the `printf` model: it takes a format string with
textual placeholders for substitution, and an arbitrary list of other arguments.
The syntax and meaning of these placeholders has a long history in
C, but for anyone who doesn't use them regularly they are cryptic and complex,
as the `printf (3)` man page attests.

Aside from complexity, this style of API has two major problems: First, the
spelling of these placeholders must match up to the types of the arguments, in
the right order, or the behavior is undefined. Some limited support for
compile-time checking of this correspondence could be implemented, but only for
the cases where the format string is a literal. Second, there's no reasonable
way to extend the formatting vocabulary to cover the needs of new types: you are
stuck with what's in the box.

#### Foundation Formatters

The formatters supplied by Foundation are highly capable and versatile, offering
both formatting and parsing services. When used for formatting, though, the
design pattern demands more from users than it should:

* Matching the type of data being formatted to a formatter type
* Creating an instance of that type
* Setting stateful options (`currency`, `dateStyle`) on the type. Note: the
   need for this step prevents the instance from being used and discarded in
   the same expression where it is created.
* Overall, introduction of needless verbosity into source

These may seem like small issues, but the experience of Apple localization
experts is that the total drag of these factors on programmers is such that they
tend to reach for `String.format` instead.

#### String Interpolation

Swift string interpolation provides a user-friendly alternative to printf's
domain-specific language (just write ordinary swift code!) and its type safety
problems (put the data right where it belongs!) but the following issues prevent
it from being useful for localized formatting (among other jobs):

* [SR-2303](https://bugs.swift.org/browse/SR-2303\) We are unable to restrict
   types used in string interpolation.
* [SR-1260](https://bugs.swift.org/browse/SR-1260\) String interpolation can't
   distinguish (fragments of) the base string from the string substitutions.

In the long run, we should improve Swift string interpolation to the point where
it can participate in most any formatting job. Mostly this centers around
fixing the interpolation protocols per the previous item, and supporting
localization.

To be able to use formatting effectively inside interpolations, it needs to be
both lightweight (because it all happens in-situ) and discoverable. One
approach would be to standardize on `format` methods, e.g.:

"Column 1: \(n.format(radix:16, width:8)) *** \(message)"

"Something with leading zeroes: \(x.format(fill: zero, width:8))"

### C String Interop

Our support for interoperation with nul-terminated C strings is scattered and
incoherent, with 6 ways to transform a C string into a `String` and four ways to
do the inverse. These APIs should be replaced with the following

extension String {
 /// Constructs a `String` having the same contents as `nulTerminatedUTF8`.
 ///
 /// - Parameter nulTerminatedUTF8: a sequence of contiguous UTF-8 encoded 
 ///   bytes ending just before the first zero byte (NUL character).
 init(cString nulTerminatedUTF8: UnsafePointer<CChar>)

 /// Constructs a `String` having the same contents as `nulTerminatedCodeUnits`.
 ///
 /// - Parameter nulTerminatedCodeUnits: a sequence of contiguous code units in
 ///   the given `encoding`, ending just before the first zero code unit.
 /// - Parameter encoding: describes the encoding in which the code units
 ///   should be interpreted.
 init<Encoding: UnicodeEncoding>(
   cString nulTerminatedCodeUnits: UnsafePointer<Encoding.CodeUnit>,
   encoding: Encoding)

 /// Invokes the given closure on the contents of the string, represented as a
 /// pointer to a null-terminated sequence of UTF-8 code units.
 func withCString<Result>(
   _ body: (UnsafePointer<CChar>) throws -> Result) rethrows -> Result
}

In both of the construction APIs, any invalid encoding sequence detected will
have its longest valid prefix replaced by U+FFFD, the Unicode replacement
character, per Unicode specification. This covers the common case. The
replacement is done *physically* in the underlying storage and the validity of
the result is recorded in the `String`'s `encoding` such that future accesses
need not be slowed down by possible error repair separately.

Construction that is aborted when encoding errors are detected can be
accomplished using APIs on the `encoding`. String types that retain their
physical encoding even in the presence of errors and are repaired on-the-fly can
be built as different instances of the `Unicode` protocol.

### Unicode 9 Conformance

Unicode 9 (and MacOS 10.11) brought us support for family emoji, which changes
the process of properly identifying `Character` boundaries. We need to update
`String` to account for this change.

### High-Performance String Processing

Many strings are short enough to store in 64 bits, many can be stored using only
8 bits per unicode scalar, others are best encoded in UTF-16, and some come to
us already in some other encoding, such as UTF-8, that would be costly to
translate. Supporting these formats while maintaining usability for
general-purpose APIs demands that a single `String` type can be backed by many
different representations.

That said, the highest performance code always requires static knowledge of the
data structures on which it operates, and for this code, dynamic selection of
representation comes at too high a cost. Heavy-duty text processing demands a
way to opt out of dynamism and directly use known encodings. Having this
ability can also make it easy to cleanly specialize code that handles dynamic
cases for maximal efficiency on the most common representations.

To address this need, we can build models of the `Unicode` protocol that encode
representation information into the type, such as `NFCNormalizedUTF16String`.

### Parsing ASCII Structure

Although many machine-readable formats support the inclusion of arbitrary
Unicode text, it is also common that their fundamental structure lies entirely
within the ASCII subset (JSON, YAML, many XML formats). These formats are often
processed most efficiently by recognizing ASCII structural elements as ASCII,
and capturing the arbitrary sections between them in more-general strings. The
current String API offers no way to efficiently recognize ASCII and skip past
everything else without the overhead of full decoding into unicode scalars.

For these purposes, strings should supply an `extendedASCII` view that is a
collection of `UInt32`, where values less than `0x80` represent the
corresponding ASCII character, and other values represent data that is specific
to the underlying encoding of the string.

## Language Support

This proposal depends on two new features in the Swift language:

1. **Generic subscripts**, to
  enable unified slicing syntax.

2. **A subtype relationship** between
  `Substring` and `String`, enabling framework APIs to traffic solely in
  `String` while still making it possible to avoid copies by handling
  `Substring`s where necessary.

Additionally, **the ability to nest types and protocols inside
protocols** could significantly shrink the footprint of this proposal
on the top-level Swift namespace.

## Open Questions

### Must `String` be limited to storing UTF-16 subset encodings?

- The ability to handle `UTF-8`-encoded strings (models of `Unicode`) is not in
question here; this is about what encodings must be storable, without
transcoding, in the common currency type called “`String`”.
- ASCII, Latin-1, UCS-2, and UTF-16 are UTF-16 subsets. UTF-8 is not.
- If we have a way to get at a `String`'s code units, we need a concrete type in
which to express them in the API of `String`, which is a concrete type
- If String needs to be able to represent UTF-32, presumably the code units need
to be `UInt32`.
- Not supporting UTF-32-encoded text seems like one reasonable design choice.
- Maybe we can allow UTF-8 storage in `String` and expose its code units as
`UInt16`, just as we would for Latin-1.
- Supporting only UTF-16-subset encodings would imply that `String` indices can
be serialized without recording the `String`'s underlying encoding.

### Do we need a type-erasable base protocol for UnicodeEncoding?

UnicodeEncoding has an associated type, but it may be important to be able to
traffic in completely dynamic encoding values, e.g. for “tell me the most
efficient encoding for this string.”

### Should there be a string “facade?”

One possible design alternative makes `Unicode` a vehicle for expressing
the storage and encoding of code units, but does not attempt to give it an API
appropriate for `String`. Instead, string APIs would be provided by a generic
wrapper around an instance of `Unicode`:

struct StringFacade<U: Unicode> : BidirectionalCollection {

 // ...APIs for high-level string processing here...

 var unicode: U // access to lower-level unicode details
}

typealias String = StringFacade<StringStorage>
typealias Substring = StringFacade<StringStorage.SubSequence>

This design would allow us to de-emphasize lower-level `String` APIs such as
access to the specific encoding, by putting them behind a `.unicode` property.
A similar effect in a facade-less design would require a new top-level
`StringProtocol` playing the role of the facade with an an `associatedtype
Storage : Unicode`.

An interesting variation on this design is possible if defaulted generic
parameters are introduced to the language:

struct String<U: Unicode = StringStorage> 
 : BidirectionalCollection {

 // ...APIs for high-level string processing here...

 var unicode: U // access to lower-level unicode details
}

typealias Substring = String<StringStorage.SubSequence>

One advantage of such a design is that naïve users will always extend “the right
type” (`String`) without thinking, and the new APIs will show up on `Substring`,
`MyUTF8String`, etc. That said, it also has downsides that should not be
overlooked, not least of which is the confusability of the meaning of the word
“string.” Is it referring to the generic or the concrete type?

### `TextOutputStream` and `TextOutputStreamable`

`TextOutputStreamable` is intended to provide a vehicle for
efficiently transporting formatted representations to an output stream
without forcing the allocation of storage. Its use of `String`, a
type with multiple representations, at the lowest-level unit of
communication, conflicts with this goal. It might be sufficient to
change `TextOutputStream` and `TextOutputStreamable` to traffic in an
associated type conforming to `Unicode`, but that is not yet clear.
This area will require some design work.

### `description` and `debugDescription`

* Should these be creating localized or non-localized representations?
* Is returning a `String` efficient enough?
* Is `debugDescription` pulling the weight of the API surface area it adds?

### `StaticString`

`StaticString` was added as a byproduct of standard library developed and kept
around because it seemed useful, but it was never truly *designed* for client
programmers. We need to decide what happens with it. Presumably *something*
should fill its role, and that should conform to `Unicode`.

## Footnotes

<b id="f0">0</b> The integers rewrite currently underway is expected to
   substantially reduce the scope of `Int`'s API by using more
   generics. [:leftwards_arrow_with_hook:](#a0)

<b id="f1">1</b> In practice, these semantics will usually be tied to the
version of the installed [ICU](http://icu-project.org) library, which
programmatically encodes the most complex rules of the Unicode Standard and its
de-facto extension, CLDR.[:leftwards_arrow_with_hook:](#a1)

<b id="f2">2</b>
See
[UAX #29: Unicode Text Segmentation](UAX #29: Unicode Text Segmentation). Note
that inserting Unicode scalar values to prevent merging of grapheme clusters would
also constitute a kind of misbehavior (one of the clusters at the boundary would
not be found in the result), so would be relatively costly to implement, with
little benefit. [:leftwards_arrow_with_hook:](#a2)

<b id="f4">4</b> The use of non-UCA-compliant ordering is fully sanctioned by
the Unicode standard for this purpose. In fact there's
a [whole chapter](http://www.unicode.org/versions/Unicode9.0.0/ch05.pdf\)
dedicated to it. In particular, §5.17 says:

When comparing text that is visible to end users, a correct linguistic sort
should be used, as described in _Section 5.16, Sorting and
Searching_. However, in many circumstances the only requirement is for a
fast, well-defined ordering. In such cases, a binary ordering can be used.

[:leftwards_arrow_with_hook:](#a4)

<b id="f5">5</b> The queries supported by `NSCharacterSet` map directly onto
properties in a table that's indexed by unicode scalar value. This table is
part of the Unicode standard. Some of these queries (e.g., “is this an
uppercase character?”) may have fairly obvious generalizations to grapheme
clusters, but exactly how to do it is a research topic and *ideally* we'd either
establish the existing practice that the Unicode committee would standardize, or
the Unicode committee would do the research and we'd implement their
result.[:leftwards_arrow_with_hook:](#a5)

_______________________________________________
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https://lists.swift.org/mailman/listinfo/swift-evolution

Maybe (though are those extra characters really telling you much?).

But you can’t write that in Swift – you’d need a 0-argument operator.

(Or a […] postfix operator I guess if you wanted to try and sneak that through, but that is also not allowed… :)

···

On Jan 20, 2017, at 3:29 PM, Jaden Geller via swift-evolution <swift-evolution@swift.org> wrote:

Wouldn’t `x[…]` be more consistent with these other syntaxes?

Technically, you can, since operators are function values:

struct Foo {}
struct Woo { subscript(_: (Foo, Foo) -> Foo) -> Int { return 0 } }
func ...(_ x: Foo, _ y: Foo) -> Foo { return x }

Woo()[...]

Whether you *want* to, though…

-Joe

···

On Jan 20, 2017, at 5:15 PM, Ben Cohen via swift-evolution <swift-evolution@swift.org> wrote:

On Jan 20, 2017, at 3:29 PM, Jaden Geller via swift-evolution <swift-evolution@swift.org> wrote:

Wouldn’t `x[…]` be more consistent with these other syntaxes?

Maybe (though are those extra characters really telling you much?).

But you can’t write that in Swift – you’d need a 0-argument operator.

(Or a […] postfix operator I guess if you wanted to try and sneak that through, but that is also not allowed… :)