# Pitch: String Index Overhaul

**URL:** <https://forums.swift.org/t/pitch-string-index-overhaul/6017>\
**Category:** Pitches\
**Created:** [May 27, 2017, 5:40pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017 "2017-05-27T17:40:40Z")\
**Posts on this page:** 15\
**Page:** 1

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [May 27, 2017, 5:40pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/1 "2017-05-27T17:40:40Z")

</div>

Pretty version: [https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md](https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md)

> **···**
>
> ----
> 
> # String Index Overhaul
> 
> \* Proposal: [SE-NNNN](NNNN-string-index-overhaul.md)  
> \* Authors: [Dave Abrahams]([https://github.com/dabrahams\](https://github.com/dabrahams%5C))  
> \* Review Manager: TBD  
> \* Status: \*\*Awaiting review\*\*  
> \* Pull Request Implementing This Proposal: [https://github.com/apple/swift/pull/9806](https://github.com/apple/swift/pull/9806)
> 
> \*During the review process, add the following fields as needed:\*
> 
> ## Introduction
> 
> Today `String` shares an `Index` type with its `CharacterView` but not  
> with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> and `String.CharacterView.Index` as typealiases for `String.Index`,  
> and exposes a public `encodedOffset` property and initializer that can  
> be used to serialize and deserialize positions in a `String` or  
> `Substring`.
> 
> Swift-evolution thread: [Discussion thread topic for that proposal]([https://lists.swift.org/pipermail/swift-evolution/\](https://lists.swift.org/pipermail/swift-evolution/%5C))
> 
> ## Motivation
> 
> The different index types are supported by a set of `Index`  
> initializers, which are failable whenever the source index might not  
> correspond to a position in the target view:
> 
> ```swift
> if let j = String.UnicodeScalarView.Index(
> someUTF16Position, within: s.unicodeScalars) {
> ... 
> }
> 
> ```
> 
> The current API is as follows:
> 
> ```swift
> public extension String.Index {
> init?(_: String.UnicodeScalarIndex, within: String)
> init?(_: String.UTF16Index, within: String)
> init?(_: String.UTF8Index, within: String)
> }
> 
> public extension String.UTF16View.Index {
> init?(_: String.UTF8Index, within: String.UTF16View)
> init(_: String.UnicodeScalarIndex, within: String.UTF16View)
> init(_: String.Index, within: String.UTF16View)
> }
> 
> public extension String.UTF8View.Index {
> init?(_: String.UTF16Index, within: String.UTF8View)
> init(_: String.UnicodeScalarIndex, within: String.UTF8View)
> init(_: String.Index, within: String.UTF8View)
> }
> 
> public extension String.UnicodeScalarView.Index {
> init?(_: String.UTF16Index, within: String.UnicodeScalarView)
> init?(_: String.UTF8Index, within: String.UnicodeScalarView)
> init(_: String.Index, within: String.UnicodeScalarView)
> }
> 
> ```
> 
> These initializers are supplemented by a corresponding set of  
> convenience conversion methods:
> 
> ```swift
> if let j = someUTF16Position.samePosition(in: s.unicodeScalars) {
> ... 
> }
> 
> ```
> 
> with the following API:
> 
> ```swift
> public extension String.Index {
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index
> }
> 
> public extension String.UTF16View.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index?
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> }
> 
> public extension String.UTF8View.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index?
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> }
> 
> public extension String.UnicodeScalarView.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> }
> 
> ```
> 
> The result is a great deal of API surface area for apparently little  
> gain in ordinary code, that normally only interchanges indices among  
> views when the positions match up exactly (i.e. when the conversion is  
> going to succeed). Also, the resulting code is needlessly awkward.
> 
> Finally, the opacity of these index types makes it difficult to record  
> `String` or `Substring` positions in files or other archival forms,  
> and reconstruct the original positions with respect to a deserialized  
> `String` or `Substring`.
> 
> ## Proposed solution
> 
> All `String` views will use a single index type (`String.Index`), so  
> that positions can be interchanged without awkward explicit  
> conversions:
> 
> ```swift
> let html: String = "See <a href=\"http://swift.org\">swift.org</a>"
> 
> // Search the UTF16, instead of characters, for performance reasons:
> let open = "<".utf16.first!, close = ">".utf16.first!
> let tagStart = s.utf16.index(of: open)
> let tagEnd = s.utf16[tagStart...].index(of: close)
> 
> // Slice the String with the UTF-16 indices to retrieve the tag.
> let tag = html[tagStart...tagEnd]
> 
> ```
> 
> A property and an intializer will be added to `String.Index`, exposing  
> the offset of the index in code units (currently only UTF-16) from the  
> beginning of the string:
> 
> ```swift
> let n: Int = html.endIndex.encodedOffset
> let end = String.Index(encodedOffset: n)
> assert(end == String.endIndex)
> 
> ```
> 
> # Comparison and Slicing Semantics
> 
> When two indices being compared correspond to positions that are valid  
> in any single `String` view, comparison semantics are already fully  
> specified by the `Collection` requirements. Where no single `String`  
> view contains both index values, the indices compare unequal and  
> ordering is determined by comparison of `encodedOffsets`. These index  
> values are not totally ordered but do satisfy strict weak ordering  
> requirements, which is sufficient for algorithms such as `sort` to  
> exhibit sensible behavior. We might consider loosening the specified  
> requirements on these algorithms and on `Comparable` to support strict  
> weak ordering, but for now we can treat such index pairs as being  
> outside the domain of comparison, like any other indices from  
> completely distinct collections.
> 
> An index that does not fall on an exact boundary in a given `String`  
> or `Substring` view will be “rounded down” to the nearest boundary  
> when used for slicing or range replacement. So, for example,
> 
> ```swift
> let s = "e\u{301}galite\u{301}" // "égalité"
> print(s[s.unicodeScalars.indices.dropFirst().first!...]) // "égalité"
> print(s[..<s.unicodeScalars.indices.last!]) // "égalit"
> 
> ```
> 
> Replacing the failable APIs listed [above](#motivation) that detect  
> whether an index represents a valid position in a given view, and  
> enhancement that explicitly round index positions to nearby boundaries  
> in a given view, are left to a later proposal. For now, we do not  
> propose to remove the existing index conversion APIs.
> 
> ## Detailed design
> 
> `String.Index` acquires an `encodedOffset` property and initializer:
> 
> ```swift
> public extension String.Index {
> /// Creates a position corresponding to the given offset in a
> /// `String`'s underlying (UTF-16) code units.
> init(encodedOffset: Int)
> 
> /// The position of this index expressed as an offset from the
> /// beginning of the `String`'s underlying (UTF-16) code units.
> var encodedOffset: Int
> }
> 
> ```
> 
> `Index` types of `String.UTF8View`, `String.UTF16View`, and  
> `String.UnicodeScalarView` are replaced by `String.Index`:
> 
> ```swift
> public extension String.UTF8View {
> typealias Index = String.Index
> }
> public extension String.UTF16View {
> typealias Index = String.Index
> }
> public extension String.UnicodeScalarView {
> typealias Index = String.Index
> }
> 
> ```
> 
> Because the index types are collapsing, index conversion methods and  
> initializers are reduced to the following:
> 
> ```swift
> public extension String.Index {
> init?(_: String.Index, within: String)
> init?(_: String.Index, within: String.UTF8View)
> init?(_: String.Index, within: String.UTF16View)
> init?(_: String.Index, within: String.UnicodeScalarView)
> 
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF8View) -> String.Index?
> func samePosition(in: String.UTF16View) -> String.Index?
> func samePosition(in: String.UnicodeScalarView) -> String.Index?
> }
> 
> ```
> 
> ## Source compatibility
> 
> Because of the collapse of index  
> types, [existing non-failable APIs](#motivation) become failable. To  
> avoid breaking Swift 3 code, the following overloads of existing  
> functions are added, allowing the resulting optional indices to be  
> used where previously non-optional indices were used. These overloads  
> were driven by making the new APIs work with existing code, including  
> the Swift source compatibility test suite, and should be viewed as  
> migration aids only, rather than additions to the Swift 3 API.
> 
> ```swift
> extension Optional where Wrapped == String.Index {
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> public static func ..<(
> lhs: String.Index?, rhs: String.Index?
> ) -> Range<String.Index> {
> return lhs! ..< rhs!
> }
> 
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> public static func ...(
> lhs: String.Index?, rhs: String.Index?
> ) -> ClosedRange<String.Index> {
> return lhs! ... rhs!
> }
> }
> 
> // backward compatibility for index interchange.  
> extension String.UTF16View {
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(after i: Index?) -> Index {
> return index(after: i)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(
> _ i: Index?, offsetBy n: IndexDistance) -> Index {
> return index(i!, offsetBy: n)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> return distance(from: i!, to: j!)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public subscript(i: Index?) -> Unicode.UTF16.CodeUnit {
> return self[i!]
> }
> }
> 
> extension String.UTF8View {
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(after i: Index?) -> Index {
> return index(after: i!)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> return index(i!, offsetBy: n)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> public func distance(
> from i: Index?, to j: Index?) -> IndexDistance {
> return distance(from: i!, to: j!)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public subscript(i: Index?) -> Unicode.UTF8.CodeUnit {
> return self[i!]
> }
> }
> 
> // backward compatibility for index interchange.  
> extension String.UnicodeScalarView {
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(after i: Index?) -> Index {
> return index(after: i)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> return index(i!, offsetBy: n)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> return distance(from: i!, to: j!)
> }
> @available(
> swift, deprecated: 3.2, obsoleted: 4.0,
> message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> public subscript(i: Index?) -> Unicode.Scalar {
> return self[i!]
> }
> }
> 
> ```
> 
> - \*\*Q\*\*: Will existing correct Swift 3 applications stop compiling due  
> &nbsp;&nbsp;to this change?
> 
> &nbsp;&nbsp;\*\*A\*\*: it is possible but unlikely. The existing index conversion  
> &nbsp;&nbsp;APIs are relatively rarely used, and the overloads listed above  
> &nbsp;&nbsp;handle the common cases in Swift 3 compatibility mode.  
> &nbsp;&nbsp;  
> - \*\*Q\*\*: Will applications still compile but produce  
> &nbsp;&nbsp;different behavior than they used to?
> 
> &nbsp;&nbsp;\*\*A\*\*: No.  
> &nbsp;&nbsp;  
> - \*\*Q\*\*: Is it possible to automatically migrate from the old syntax  
> &nbsp;&nbsp;to the new syntax?
> 
> &nbsp;&nbsp;\*\*A\*\*: Yes, although usages of these APIs may be rare enough that it  
> &nbsp;&nbsp;isn't worth the trouble.
> 
> - \*\*Q\*\*: Can Swift applications be written in a common subset that works  
> &nbsp;&nbsp;&nbsp;both with Swift 3 and Swift 4 to aid in migration?
> 
> &nbsp;&nbsp;\*\*A\*\*: Yes, the Swift 4 APIs will all be available in Swift 3 mode.
> 
> ## Effect on ABI stability
> 
> This proposal changes the ABI of the standard library.
> 
> ## Effect on API resilience
> 
> This proposal makes no changes to the resilience of any APIs.
> 
> ## Alternatives considered
> 
> The only alternative considered was no action.
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![Michael\_Ilseman](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/michael_ilseman/32/44640_2.png) [@Michael\_Ilseman](https://forums.swift.org/u/Michael_Ilseman)\
**Post date:** [May 30, 2017, 8:18pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/2 "2017-05-30T20:18:11Z")

</div>

> Pretty version: [https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md](https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md)
> 
> ----
> 
> # String Index Overhaul
> 
> \* Proposal: [SE-NNNN](NNNN-string-index-overhaul.md)  
> \* Authors: [Dave Abrahams]([https://github.com/dabrahams\](https://github.com/dabrahams%5C))  
> \* Review Manager: TBD  
> \* Status: \*\*Awaiting review\*\*  
> \* Pull Request Implementing This Proposal: [[stdlib] String index interchange, etc. by dabrahams · Pull Request #9806 · apple/swift · GitHub](https://github.com/apple/swift/pull/9806)
> 
> \*During the review process, add the following fields as needed:\*
> 
> ## Introduction
> 
> Today `String` shares an `Index` type with its `CharacterView` but not  
> with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> and `String.CharacterView.Index` as typealiases for `String.Index`,  
> and exposes a public `encodedOffset` property and initializer that can  
> be used to serialize and deserialize positions in a `String` or  
> `Substring`.
> 
> Swift-evolution thread: [Discussion thread topic for that proposal]([https://lists.swift.org/pipermail/swift-evolution/\](https://lists.swift.org/pipermail/swift-evolution/%5C))
> 
> ## Motivation
> 
> The different index types are supported by a set of `Index`  
> initializers, which are failable whenever the source index might not  
> correspond to a position in the target view:
> 
> ```swift
> if let j = String.UnicodeScalarView.Index(
> someUTF16Position, within: s.unicodeScalars) {
> ... 
> }
> 
> ```
> 
> The current API is as follows:
> 
> ```swift
> public extension String.Index {
> init?(_: String.UnicodeScalarIndex, within: String)
> init?(_: String.UTF16Index, within: String)
> init?(_: String.UTF8Index, within: String)
> }
> 
> public extension String.UTF16View.Index {
> init?(_: String.UTF8Index, within: String.UTF16View)
> init(_: String.UnicodeScalarIndex, within: String.UTF16View)
> init(_: String.Index, within: String.UTF16View)
> }
> 
> public extension String.UTF8View.Index {
> init?(_: String.UTF16Index, within: String.UTF8View)
> init(_: String.UnicodeScalarIndex, within: String.UTF8View)
> init(_: String.Index, within: String.UTF8View)
> }
> 
> public extension String.UnicodeScalarView.Index {
> init?(_: String.UTF16Index, within: String.UnicodeScalarView)
> init?(_: String.UTF8Index, within: String.UnicodeScalarView)
> init(_: String.Index, within: String.UnicodeScalarView)
> }
> 
> ```
> 
> These initializers are supplemented by a corresponding set of  
> convenience conversion methods:
> 
> ```swift
> if let j = someUTF16Position.samePosition(in: s.unicodeScalars) {
> ... 
> }
> 
> ```
> 
> with the following API:
> 
> ```swift
> public extension String.Index {
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index
> }
> 
> public extension String.UTF16View.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index?
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> }
> 
> public extension String.UTF8View.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index?
> func samePosition(
> in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> }
> 
> public extension String.UnicodeScalarView.Index {
> func samePosition(in: String) -> String.Index?
> func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> }
> 
> ```
> 
> The result is a great deal of API surface area for apparently little  
> gain in ordinary code, that normally only interchanges indices among  
> views when the positions match up exactly (i.e. when the conversion is  
> going to succeed). Also, the resulting code is needlessly awkward.
> 
> Finally, the opacity of these index types makes it difficult to record  
> `String` or `Substring` positions in files or other archival forms,  
> and reconstruct the original positions with respect to a deserialized  
> `String` or `Substring`.
> 
> ## Proposed solution
> 
> All `String` views will use a single index type (`String.Index`), so  
> that positions can be interchanged without awkward explicit  
> conversions:
> 
> ```swift
> let html: String = "See <a href=\"http://swift.org\">swift.org</a>"
> 
> // Search the UTF16, instead of characters, for performance reasons:
> let open = "<".utf16.first!, close = ">".utf16.first!
> let tagStart = s.utf16.index(of: open)
> let tagEnd = s.utf16[tagStart...].index(of: close)
> 
> // Slice the String with the UTF-16 indices to retrieve the tag.
> let tag = html[tagStart...tagEnd]
> 
> ```
> 
> A property and an intializer will be added to `String.Index`, exposing  
> the offset of the index in code units (currently only UTF-16) from the  
> beginning of the string:
> 
> ```swift
> let n: Int = html.endIndex.encodedOffset
> let end = String.Index(encodedOffset: n)
> assert(end == String.endIndex)
> 
> ```
> 
> # Comparison and Slicing Semantics
> 
> When two indices being compared correspond to positions that are valid  
> in any single `String` view, comparison semantics are already fully  
> specified by the `Collection` requirements. Where no single `String`  
> view contains both index values, the indices compare unequal and  
> ordering is determined by comparison of `encodedOffsets`. These index  
> values are not totally ordered but do satisfy strict weak ordering  
> requirements, which is sufficient for algorithms such as `sort` to  
> exhibit sensible behavior. We might consider loosening the specified  
> requirements on these algorithms and on `Comparable` to support strict  
> weak ordering, but for now we can treat such index pairs as being  
> outside the domain of comparison, like any other indices from  
> completely distinct collections.
> 
> An index that does not fall on an exact boundary in a given `String`  
> or `Substring` view will be “rounded down” to the nearest boundary  
> when used for slicing or range replacement. So, for example,

What about normal subscript? I.e. what would the following print?

print(s[s.unicodeScalars.indices.dropFirst().first!]) // “é”, or just the combining scalar?

Would unifying under the same type require that indices be less stateful than they currently are?

> **···**
>
> > On May 27, 2017, at 10:40 AM, Dave Abrahams via swift-evolution \<swift-evolution@swift.org\> wrote:
> 
> > ```swift
> > let s = "e\u{301}galite\u{301}" // "égalité"
> > print(s[s.unicodeScalars.indices.dropFirst().first!...]) // “égalité"
> > print(s[..<s.unicodeScalars.indices.last!]) // "égalit"
> > 
> > ```
> > 
> > Replacing the failable APIs listed [above](#motivation) that detect  
> > whether an index represents a valid position in a given view, and  
> > enhancement that explicitly round index positions to nearby boundaries  
> > in a given view, are left to a later proposal. For now, we do not  
> > propose to remove the existing index conversion APIs.
> > 
> > ## Detailed design
> > 
> > `String.Index` acquires an `encodedOffset` property and initializer:
> > 
> > ```swift
> > public extension String.Index {
> > /// Creates a position corresponding to the given offset in a
> > /// `String`'s underlying (UTF-16) code units.
> > init(encodedOffset: Int)
> > 
> > /// The position of this index expressed as an offset from the
> > /// beginning of the `String`'s underlying (UTF-16) code units.
> > var encodedOffset: Int
> > }
> > 
> > ```
> > 
> > `Index` types of `String.UTF8View`, `String.UTF16View`, and  
> > `String.UnicodeScalarView` are replaced by `String.Index`:
> > 
> > ```swift
> > public extension String.UTF8View {
> > typealias Index = String.Index
> > }
> > public extension String.UTF16View {
> > typealias Index = String.Index
> > }
> > public extension String.UnicodeScalarView {
> > typealias Index = String.Index
> > }
> > 
> > ```
> > 
> > Because the index types are collapsing, index conversion methods and  
> > initializers are reduced to the following:
> > 
> > ```swift
> > public extension String.Index {
> > init?(_: String.Index, within: String)
> > init?(_: String.Index, within: String.UTF8View)
> > init?(_: String.Index, within: String.UTF16View)
> > init?(_: String.Index, within: String.UnicodeScalarView)
> > 
> > func samePosition(in: String) -> String.Index?
> > func samePosition(in: String.UTF8View) -> String.Index?
> > func samePosition(in: String.UTF16View) -> String.Index?
> > func samePosition(in: String.UnicodeScalarView) -> String.Index?
> > }
> > 
> > ```
> > 
> > ## Source compatibility
> > 
> > Because of the collapse of index  
> > types, [existing non-failable APIs](#motivation) become failable. To  
> > avoid breaking Swift 3 code, the following overloads of existing  
> > functions are added, allowing the resulting optional indices to be  
> > used where previously non-optional indices were used. These overloads  
> > were driven by making the new APIs work with existing code, including  
> > the Swift source compatibility test suite, and should be viewed as  
> > migration aids only, rather than additions to the Swift 3 API.
> > 
> > ```swift
> > extension Optional where Wrapped == String.Index {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public static func ..<(
> > lhs: String.Index?, rhs: String.Index?
> > ) -> Range<String.Index> {
> > return lhs! ..< rhs!
> > }
> > 
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public static func ...(
> > lhs: String.Index?, rhs: String.Index?
> > ) -> ClosedRange<String.Index> {
> > return lhs! ... rhs!
> > }
> > }
> > 
> > // backward compatibility for index interchange.  
> > extension String.UTF16View {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(
> > _ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.UTF16.CodeUnit {
> > return self[i!]
> > }
> > }
> > 
> > extension String.UTF8View {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(
> > from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.UTF8.CodeUnit {
> > return self[i!]
> > }
> > }
> > 
> > // backward compatibility for index interchange.  
> > extension String.UnicodeScalarView {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.Scalar {
> > return self[i!]
> > }
> > }
> > 
> > ```
> > 
> > - \*\*Q\*\*: Will existing correct Swift 3 applications stop compiling due  
> > to this change?
> > 
> > \*\*A\*\*: it is possible but unlikely. The existing index conversion  
> > APIs are relatively rarely used, and the overloads listed above  
> > handle the common cases in Swift 3 compatibility mode.
> > 
> > - \*\*Q\*\*: Will applications still compile but produce  
> > different behavior than they used to?
> > 
> > \*\*A\*\*: No.
> > 
> > - \*\*Q\*\*: Is it possible to automatically migrate from the old syntax  
> > to the new syntax?
> > 
> > \*\*A\*\*: Yes, although usages of these APIs may be rare enough that it  
> > isn't worth the trouble.
> > 
> > - \*\*Q\*\*: Can Swift applications be written in a common subset that works  
> > &nbsp;&nbsp;both with Swift 3 and Swift 4 to aid in migration?
> > 
> > \*\*A\*\*: Yes, the Swift 4 APIs will all be available in Swift 3 mode.
> > 
> > ## Effect on ABI stability
> > 
> > This proposal changes the ABI of the standard library.
> > 
> > ## Effect on API resilience
> > 
> > This proposal makes no changes to the resilience of any APIs.
> > 
> > ## Alternatives considered
> > 
> > The only alternative considered was no action.
> > 
> > --  
> > -Dave
> > 
> > \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_  
> > swift-evolution mailing list  
> > swift-evolution@swift.org  
> > [https://lists.swift.org/mailman/listinfo/swift-evolution](https://lists.swift.org/mailman/listinfo/swift-evolution)

---

<div class="post-metadata">

**Author:** ![jrose](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jrose/32/408_2.png) [@jrose](https://forums.swift.org/u/jrose)\
**Post date:** [May 30, 2017, 9:23pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/3 "2017-05-30T21:23:31Z")

</div>

My knee-jerk reaction is to say it's too late in Swift 4 for this kind of change, but with that out of the way, I'm most concerned about what it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.

let str = "言"  
let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
let trailingBytes = str.utf8[oneUnitIn...]

What can I do with 'oneUnitIn'? How do I test to see if it's on a Character boundary or a UnicodeScalar boundary?

Jordan

> **···**
>
> > On May 27, 2017, at 10:40, Dave Abrahams via swift-evolution \<swift-evolution@swift.org\> wrote:
> > 
> > Pretty version: [https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md](https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md)
> > 
> > ----
> > 
> > # String Index Overhaul
> > 
> > \* Proposal: [SE-NNNN](NNNN-string-index-overhaul.md)  
> > \* Authors: [Dave Abrahams]([https://github.com/dabrahams\](https://github.com/dabrahams%5C))  
> > \* Review Manager: TBD  
> > \* Status: \*\*Awaiting review\*\*  
> > \* Pull Request Implementing This Proposal: [[stdlib] String index interchange, etc. by dabrahams · Pull Request #9806 · apple/swift · GitHub](https://github.com/apple/swift/pull/9806)
> > 
> > \*During the review process, add the following fields as needed:\*
> > 
> > ## Introduction
> > 
> > Today `String` shares an `Index` type with its `CharacterView` but not  
> > with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> > proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> > and `String.CharacterView.Index` as typealiases for `String.Index`,  
> > and exposes a public `encodedOffset` property and initializer that can  
> > be used to serialize and deserialize positions in a `String` or  
> > `Substring`.
> > 
> > Swift-evolution thread: [Discussion thread topic for that proposal]([https://lists.swift.org/pipermail/swift-evolution/\](https://lists.swift.org/pipermail/swift-evolution/%5C))

---

<div class="post-metadata">

**Author:** ![Philippe\_Hausler](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/philippe_hausler/32/22569_2.png) [@Philippe\_Hausler](https://forums.swift.org/u/Philippe_Hausler)\
**Post date:** [May 31, 2017, 6:13pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/4 "2017-05-31T18:13:38Z")

</div>

I would presume that the index type will still be shared between String and SubString, will this mean that we will now be able to express index manipulation in StringProtocol?

I find StringProtocol a bit hard to deal with when attempting to make range conversions; it would be really nice if we could make this possible (or perhaps more intuitive... since, for the life of me I can’t figure out a way to generically convert indexes for StringProtocol adoption)

So lets say you have a function as such:

func foo\<S: StringProtocol\>(\_ str: S, range: Range\<S.Index\>) {  
&nbsp;&nbsp;&nbsp;&nbsp;range.lowerBound.samePosition(in: str.utf16)  
}

results in the error error: value of type 'S.Index' has no member ‘samePosition’

This of course is an intended target of something that deals with strings and wants to deal with both strings and substrings uniformly since it is reasonable to pass either.

In short: are StringProtocol accessors a consideration for conversion in this change?

> **···**
>
> > On May 27, 2017, at 10:40 AM, Dave Abrahams via swift-evolution \<swift-evolution@swift.org\> wrote:
> > 
> > Pretty version: [https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md](https://github.com/dabrahams/swift-evolution/blob/string-index-overhaul/proposals/NNNN-string-index-overhaul.md)
> > 
> > ----
> > 
> > # String Index Overhaul
> > 
> > \* Proposal: [SE-NNNN](NNNN-string-index-overhaul.md)  
> > \* Authors: [Dave Abrahams]([https://github.com/dabrahams\](https://github.com/dabrahams%5C))  
> > \* Review Manager: TBD  
> > \* Status: \*\*Awaiting review\*\*  
> > \* Pull Request Implementing This Proposal: [[stdlib] String index interchange, etc. by dabrahams · Pull Request #9806 · apple/swift · GitHub](https://github.com/apple/swift/pull/9806)
> > 
> > \*During the review process, add the following fields as needed:\*
> > 
> > ## Introduction
> > 
> > Today `String` shares an `Index` type with its `CharacterView` but not  
> > with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> > proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> > and `String.CharacterView.Index` as typealiases for `String.Index`,  
> > and exposes a public `encodedOffset` property and initializer that can  
> > be used to serialize and deserialize positions in a `String` or  
> > `Substring`.
> > 
> > Swift-evolution thread: [Discussion thread topic for that proposal]([https://lists.swift.org/pipermail/swift-evolution/\](https://lists.swift.org/pipermail/swift-evolution/%5C))
> > 
> > ## Motivation
> > 
> > The different index types are supported by a set of `Index`  
> > initializers, which are failable whenever the source index might not  
> > correspond to a position in the target view:
> > 
> > ```swift
> > if let j = String.UnicodeScalarView.Index(
> > someUTF16Position, within: s.unicodeScalars) {
> > ... 
> > }
> > 
> > ```
> > 
> > The current API is as follows:
> > 
> > ```swift
> > public extension String.Index {
> > init?(_: String.UnicodeScalarIndex, within: String)
> > init?(_: String.UTF16Index, within: String)
> > init?(_: String.UTF8Index, within: String)
> > }
> > 
> > public extension String.UTF16View.Index {
> > init?(_: String.UTF8Index, within: String.UTF16View)
> > init(_: String.UnicodeScalarIndex, within: String.UTF16View)
> > init(_: String.Index, within: String.UTF16View)
> > }
> > 
> > public extension String.UTF8View.Index {
> > init?(_: String.UTF16Index, within: String.UTF8View)
> > init(_: String.UnicodeScalarIndex, within: String.UTF8View)
> > init(_: String.Index, within: String.UTF8View)
> > }
> > 
> > public extension String.UnicodeScalarView.Index {
> > init?(_: String.UTF16Index, within: String.UnicodeScalarView)
> > init?(_: String.UTF8Index, within: String.UnicodeScalarView)
> > init(_: String.Index, within: String.UnicodeScalarView)
> > }
> > 
> > ```
> > 
> > These initializers are supplemented by a corresponding set of  
> > convenience conversion methods:
> > 
> > ```swift
> > if let j = someUTF16Position.samePosition(in: s.unicodeScalars) {
> > ... 
> > }
> > 
> > ```
> > 
> > with the following API:
> > 
> > ```swift
> > public extension String.Index {
> > func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> > func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> > func samePosition(
> > in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index
> > }
> > 
> > public extension String.UTF16View.Index {
> > func samePosition(in: String) -> String.Index?
> > func samePosition(in: String.UTF8View) -> String.UTF8View.Index?
> > func samePosition(
> > in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> > }
> > 
> > public extension String.UTF8View.Index {
> > func samePosition(in: String) -> String.Index?
> > func samePosition(in: String.UTF16View) -> String.UTF16View.Index?
> > func samePosition(
> > in: String.UnicodeScalarView) -> String.UnicodeScalarView.Index?
> > }
> > 
> > public extension String.UnicodeScalarView.Index {
> > func samePosition(in: String) -> String.Index?
> > func samePosition(in: String.UTF8View) -> String.UTF8View.Index
> > func samePosition(in: String.UTF16View) -> String.UTF16View.Index
> > }
> > 
> > ```
> > 
> > The result is a great deal of API surface area for apparently little  
> > gain in ordinary code, that normally only interchanges indices among  
> > views when the positions match up exactly (i.e. when the conversion is  
> > going to succeed). Also, the resulting code is needlessly awkward.
> > 
> > Finally, the opacity of these index types makes it difficult to record  
> > `String` or `Substring` positions in files or other archival forms,  
> > and reconstruct the original positions with respect to a deserialized  
> > `String` or `Substring`.
> > 
> > ## Proposed solution
> > 
> > All `String` views will use a single index type (`String.Index`), so  
> > that positions can be interchanged without awkward explicit  
> > conversions:
> > 
> > ```swift
> > let html: String = "See <a href=\"http://swift.org\">swift.org</a>"
> > 
> > // Search the UTF16, instead of characters, for performance reasons:
> > let open = "<".utf16.first!, close = ">".utf16.first!
> > let tagStart = s.utf16.index(of: open)
> > let tagEnd = s.utf16[tagStart...].index(of: close)
> > 
> > // Slice the String with the UTF-16 indices to retrieve the tag.
> > let tag = html[tagStart...tagEnd]
> > 
> > ```
> > 
> > A property and an intializer will be added to `String.Index`, exposing  
> > the offset of the index in code units (currently only UTF-16) from the  
> > beginning of the string:
> > 
> > ```swift
> > let n: Int = html.endIndex.encodedOffset
> > let end = String.Index(encodedOffset: n)
> > assert(end == String.endIndex)
> > 
> > ```
> > 
> > # Comparison and Slicing Semantics
> > 
> > When two indices being compared correspond to positions that are valid  
> > in any single `String` view, comparison semantics are already fully  
> > specified by the `Collection` requirements. Where no single `String`  
> > view contains both index values, the indices compare unequal and  
> > ordering is determined by comparison of `encodedOffsets`. These index  
> > values are not totally ordered but do satisfy strict weak ordering  
> > requirements, which is sufficient for algorithms such as `sort` to  
> > exhibit sensible behavior. We might consider loosening the specified  
> > requirements on these algorithms and on `Comparable` to support strict  
> > weak ordering, but for now we can treat such index pairs as being  
> > outside the domain of comparison, like any other indices from  
> > completely distinct collections.
> > 
> > An index that does not fall on an exact boundary in a given `String`  
> > or `Substring` view will be “rounded down” to the nearest boundary  
> > when used for slicing or range replacement. So, for example,
> > 
> > ```swift
> > let s = "e\u{301}galite\u{301}" // "égalité"
> > print(s[s.unicodeScalars.indices.dropFirst().first!...]) // "égalité"
> > print(s[..<s.unicodeScalars.indices.last!]) // "égalit"
> > 
> > ```
> > 
> > Replacing the failable APIs listed [above](#motivation) that detect  
> > whether an index represents a valid position in a given view, and  
> > enhancement that explicitly round index positions to nearby boundaries  
> > in a given view, are left to a later proposal. For now, we do not  
> > propose to remove the existing index conversion APIs.
> > 
> > ## Detailed design
> > 
> > `String.Index` acquires an `encodedOffset` property and initializer:
> > 
> > ```swift
> > public extension String.Index {
> > /// Creates a position corresponding to the given offset in a
> > /// `String`'s underlying (UTF-16) code units.
> > init(encodedOffset: Int)
> > 
> > /// The position of this index expressed as an offset from the
> > /// beginning of the `String`'s underlying (UTF-16) code units.
> > var encodedOffset: Int
> > }
> > 
> > ```
> > 
> > `Index` types of `String.UTF8View`, `String.UTF16View`, and  
> > `String.UnicodeScalarView` are replaced by `String.Index`:
> > 
> > ```swift
> > public extension String.UTF8View {
> > typealias Index = String.Index
> > }
> > public extension String.UTF16View {
> > typealias Index = String.Index
> > }
> > public extension String.UnicodeScalarView {
> > typealias Index = String.Index
> > }
> > 
> > ```
> > 
> > Because the index types are collapsing, index conversion methods and  
> > initializers are reduced to the following:
> > 
> > ```swift
> > public extension String.Index {
> > init?(_: String.Index, within: String)
> > init?(_: String.Index, within: String.UTF8View)
> > init?(_: String.Index, within: String.UTF16View)
> > init?(_: String.Index, within: String.UnicodeScalarView)
> > 
> > func samePosition(in: String) -> String.Index?
> > func samePosition(in: String.UTF8View) -> String.Index?
> > func samePosition(in: String.UTF16View) -> String.Index?
> > func samePosition(in: String.UnicodeScalarView) -> String.Index?
> > }
> > 
> > ```
> > 
> > ## Source compatibility
> > 
> > Because of the collapse of index  
> > types, [existing non-failable APIs](#motivation) become failable. To  
> > avoid breaking Swift 3 code, the following overloads of existing  
> > functions are added, allowing the resulting optional indices to be  
> > used where previously non-optional indices were used. These overloads  
> > were driven by making the new APIs work with existing code, including  
> > the Swift source compatibility test suite, and should be viewed as  
> > migration aids only, rather than additions to the Swift 3 API.
> > 
> > ```swift
> > extension Optional where Wrapped == String.Index {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public static func ..<(
> > lhs: String.Index?, rhs: String.Index?
> > ) -> Range<String.Index> {
> > return lhs! ..< rhs!
> > }
> > 
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public static func ...(
> > lhs: String.Index?, rhs: String.Index?
> > ) -> ClosedRange<String.Index> {
> > return lhs! ... rhs!
> > }
> > }
> > 
> > // backward compatibility for index interchange.  
> > extension String.UTF16View {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(
> > _ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.UTF16.CodeUnit {
> > return self[i!]
> > }
> > }
> > 
> > extension String.UTF8View {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(
> > from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.UTF8.CodeUnit {
> > return self[i!]
> > }
> > }
> > 
> > // backward compatibility for index interchange.  
> > extension String.UnicodeScalarView {
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(after i: Index?) -> Index {
> > return index(after: i)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public func index(_ i: Index?, offsetBy n: IndexDistance) -> Index {
> > return index(i!, offsetBy: n)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
> > public func distance(from i: Index?, to j: Index?) -> IndexDistance {
> > return distance(from: i!, to: j!)
> > }
> > @available(
> > swift, deprecated: 3.2, obsoleted: 4.0,
> > message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
> > public subscript(i: Index?) -> Unicode.Scalar {
> > return self[i!]
> > }
> > }
> > 
> > ```
> > 
> > - \*\*Q\*\*: Will existing correct Swift 3 applications stop compiling due  
> > to this change?
> > 
> > \*\*A\*\*: it is possible but unlikely. The existing index conversion  
> > APIs are relatively rarely used, and the overloads listed above  
> > handle the common cases in Swift 3 compatibility mode.
> > 
> > - \*\*Q\*\*: Will applications still compile but produce  
> > different behavior than they used to?
> > 
> > \*\*A\*\*: No.
> > 
> > - \*\*Q\*\*: Is it possible to automatically migrate from the old syntax  
> > to the new syntax?
> > 
> > \*\*A\*\*: Yes, although usages of these APIs may be rare enough that it  
> > isn't worth the trouble.
> > 
> > - \*\*Q\*\*: Can Swift applications be written in a common subset that works  
> > &nbsp;&nbsp;both with Swift 3 and Swift 4 to aid in migration?
> > 
> > \*\*A\*\*: Yes, the Swift 4 APIs will all be available in Swift 3 mode.
> > 
> > ## Effect on ABI stability
> > 
> > This proposal changes the ABI of the standard library.
> > 
> > ## Effect on API resilience
> > 
> > This proposal makes no changes to the resilience of any APIs.
> > 
> > ## Alternatives considered
> > 
> > The only alternative considered was no action.
> > 
> > --  
> > -Dave
> > 
> > \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_  
> > swift-evolution mailing list  
> > swift-evolution@swift.org  
> > [https://lists.swift.org/mailman/listinfo/swift-evolution](https://lists.swift.org/mailman/listinfo/swift-evolution)

---

<div class="post-metadata">

**Author:** ![Brent\_Royal-Gordon](https://avatars.discourse-cdn.com/v4/letter/b/53a042/32.png) [@Brent\_Royal-Gordon](https://forums.swift.org/u/Brent_Royal-Gordon)\
**Post date:** [June 2, 2017, 11:37pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/5 "2017-06-02T23:37:01Z")

</div>

Do you intend to allow users to serialize an `encodedOffset` and deserialize it later, perhaps in a later version of Swift, to represent the same position? If so, I'm not sure how you intend to maintain compatibility once the "currently only UTF-16" notation is no longer true.

> **···**
>
> > On May 27, 2017, at 10:40 AM, Dave Abrahams via swift-evolution \<swift-evolution@swift.org\> wrote:
> > 
> > A property and an intializer will be added to `String.Index`, exposing  
> > the offset of the index in code units (currently only UTF-16) from the  
> > beginning of the string:
> > 
> > ```swift
> > let n: Int = html.endIndex.encodedOffset
> > let end = String.Index(encodedOffset: n)
> > assert(end == String.endIndex)
> > 
> > ```
> 
> --  
> Brent Royal-Gordon  
> Architechies

---

<div class="post-metadata">

**Author:** ![benrimmington](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/benrimmington/32/695_2.png) [@benrimmington](https://forums.swift.org/u/benrimmington)\
**Post date:** [June 3, 2017, 11:32pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/6 "2017-06-03T23:32:14Z")

</div>

> ## Introduction
> 
> Today `String` shares an `Index` type with its `CharacterView` but not  
> with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> and `String.CharacterView.Index` as typealiases for `String.Index`,  
> and exposes a public `encodedOffset` property and initializer that can  
> be used to serialize and deserialize positions in a `String` or  
> `Substring`.

If `encodedOffset` is only needed for serialization of indices,  
could `String.Index` conform to `Codable` from SE-0166 instead?

> ## Proposed solution
> 
> All `String` views will use a single index type (`String.Index`), so  
> that positions can be interchanged without awkward explicit  
> conversions:
> 
> ```swift  
> let html: String = "See \<a href=\"[http://swift.org](http://swift.org)&quot;\>swift.org\</a\>"
> 
> // Search the UTF16, instead of characters, for performance reasons:  
> let open = "\<".utf16.first!, close = "\>".utf16.first!  
> let tagStart = s.utf16.index(of: open)  
> let tagEnd = s.utf16[tagStart...].index(of: close)

I think `s` should be `html` in the previous two lines.

-- Ben

> **···**
>
> > On 27 May 2017, at 18:40, Dave Abrahams wrote:

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [May 30, 2017, 9:48pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/7 "2017-05-30T21:48:46Z")

</div>

> > An index that does not fall on an exact boundary in a given `String`  
> > or `Substring` view will be “rounded down” to the nearest boundary  
> > when used for slicing or range replacement. So, for example,
> 
> What about normal subscript? I.e. what would the following print?
> 
> print(s[s.unicodeScalars.indices.dropFirst().first!]) // “é”, or just  
> the combining scalar?

I am proposing that it would be “é”

> Would unifying under the same type require that indices be less  
> stateful than they currently are?

No; it's just a matter of unifying the states (in an enum). You can  
look at the implementation in [https://github.com/apple/swift/pull/9806](https://github.com/apple/swift/pull/9806)  
for details.

> **···**
>
> on Tue May 30 2017, Michael Ilseman \<milseman-AT-apple.com\> wrote:
> 
> > > On May 27, 2017, at 10:40 AM, Dave Abrahams via swift-evolution \<swift-evolution@swift.org\> wrote:
> 
> > > ```swift
> > > let s = "e\u{301}galite\u{301}" // "égalité"
> > > print(s[s.unicodeScalars.indices.dropFirst().first!...]) // “égalité"
> > > print(s[..<s.unicodeScalars.indices.last!]) // "égalit"
> > > 
> > > ```
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [May 30, 2017, 9:53pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/8 "2017-05-30T21:53:34Z")

</div>

> My knee-jerk reaction is to say it's too late in Swift 4 for this kind  
> of change, but with that out of the way, I'm most concerned about what  
> it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.
> 
> let str = "言"  
> let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
> let trailingBytes = str.utf8[oneUnitIn...]

This is not new; it exists today.

> What can I do with 'oneUnitIn'?

All the usual stuff; we're not proposing to change what you can do with  
it.

> How do I test to see if it's on a Character boundary or a  
> UnicodeScalar boundary?

as noted,

&nbsp;&nbsp;Replacing the failable APIs listed [above](#motivation) that detect  
&nbsp;&nbsp;whether an index represents a valid position in a given view, and  
&nbsp;&nbsp;enhancement that explicitly round index positions to nearby boundaries  
&nbsp;&nbsp;in a given view, are left to a later proposal. For now, we do not  
&nbsp;&nbsp;propose to remove the existing index conversion APIs.

That means you can use oneUnitIn.samePosition(in: str) or  
oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta  
character or unicode scalar boundary.

> **···**
>
> on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [June 3, 2017, 12:06am UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/9 "2017-06-03T00:06:32Z")

</div>

Strings will offer access to their underlying code units, and can be  
serialized and deserialized however they are represented. A String  
stored as UTF-8 will be serialized and deserialized as UTF-8, so  
encodedOffsets will maintain their meanings.

Cheers,

> **···**
>
> on Fri Jun 02 2017, Brent Royal-Gordon \<swift-evolution@swift.org\> wrote:
> 
> > > On May 27, 2017, at 10:40 AM, Dave Abrahams via swift-evolution \> \<swift-evolution@swift.org\> wrote:
> > > 
> > > A property and an intializer will be added to `String.Index`, exposing  
> > > the offset of the index in code units (currently only UTF-16) from the  
> > > beginning of the string:
> > > 
> > > ```swift
> > > let n: Int = html.endIndex.encodedOffset
> > > let end = String.Index(encodedOffset: n)
> > > assert(end == String.endIndex)
> > > 
> > > ```
> > 
> > Do you intend to allow users to serialize an `encodedOffset` and  
> > deserialize it later, perhaps in a later version of Swift, to  
> > represent the same position? If so, I'm not sure how you intend to  
> > maintain compatibility once the "currently only UTF-16" notation is no  
> > longer true.
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [June 4, 2017, 12:10am UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/10 "2017-06-04T00:10:00Z")

</div>

> > ## Introduction
> > 
> > Today `String` shares an `Index` type with its `CharacterView` but not  
> > with its `UTF8View`, `UTF16View`, or `UnicodeScalarView`. This  
> > proposal redefines `String.UTF8View.Index`, `String.UTF16View.Index`,  
> > and `String.CharacterView.Index` as typealiases for `String.Index`,  
> > and exposes a public `encodedOffset` property and initializer that can  
> > be used to serialize and deserialize positions in a `String` or  
> > `Substring`.
> 
> If `encodedOffset` is only needed for serialization of indices,

It's not only needed for that. Once StringProtocol provides access to  
its underlying code units, the encodedOffset will be useful information  
in other ways.

> could `String.Index` conform to `Codable` from SE-0166 instead?

Not instead, definitely. Additionally, maybe; that's a separate  
proposal.

> > ## Proposed solution
> > 
> > All `String` views will use a single index type (`String.Index`), so  
> > that positions can be interchanged without awkward explicit  
> > conversions:
> > 
> > ```swift  
> > let html: String = "See \<a href=\"[http://swift.org](http://swift.org)&quot;\>swift.org\</a\>"
> > 
> > // Search the UTF16, instead of characters, for performance reasons:  
> > let open = "\<".utf16.first!, close = "\>".utf16.first!  
> > let tagStart = s.utf16.index(of: open)  
> > let tagEnd = s.utf16[tagStart...].index(of: close)
> 
> I think `s` should be `html` in the previous two lines.

Ah, yeah, thanks; good catch!

> **···**
>
> on Sat Jun 03 2017, Ben Rimmington \<me-AT-benrimmington.com\> wrote:
> 
> > > On 27 May 2017, at 18:40, Dave Abrahams wrote:
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [June 3, 2017, 5:40pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/11 "2017-06-03T17:40:05Z")

</div>

Philippe Hausler via swift-evolution

> I would presume that the index type will still be shared between String and SubString,

Yes; that's a requirement of Collection conformance.

> will this mean that we will now be able to express index manipulation in StringProtocol?

Depends what you mean by that I guess ;-)

> I find StringProtocol a bit hard to deal with when attempting to make range conversions;

StringProtocol is pretty far from being in its indented final form, if  
that's any consolation…

> it would be really nice if we could make this possible (or perhaps more  
> intuitive... since, for the life of me I can’t figure out a way to  
> generically convert indexes for StringProtocol adoption)
> 
> So lets say you have a function as such:
> 
> func foo\<S: StringProtocol\>(\_ str: S, range: Range\<S.Index\>) {  
> &nbsp;&nbsp;&nbsp;&nbsp;range.lowerBound.samePosition(in: str.utf16)  
> }
> 
> results in the error error: value of type 'S.Index' has no member ‘samePosition’

> This of course is an intended target of something that deals with strings  
> and wants to deal with both strings and substrings uniformly since it is  
> reasonable to pass either.
> 
> In short: are StringProtocol accessors a consideration for conversion in this change?

I'm sorry but I don't understand the question. If you're asking whether  
this particular proposal is designed to address index translation on models  
of StringProtocol, the answer is no. It will appear to do so for the moment  
but only because StringProtocol is currently over-constrained.

The intention is that eventually StringProtocol does not constrain its  
views to all have the same index type, but does constrain them to  
Unicode.ViewIndex, which will require the encodedOffset initializer and  
property. Also the index type of a StringProtocol's SubSequence's XXXView  
will be constrained to be the same as that of the StringProtocol's own  
XXXView.

Hope this helps,  
Dave

> **···**
>
> \<swift-evolution@swift.org\> wrote:

---

<div class="post-metadata">

**Author:** ![jrose](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jrose/32/408_2.png) [@jrose](https://forums.swift.org/u/jrose)\
**Post date:** [May 30, 2017, 9:58pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/12 "2017-05-30T21:58:33Z")

</div>

> > My knee-jerk reaction is to say it's too late in Swift 4 for this kind  
> > of change, but with that out of the way, I'm most concerned about what  
> > it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.
> > 
> > let str = "言"  
> > let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
> > let trailingBytes = str.utf8[oneUnitIn...]
> 
> This is not new; it exists today.

Yes, I think that’s valuable. What’s different is that it’s not a String.Index.

> > What can I do with 'oneUnitIn'?
> 
> All the usual stuff; we're not proposing to change what you can do with  
> it.

By changing the type, you have increased the scope of where an index can be used. What happens when I use it in one of the other views and it’s not on a boundary?

(I suspect the answer is “it traps” but the proposal should spell that out explicitly.)

> > How do I test to see if it's on a Character boundary or a  
> > UnicodeScalar boundary?
> 
> as noted,
> 
> Replacing the failable APIs listed [above](#motivation) that detect  
> whether an index represents a valid position in a given view, and  
> enhancement that explicitly round index positions to nearby boundaries  
> in a given view, are left to a later proposal. For now, we do not  
> propose to remove the existing index conversion APIs.
> 
> That means you can use oneUnitIn.samePosition(in: str) or  
> oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta  
> character or unicode scalar boundary.

I’m sorry, I completely missed that. This part of the question is withdrawn.

I’m also concerned about putting “UTF-16” in the documentation for encodedOffset. Either it’s a ‘utf16Offset’ or it isn’t; if it’s an opaque value then it should be treated as such. (It’s also a little disturbing that round-tripping through encodedOffset isn’t guaranteed to give you the same index back.)

Jordan

> **···**
>
> > On May 30, 2017, at 14:53, Dave Abrahams \<dabrahams@apple.com\> wrote:  
> > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [May 30, 2017, 11:13pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/13 "2017-05-30T23:13:04Z")

</div>

> > > My knee-jerk reaction is to say it's too late in Swift 4 for this kind  
> > > of change, but with that out of the way, I'm most concerned about what  
> > > it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.
> > > 
> > > let str = "言"  
> > > let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
> > > let trailingBytes = str.utf8[oneUnitIn...]
> > 
> > This is not new; it exists today.
> 
> Yes, I think that’s valuable. What’s different is that it’s not a String.Index.
> 
> > > What can I do with 'oneUnitIn'?
> > 
> > All the usual stuff; we're not proposing to change what you can do with  
> > it.
> 
> By changing the type, you have increased the scope of where an index  
> can be used. What happens when I use it in one of the other views and  
> it’s not on a boundary?
> 
> (I suspect the answer is “it traps” but the proposal should spell that  
> out explicitly.)

Sorry, I mistakenly limited the “rounding down” behavior to slicing and  
range replacement. The index would be rounded down to the previous  
boundary, and then used as ever.

> > > How do I test to see if it's on a Character boundary or a  
> > > UnicodeScalar boundary?
> > 
> > as noted,
> > 
> > Replacing the failable APIs listed [above](#motivation) that detect  
> > whether an index represents a valid position in a given view, and  
> > enhancement that explicitly round index positions to nearby boundaries  
> > in a given view, are left to a later proposal. For now, we do not  
> > propose to remove the existing index conversion APIs.
> > 
> > That means you can use oneUnitIn.samePosition(in: str) or  
> > oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta  
> > character or unicode scalar boundary.
> 
> I’m sorry, I completely missed that. This part of the question is withdrawn.
> 
> I’m also concerned about putting “UTF-16” in the documentation for  
> encodedOffset. Either it’s a ‘utf16Offset’ or it isn’t

It is today; hopefully it won't be someday

> ; if it’s an opaque value then it should be treated as such.

Today a String has underlying UTF-16-compatible storage and that's  
documented as such, but we intend to lift that restriction and don't  
want the names to lock us into semantics.

> (It’s also a little disturbing that round-tripping through  
> encodedOffset isn’t guaranteed to give you the same index back.)

Define “same.”

The encodedOffset is not the full value of an \*arbitrary\* index, and  
doesn't claim to be. The indices that can be serialized and  
reconstructed exactly using encodedOffset are those that fall on code  
unit boundaries. Today, that means everything but UTF-8 indices. We  
could consider exposing the transcodedOffset (offset within the UTF8  
encoding of the scalar) as well, but I want to be conservative.

> **···**
>
> on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:
> 
> > > On May 30, 2017, at 14:53, Dave Abrahams \<dabrahams@apple.com\> wrote:  
> > > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:
> 
> --  
> -Dave

---

<div class="post-metadata">

**Author:** ![jrose](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/jrose/32/408_2.png) [@jrose](https://forums.swift.org/u/jrose)\
**Post date:** [May 31, 2017, 2:39am UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/14 "2017-05-31T02:39:19Z")

</div>

> > > > My knee-jerk reaction is to say it's too late in Swift 4 for this kind  
> > > > of change, but with that out of the way, I'm most concerned about what  
> > > > it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.
> > > > 
> > > > let str = "言"  
> > > > let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
> > > > let trailingBytes = str.utf8[oneUnitIn...]
> > > 
> > > This is not new; it exists today.
> > 
> > Yes, I think that’s valuable. What’s different is that it’s not a String.Index.
> > 
> > > > What can I do with 'oneUnitIn'?
> > > 
> > > All the usual stuff; we're not proposing to change what you can do with  
> > > it.
> > 
> > By changing the type, you have increased the scope of where an index  
> > can be used. What happens when I use it in one of the other views and  
> > it’s not on a boundary?
> > 
> > (I suspect the answer is “it traps” but the proposal should spell that  
> > out explicitly.)
> 
> Sorry, I mistakenly limited the “rounding down” behavior to slicing and  
> range replacement. The index would be rounded down to the previous  
> boundary, and then used as ever.

Makes sense!

> > > > How do I test to see if it's on a Character boundary or a  
> > > > UnicodeScalar boundary?
> > > 
> > > as noted,
> > > 
> > > Replacing the failable APIs listed [above](#motivation) that detect  
> > > whether an index represents a valid position in a given view, and  
> > > enhancement that explicitly round index positions to nearby boundaries  
> > > in a given view, are left to a later proposal. For now, we do not  
> > > propose to remove the existing index conversion APIs.
> > > 
> > > That means you can use oneUnitIn.samePosition(in: str) or  
> > > oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta  
> > > character or unicode scalar boundary.
> > 
> > I’m sorry, I completely missed that. This part of the question is withdrawn.
> > 
> > I’m also concerned about putting “UTF-16” in the documentation for  
> > encodedOffset. Either it’s a ‘utf16Offset’ or it isn’t
> 
> It is today; hopefully it won't be someday
> 
> > ; if it’s an opaque value then it should be treated as such.
> 
> Today a String has underlying UTF-16-compatible storage and that's  
> documented as such, but we intend to lift that restriction and don't  
> want the names to lock us into semantics.

I don’t think you should promise that about new APIs, then, or someone will start relying on it.

> > (It’s also a little disturbing that round-tripping through  
> > encodedOffset isn’t guaranteed to give you the same index back.)
> 
> Define “same.”
> 
> The encodedOffset is not the full value of an \*arbitrary\* index, and  
> doesn't claim to be. The indices that can be serialized and  
> reconstructed exactly using encodedOffset are those that fall on code  
> unit boundaries. Today, that means everything but UTF-8 indices. We  
> could consider exposing the transcodedOffset (offset within the UTF8  
> encoding of the scalar) as well, but I want to be conservative.

I’m not sure it’s clear from the name “encodedOffset” that this is a lossy conversion. I’d say it should be an optional property, but that’s probably too annoying in the invalid case. Maybe it should trap.

Jordan

> **···**
>
> > On May 30, 2017, at 16:13, Dave Abrahams \<dabrahams@apple.com\> wrote:  
> > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com \<[http://at-apple.com/&gt;&gt](http://at-apple.com/&gt;&gt); wrote:
> > 
> > > > On May 30, 2017, at 14:53, Dave Abrahams \<dabrahams@apple.com\> wrote:  
> > > > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:

---

<div class="post-metadata">

**Author:** ![dabrahams](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/dabrahams/32/9815_2.png) [@dabrahams](https://forums.swift.org/u/dabrahams)\
**Post date:** [May 31, 2017, 8:37pm UTC](https://forums.swift.org/t/pitch-string-index-overhaul/6017/15 "2017-05-31T20:37:41Z")

</div>

> > > > > My knee-jerk reaction is to say it's too late in Swift 4 for this kind  
> > > > > of change, but with that out of the way, I'm most concerned about what  
> > > > > it means to have, say, a UTF-8 index that's not on a UTF-16 boundary.
> > > > > 
> > > > > let str = "言"  
> > > > > let oneUnitIn = str.utf8.index(after: str.utf8.startIndex)  
> > > > > let trailingBytes = str.utf8[oneUnitIn...]
> > > > 
> > > > This is not new; it exists today.
> > > 
> > > Yes, I think that’s valuable. What’s different is that it’s not a String.Index.
> > > 
> > > > > What can I do with 'oneUnitIn'?
> > > > 
> > > > All the usual stuff; we're not proposing to change what you can do with  
> > > > it.
> > > 
> > > By changing the type, you have increased the scope of where an index  
> > > can be used. What happens when I use it in one of the other views and  
> > > it’s not on a boundary?
> > > 
> > > (I suspect the answer is “it traps” but the proposal should spell that  
> > > out explicitly.)
> > 
> > Sorry, I mistakenly limited the “rounding down” behavior to slicing and  
> > range replacement. The index would be rounded down to the previous  
> > boundary, and then used as ever.
> 
> Makes sense!
> 
> > > > > How do I test to see if it's on a Character boundary or a  
> > > > > UnicodeScalar boundary?
> > > > 
> > > > as noted,
> > > > 
> > > > Replacing the failable APIs listed [above](#motivation) that detect  
> > > > whether an index represents a valid position in a given view, and  
> > > > enhancement that explicitly round index positions to nearby boundaries  
> > > > in a given view, are left to a later proposal. For now, we do not  
> > > > propose to remove the existing index conversion APIs.
> > > > 
> > > > That means you can use oneUnitIn.samePosition(in: str) or  
> > > > oneUnitIn.samePosition(in: str.unicodeScalars) to find out if it's on ta  
> > > > character or unicode scalar boundary.
> > > 
> > > I’m sorry, I completely missed that. This part of the question is withdrawn.
> > > 
> > > I’m also concerned about putting “UTF-16” in the documentation for  
> > > encodedOffset. Either it’s a ‘utf16Offset’ or it isn’t
> > 
> > It is today; hopefully it won't be someday
> > 
> > > ; if it’s an opaque value then it should be treated as such.
> > 
> > Today a String has underlying UTF-16-compatible storage and that's  
> > documented as such, but we intend to lift that restriction and don't  
> > want the names to lock us into semantics.
> 
> I don’t think you should promise that about new APIs, then, or someone  
> will start relying on it.

Okay, we could leave it out of this doc comment. But as long as  
something documents that Strings are stored as UTF-16 (e.g. we say you  
get random-access performance for the utf16 view when Foundation is  
loaded), the implication is there.

> > > (It’s also a little disturbing that round-tripping through  
> > > encodedOffset isn’t guaranteed to give you the same index back.)
> > 
> > Define “same.”
> > 
> > The encodedOffset is not the full value of an \*arbitrary\* index, and  
> > doesn't claim to be. The indices that can be serialized and  
> > reconstructed exactly using encodedOffset are those that fall on code  
> > unit boundaries. Today, that means everything but UTF-8 indices. We  
> > could consider exposing the transcodedOffset (offset within the UTF8  
> > encoding of the scalar) as well, but I want to be conservative.
> 
> I’m not sure it’s clear from the name “encodedOffset” that this is a  
> lossy conversion.

It's not a conversion :-)

> I’d say it should be an optional property, but that’s probably too  
> annoying in the invalid case. Maybe it should trap.

I really don't think so; IMO that would be inconsistent with the  
“rounding down” behavior proposed. I think either all misaligned  
accesses should trap or they should do something lenient. I proposed  
lenience, but trapping is still an option.

> **···**
>
> on Tue May 30 2017, Jordan Rose \<swift-evolution@swift.org\> wrote:
> 
> > > On May 30, 2017, at 16:13, Dave Abrahams \<dabrahams@apple.com\> \> wrote:  
> > > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com \<[http://at-apple.com/&gt;&gt](http://at-apple.com/&gt;&gt); wrote:
> > > 
> > > > > On May 30, 2017, at 14:53, Dave Abrahams \<dabrahams@apple.com\> wrote:  
> > > > > on Tue May 30 2017, Jordan Rose \<jordan\_rose-AT-apple.com\> wrote:
> 
> --  
> -Dave