[Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching

## Title

Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.

However, the `switch` statement has several unique behaviors unrelated to
pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases
are not executed.
- `default:` case is required, even for expressions where a default case
does not make sense.

These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does
not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical
branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.

The programmer would choose between `switch` or `match` based on the goal
of the pattern matching. For example, pattern matching a String. `switch`
would be appropriate for evaluating a String that represents the rawValue
of an enum. But `match` would be more appropriate for evaluating a single
input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.

- A syntax that exactly aligns with the familiar, succinct, elegant, and
understandable `switch` syntax.

- The keyword "match" highlights that pattern matching will occur. Would be
even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as typos
or changes to the expression being evaluated in a *single* `if case` from
the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy "just
delete whitespace" conversion from standard multi-line switch/match syntax,
whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for non-exhaustible
expressions. Pretty much any non-Enum type being evaluated is
non-exhaustible. (This is not the *main* goal of this proposal.)

## Prototype

A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }

    print(output)
}

// `15` prints "FizzBuzz"
1 Like

Having spent a lot of time with ‘switch’, I don’t understand any of the motivations or corresponding justifications for this idea. Comments inline:

~Robert Widmann

2017/11/17 15:06、Peter Kamb via swift-evolution <swift-evolution@swift.org>のメール:

## Title

Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.

However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases are not executed.

This is not unrelated to pattern matching, this is the expected behavior of every pattern matching algorithm. The intent is to compile a switch-case tree down to (conceptually) a (hopefully minimal) if-else tree. You may be thinking of the behavior of switch in C and C-likes which is most certainly not pattern matching and includes behavior Swift has explicitly chosen to avoid like implicit fallthroughs.

- `default:` case is required, even for expressions where a default case does not make sense.

Expression patterns may look to be covered “at first glance”, but the analysis required to prove that is equivalent to solving the halting problem in the general case. Further, your proposed idea has absolutely nothing to do with this.

These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.

The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.

Again, this is not pattern matching.

- A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.

A syntax that duplicates the existing if-case construct which, I should note, takes the same number of lines and roughly the same number of columns to express as your match. Even less, considering you unwrap in the if-case to exaggerate the example but not in the match.

- The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)

This is not boilerplate!

## Prototype

A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }
   
    print(output)
}

// `15` prints "FizzBuzz"

for i in 1...100 {
  var output = “”
  output += (i % 3 == 0) ? “Fizz” : “”
  output += (i % 5 == 0) ? “Buzz” : “”
  print(output.isEmpty ? “\(i)” : output)
}

If control flow should branch multiple times, why not write just write it that way!

···

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

There have been earlier suggestions for an alternative to `fallthrough` that would continue matching cases; I think that is much more likely to get support than a whole new construct with only a subtle difference from an existing one—would that be an acceptable alternative to you?

···

On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <swift-evolution@swift.org> wrote:

## Title

Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.

However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases are not executed.
- `default:` case is required, even for expressions where a default case does not make sense.

These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.

The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.

- A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.

- The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)

## Prototype

A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }
   
    print(output)
}

// `15` prints "FizzBuzz"

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

1 Like

I haven’t spend that much time on thinking about the implications, but at first sight, it sounds like a sensible idea — maybe even more useful than switch (whose behavior could be mimicked).
But switch is established, and I don’t see a realistic chance to either replace it or add a construct that is quite similar.
Afaics, it will already be hard to collect feedback, so it might be a good idea to delay the pitch until the move to Discourse is done.

1 Like

Thanks for the review.

Motivation is essentially to provide a low-friction way to write `if case /
if case / if case` statements that are all matching on the same expression,
much in the same way that `switch` offers a way to write `if / else if /
else` statements against the same expression.

> > Only the *first* matching case is executed. Subsequent matching cases
are not executed.
> This is not unrelated to pattern matching, this is the expected behavior
of every pattern matching algorithm.

By that I meant that a string of `if case / if case / if case` statements,
each individually matching against the same expression, would match and
execute all 3 cases. A `switch` using the same 3 cases would execute only
the first matching case. I took the "only first case" behavior to be a
property of the *switch*, not of "pattern matching" in general? But perhaps
I'm using the wrong terminology.

> > Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
> Again, this is not pattern matching.

A switch can have multiple cases that "would" match and execute, if you
were to comment out the successful case above them. Why would a
hypothetical statement that executed *all* matching cases, rather than just
the first case, not be pattern matching? It wouldn't be a `switch`, for
sure, but it seems like it would be just as much "pattern matching" as an
`if-case`.

> This is not boilerplate!

An `if case / if case` pair would not have the same concept of a shared
`else/default` case used by a `switch` or `if/else`, which is why I said it
would not be needed. `if-case` does not require an else/default. (Perhaps
boilerplate was the wrong word, and I definitely didn't mean to suggest
that switches should no longer require `default:`).

> A syntax that duplicates the existing if-case construct

Right, it would duplicate `if-case` just as `switch` duplicates `if-else`.

Cheers, thanks.

Peter

···

On Sat, Nov 18, 2017 at 12:19 AM, Robert Widmann <devteam.codafi@gmail.com> wrote:

Having spent a lot of time with ‘switch’, I don’t understand any of the
motivations or corresponding justifications for this idea. Comments inline:

~Robert Widmann

2017/11/17 15:06、Peter Kamb via swift-evolution <swift-evolution@swift.org
>のメール:

## Title

Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.

However, the `switch` statement has several unique behaviors unrelated to
pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases
are not executed.

This is not unrelated to pattern matching, this is the expected behavior
of every pattern matching algorithm. The intent is to compile a
switch-case tree down to (conceptually) a (hopefully minimal) if-else
tree. You may be thinking of the behavior of switch in C and C-likes which
is most certainly not pattern matching and includes behavior Swift has
explicitly chosen to avoid like implicit fallthroughs.

- `default:` case is required, even for expressions where a default case
does not make sense.

Expression patterns may look to be covered “at first glance”, but the
analysis required to prove that is equivalent to solving the halting
problem in the general case. Further, your proposed idea has absolutely
nothing to do with this.

These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does
not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical
branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.

The programmer would choose between `switch` or `match` based on the goal
of the pattern matching. For example, pattern matching a String. `switch`
would be appropriate for evaluating a String that represents the rawValue
of an enum. But `match` would be more appropriate for evaluating a single
input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.

Again, this is not pattern matching.

- A syntax that exactly aligns with the familiar, succinct, elegant, and
understandable `switch` syntax.

A syntax that duplicates the existing if-case construct which, I should
note, takes the same number of lines and roughly the same number of columns
to express as your match. Even less, considering you unwrap in the if-case
to exaggerate the example but not in the match.

- The keyword "match" highlights that pattern matching will occur. Would
be even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as typos
or changes to the expression being evaluated in a *single* `if case` from
the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)

This is not boilerplate!

## Prototype

A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }

    print(output)
}

// `15` prints "FizzBuzz"

for i in 1...100 {
  var output = “”
  output += (i % 3 == 0) ? “Fizz” : “”
  output += (i % 5 == 0) ? “Buzz” : “”
  print(output.isEmpty ? “\(i)” : output)
}

If control flow should branch multiple times, *why not write just write
it that way!*

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

Robert is quite right--I'm not sure what we're designing for here. There's
a very high bar for introducing new syntax and a distaste for the existing
syntax is not a motivating use case.

···

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution < swift-evolution@swift.org> wrote:

There have been earlier suggestions for an alternative to `fallthrough`
that would continue matching cases; I think that is much more likely to get
support than a whole new construct with only a subtle difference from an
existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < > swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.
>
> However, the `switch` statement has several unique behaviors unrelated
to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching cases
are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant,
and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur. Would
be even better than `switch` for initial introductions to pattern-matching.
>
> - No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

Thanks for the review.

Motivation is essentially to provide a low-friction way to write `if case / if case / if case` statements that are all matching on the same expression, much in the same way that `switch` offers a way to write `if / else if / else` statements against the same expression.

That much I understand, but there’s no motivation as to why this kind of change needs to occur over a line of if-case statements. There isn’t anything in the pitch that demonstrates friction outside of a personal distaste for the syntax.

> > Only the *first* matching case is executed. Subsequent matching cases are not executed.
> This is not unrelated to pattern matching, this is the expected behavior of every pattern matching algorithm.

By that I meant that a string of `if case / if case / if case` statements, each individually matching against the same expression, would match and execute all 3 cases. A `switch` using the same 3 cases would execute only the first matching case. I took the "only first case" behavior to be a property of the *switch*, not of "pattern matching" in general? But perhaps I'm using the wrong terminology.

> > Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.
> Again, this is not pattern matching.

A switch can have multiple cases that "would" match and execute, if you were to comment out the successful case above them. Why would a hypothetical statement that executed *all* matching cases, rather than just the first case, not be pattern matching? It wouldn't be a `switch`, for sure, but it seems like it would be just as much "pattern matching" as an `if-case`.

It is not pattern matching because it violates minimality - and we have a warning today when you write switches of overlapping patterns. Consider the semantics of pattern matching in the context of a tree - but it just so happens the nodes of this tree are your patterns and the branches, actual branches where control flow splits along the case-matrix. The goal is not to execute a find-all, the goal is to execute a find - exactly like a regular expression.

> This is not boilerplate!

An `if case / if case` pair would not have the same concept of a shared `else/default` case used by a `switch` or `if/else`, which is why I said it would not be needed. `if-case` does not require an else/default. (Perhaps boilerplate was the wrong word, and I definitely didn't mean to suggest that switches should no longer require `default:`).

You misunderstand me. This is the same problem as the one I brought up before: You are subject to the halting problem in the general case. The catch-all clause isn’t just noise, it’s a fundamental necessity to guarantee sound semantics for this particular class of switch statements that, according to a fundamental barrier in computer science, cannot be checked for exhaustiveness.

On top of that, we cannot allow you to write an uncovered switch statement full of expression patterns because you would most-assuredly not handle all possible cases (suppose I extend your OptionSet with a new bit-pattern in a different module - your code will miscompile).

···

On Nov 18, 2017, at 4:42 AM, Peter Kamb <peterkamb@gmail.com> wrote:

On Sat, Nov 18, 2017 at 12:19 AM, Robert Widmann <devteam.codafi@gmail.com <mailto:devteam.codafi@gmail.com>> wrote:
Having spent a lot of time with ‘switch’, I don’t understand any of the motivations or corresponding justifications for this idea. Comments inline:

~Robert Widmann

2017/11/17 15:06、Peter Kamb via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>>のメール:

## Title

Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.

However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases are not executed.

This is not unrelated to pattern matching, this is the expected behavior of every pattern matching algorithm. The intent is to compile a switch-case tree down to (conceptually) a (hopefully minimal) if-else tree. You may be thinking of the behavior of switch in C and C-likes which is most certainly not pattern matching and includes behavior Swift has explicitly chosen to avoid like implicit fallthroughs.

- `default:` case is required, even for expressions where a default case does not make sense.

Expression patterns may look to be covered “at first glance”, but the analysis required to prove that is equivalent to solving the halting problem in the general case. Further, your proposed idea has absolutely nothing to do with this.

These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.

The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.

Again, this is not pattern matching.

- A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.

A syntax that duplicates the existing if-case construct which, I should note, takes the same number of lines and roughly the same number of columns to express as your match. Even less, considering you unwrap in the if-case to exaggerate the example but not in the match.

- The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)

This is not boilerplate!

## Prototype

A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }
   
    print(output)
}

// `15` prints "FizzBuzz"

for i in 1...100 {
  var output = “”
  output += (i % 3 == 0) ? “Fizz” : “”
  output += (i % 5 == 0) ? “Buzz” : “”
  print(output.isEmpty ? “\(i)” : output)
}

If control flow should branch multiple times, why not write just write it that way!

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

alternative to `fallthrough` that would continue matching cases

Yes, that would be interesting to look into. Do you have any references or
remember what the proposed keyword was called? Do any other languages have
this feature?

···

On Sat, Nov 18, 2017 at 10:53 AM Kevin Nattinger <swift@nattinger.net> wrote:

There have been earlier suggestions for an alternative to `fallthrough`
that would continue matching cases; I think that is much more likely to get
support than a whole new construct with only a subtle difference from an
existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < > swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.
>
> However, the `switch` statement has several unique behaviors unrelated
to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching cases
are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant,
and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur. Would
be even better than `switch` for initial introductions to pattern-matching.
>
> - No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

Aha! I found a proposal Erica put forth about this a year and a while ago—

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160704/023955.html

···

On Nov 18, 2017, at 11:30 AM, Peter Kamb <peterkamb@gmail.com> wrote:

> alternative to `fallthrough` that would continue matching cases

Yes, that would be interesting to look into. Do you have any references or remember what the proposed keyword was called? Do any other languages have this feature?

On Sat, Nov 18, 2017 at 10:53 AM Kevin Nattinger <swift@nattinger.net <mailto:swift@nattinger.net>> wrote:
There have been earlier suggestions for an alternative to `fallthrough` that would continue matching cases; I think that is much more likely to get support than a whole new construct with only a subtle difference from an existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching cases are not executed.
> - `default:` case is required, even for expressions where a default case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.
>
> The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.
>
> - No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> swift-evolution mailing list
> swift-evolution@swift.org <mailto:swift-evolution@swift.org>
> https://lists.swift.org/mailman/listinfo/swift-evolution

2 Likes

I understand your point about multiple cases + the halting problem when
applied to the implementation details of switch and pattern matching.

I think where we're missing is that what I'm envisioning would be syntactic
sugar that would be de-sugared into a a line of single-case `if-case`
statements. It would have nothing to do with matching multiple cases via
the same mechanism as `switch`.

Motivation for the change vs. a line of if-case statements is a valid
question of course :slight_smile:

···

On Sat, Nov 18, 2017 at 10:48 AM, Robert Widmann <devteam.codafi@gmail.com> wrote:

On Nov 18, 2017, at 4:42 AM, Peter Kamb <peterkamb@gmail.com> wrote:

Thanks for the review.

Motivation is essentially to provide a low-friction way to write `if case
/ if case / if case` statements that are all matching on the same
expression, much in the same way that `switch` offers a way to write `if /
else if / else` statements against the same expression.

That much I understand, but there’s no motivation as to why this kind of
change needs to occur over a line of if-case statements. There isn’t
anything in the pitch that demonstrates friction outside of a personal
distaste for the syntax.

> > Only the *first* matching case is executed. Subsequent matching cases
are not executed.
> This is not unrelated to pattern matching, this is the expected
behavior of every pattern matching algorithm.

By that I meant that a string of `if case / if case / if case` statements,
each individually matching against the same expression, would match and
execute all 3 cases. A `switch` using the same 3 cases would execute only
the first matching case. I took the "only first case" behavior to be a
property of the *switch*, not of "pattern matching" in general? But perhaps
I'm using the wrong terminology.

> > Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
> Again, this is not pattern matching.

A switch can have multiple cases that "would" match and execute, if you
were to comment out the successful case above them. Why would a
hypothetical statement that executed *all* matching cases, rather than just
the first case, not be pattern matching? It wouldn't be a `switch`, for
sure, but it seems like it would be just as much "pattern matching" as an
`if-case`.

It is not pattern matching because it violates minimality - and we have a
warning today when you write switches of overlapping patterns. Consider
the semantics of pattern matching in the context of a tree - but it just so
happens the nodes of this tree are your patterns and the branches, actual
branches where control flow splits along the case-matrix. The goal is not
to execute a find-all, the goal is to execute a find - exactly like a
regular expression.

> This is not boilerplate!

An `if case / if case` pair would not have the same concept of a shared
`else/default` case used by a `switch` or `if/else`, which is why I said it
would not be needed. `if-case` does not require an else/default. (Perhaps
boilerplate was the wrong word, and I definitely didn't mean to suggest
that switches should no longer require `default:`).

You misunderstand me. This is the same problem as the one I brought up
before: You are subject to the halting problem in the general case. The
catch-all clause isn’t just noise, it’s a fundamental necessity to
guarantee sound semantics for this particular class of switch statements
that, according to a fundamental barrier in computer science, cannot be
checked for exhaustiveness.

On top of that, we cannot allow you to write an uncovered switch statement
full of expression patterns because you would most-assuredly not handle all
possible cases (suppose I extend your OptionSet with a new bit-pattern in a
different module - your code will miscompile).

On Sat, Nov 18, 2017 at 12:19 AM, Robert Widmann <devteam.codafi@gmail.com > > wrote:

Having spent a lot of time with ‘switch’, I don’t understand any of the
motivations or corresponding justifications for this idea. Comments inline:

~Robert Widmann

2017/11/17 15:06、Peter Kamb via swift-evolution <
swift-evolution@swift.org>のメール:

## Title

Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching

## Summary:

The syntax of the `switch` statement is familiar, succinct, elegant, and
understandable. Swift pattern-matching tutorials use `switch` statements
almost exclusively, with small sections at the end for alternatives such as
`if case`.

However, the `switch` statement has several unique behaviors unrelated to
pattern matching. Namely:

- Only the *first* matching case is executed. Subsequent matching cases
are not executed.

This is not unrelated to pattern matching, this is the expected behavior
of every pattern matching algorithm. The intent is to compile a
switch-case tree down to (conceptually) a (hopefully minimal) if-else
tree. You may be thinking of the behavior of switch in C and C-likes which
is most certainly not pattern matching and includes behavior Swift has
explicitly chosen to avoid like implicit fallthroughs.

- `default:` case is required, even for expressions where a default case
does not make sense.

Expression patterns may look to be covered “at first glance”, but the
analysis required to prove that is equivalent to solving the halting
problem in the general case. Further, your proposed idea has absolutely
nothing to do with this.

These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.

Swift should contain an equally-good pattern-matching statement that does
not limit itself single-branch switching.

## Pitch:

Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.

match someValue {
case patternOne:
    always executed if pattern matches
case patternTwo:
    always executed if pattern matches
}

The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.

## Example:

struct TextFlags: OptionSet {
    let rawValue: Int
    static let italics = TextFlags(rawValue: 1 << 1)
    static let bold    = TextFlags(rawValue: 1 << 2)
}

let textFlags: TextFlags = [.italics, .bold]

// SWITCH STATEMENT
switch textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
default:
    print("forced to include a default case")
}
// prints "italics"
// Does NOT print "bold", despite .bold being set.

// MATCH STATEMENT
match textFlags {
case let x where x.contains(.italics):
    print("italics")
case let x where x.contains(.bold):
    print("bold")
}
// prints "italics"
// prints "bold"

## Enum vs. OptionSet

The basic difference between `switch` and `match` is the same conceptual
difference between `Emum` and an `OptionSet` bitmask.

`switch` is essentially designed for enums: switching to a single logical
branch based on the single distinct case represented by the enum.

`match` would be designed for OptionSet bitmasks and similar constructs.
Executing behavior for *any and all* of the following cases and patterns
that match.

The programmer would choose between `switch` or `match` based on the goal
of the pattern matching. For example, pattern matching a String. `switch`
would be appropriate for evaluating a String that represents the rawValue
of an enum. But `match` would be more appropriate for evaluating a single
input String against multiple unrelated-to-each-other regexes.

## Existing Alternatives

`switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.

Example using a string of independent `if case` statements:

if case let x = textFlags, x.contains(.italics) {
    print("italics")
}

if case let x = textFlags, x.contains(.bold) {
    print("bold")
}

## `match` statement benefits:

- Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.

Again, this is not pattern matching.

- A syntax that exactly aligns with the familiar, succinct, elegant, and
understandable `switch` syntax.

A syntax that duplicates the existing if-case construct which, I should
note, takes the same number of lines and roughly the same number of columns
to express as your match. Even less, considering you unwrap in the if-case
to exaggerate the example but not in the match.

- The keyword "match" highlights that pattern matching will occur. Would
be even better than `switch` for initial introductions to pattern-matching.

- No need to convert between the strangely slightly different syntax of
`switch` vs. `if case`, such as `case let x where x.contains(.italics):` to
`if case let x = textFlags, x.contains(.italics) {`

- Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."

- A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.

- Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.

- Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.

 match value { case pattern:
    print("matched")
}

- Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)

This is not boilerplate!

## Prototype

A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:

match: for eachCase in 0...1 {
switch (eachCase, textFlags) {
case (0, let x) where x.contains(.italics):
    print("italics")
case (1, let x) where x.contains(.bold):
    print("bold")
default: break }
}

// prints "italics"
// prints "bold"

## Notes / Discussion:

- Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!

- Should `match` allow a `default:` case? It would be easy enough to add
one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.

- FizzBuzz using proposed Swift `match` statement:

for i in 1...100 {
    var output = ""
    match 0 {
    case (i % 3): output += "Fizz"
    case (i % 3): output += "Buzz"
    default:      output = String(i)
    }

    print(output)
}

// `15` prints "FizzBuzz"

for i in 1...100 {
  var output = “”
  output += (i % 3 == 0) ? “Fizz” : “”
  output += (i % 5 == 0) ? “Buzz” : “”
  print(output.isEmpty ? “\(i)” : output)
}

If control flow should branch multiple times, *why not write just write
it that way!*

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

A high bar for new syntax is fair and expected, and by posting I was hoping
to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns, and
execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression, are
ok.

But that's definitely not as clear or concise as a switch-like construct
with the single control expression at the top. Or perhaps some other
alternative such as the mentioned `continue` or somehow enumerating a set
of cases.

···

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

Robert is quite right--I'm not sure what we're designing for here. There's
a very high bar for introducing new syntax and a distaste for the existing
syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution < > swift-evolution@swift.org> wrote:

There have been earlier suggestions for an alternative to `fallthrough`
that would continue matching cases; I think that is much more likely to get
support than a whole new construct with only a subtle difference from an
existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < >> swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant,
and understandable. Swift pattern-matching tutorials use `switch`
statements almost exclusively, with small sections at the end for
alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated
to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching
cases are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same
conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar
constructs. Executing behavior for *any and all* of the following cases and
patterns that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways
"test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant,
and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur.
Would be even better than `switch` for initial introductions to
pattern-matching.
>
> - No need to convert between the strangely slightly different syntax
of `switch` vs. `if case`, such as `case let x where x.contains(.italics):`
to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to
add one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

1 Like

A high bar for new syntax is fair and expected, and by posting I was
hoping to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns, and
execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression, are
ok.

But that's definitely not as clear or concise as a switch-like construct
with the single control expression at the top. Or perhaps some other
alternative such as the mentioned `continue` or somehow enumerating a set
of cases.

You're simply restating your proposed new syntax as the thing that's
missing. But what is the use case that motivates this construct? In what
way are multiple if-case statements "not as clear"?

···

On Sat, Nov 18, 2017 at 3:12 PM, Peter Kamb <peterkamb@gmail.com> wrote:

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

Robert is quite right--I'm not sure what we're designing for here.
There's a very high bar for introducing new syntax and a distaste for the
existing syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution < >> swift-evolution@swift.org> wrote:

There have been earlier suggestions for an alternative to `fallthrough`
that would continue matching cases; I think that is much more likely to get
support than a whole new construct with only a subtle difference from an
existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < >>> swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case`
pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant,
and understandable. Swift pattern-matching tutorials use `switch`
statements almost exclusively, with small sections at the end for
alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated
to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching
cases are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same
conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar
constructs. Executing behavior for *any and all* of the following cases and
patterns that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several
ways "test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern
matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant,
and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur.
Would be even better than `switch` for initial introductions to
pattern-matching.
>
> - No need to convert between the strangely slightly different syntax
of `switch` vs. `if case`, such as `case let x where x.contains(.italics):`
to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to
add one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

I think because it's not immediately obvious with multiple if statement,
that they all try to compare the same expression to different patterns.

match exp {
case 1
case 2
}
vs
if case 1 = exp
if case 2 = anotherexp

···

On Sat, Nov 18, 2017 at 10:43 PM, Xiaodi Wu via swift-evolution < swift-evolution@swift.org> wrote:

On Sat, Nov 18, 2017 at 3:12 PM, Peter Kamb <peterkamb@gmail.com> wrote:

A high bar for new syntax is fair and expected, and by posting I was
hoping to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns,
and execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression,
are ok.

But that's definitely not as clear or concise as a switch-like construct
with the single control expression at the top. Or perhaps some other
alternative such as the mentioned `continue` or somehow enumerating a set
of cases.

You're simply restating your proposed new syntax as the thing that's
missing. But what is the use case that motivates this construct? In what
way are multiple if-case statements "not as clear"?

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

Robert is quite right--I'm not sure what we're designing for here.
There's a very high bar for introducing new syntax and a distaste for the
existing syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution < >>> swift-evolution@swift.org> wrote:

There have been earlier suggestions for an alternative to `fallthrough`
that would continue matching cases; I think that is much more likely to get
support than a whole new construct with only a subtle difference from an
existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < >>>> swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if
case` pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant,
and understandable. Swift pattern-matching tutorials use `switch`
statements almost exclusively, with small sections at the end for
alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors
unrelated to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching
cases are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through
*multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same
conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar
constructs. Executing behavior for *any and all* of the following cases and
patterns that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several
ways "test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of
pattern matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant,
and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur.
Would be even better than `switch` for initial introductions to
pattern-matching.
>
> - No need to convert between the strangely slightly different syntax
of `switch` vs. `if case`, such as `case let x where x.contains(.italics):`
to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy
"just delete whitespace" conversion from standard multi-line switch/match
syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to
add one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

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

I think because it's not immediately obvious with multiple if statement,
that they all try to compare the same expression to different patterns.

match exp {
case 1
case 2
}
vs
if case 1 = exp
if case 2 = anotherexp

And this is a problem that requires a new syntax because...?

Consider that “exp” can be mutated in case 1; now reflect whether the
proposed syntax facilitates or hinders correct code.

···

On Sat, Nov 18, 2017 at 16:25 Benjamin G <benjamin.garrigues@gmail.com> wrote:

On Sat, Nov 18, 2017 at 10:43 PM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote:

On Sat, Nov 18, 2017 at 3:12 PM, Peter Kamb <peterkamb@gmail.com> wrote:

A high bar for new syntax is fair and expected, and by posting I was
hoping to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns,
and execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression,
are ok.

But that's definitely not as clear or concise as a switch-like construct
with the single control expression at the top. Or perhaps some other
alternative such as the mentioned `continue` or somehow enumerating a set
of cases.

You're simply restating your proposed new syntax as the thing that's
missing. But what is the use case that motivates this construct? In what
way are multiple if-case statements "not as clear"?

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:

Robert is quite right--I'm not sure what we're designing for here.
There's a very high bar for introducing new syntax and a distaste for the
existing syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution < >>>> swift-evolution@swift.org> wrote:

There have been earlier suggestions for an alternative to
`fallthrough` that would continue matching cases; I think that is much more
likely to get support than a whole new construct with only a subtle
difference from an existing one—would that be an acceptable alternative to
you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution < >>>>> swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if
case` pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant,
and understandable. Swift pattern-matching tutorials use `switch`
statements almost exclusively, with small sections at the end for
alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors
unrelated to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching
cases are not executed.
> - `default:` case is required, even for expressions where a default
case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic
match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that
does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch`
statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered
through *multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same
conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single
logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar
constructs. Executing behavior for *any and all* of the following cases and
patterns that match.
>
> The programmer would choose between `switch` or `match` based on the
goal of the pattern matching. For example, pattern matching a String.
`switch` would be appropriate for evaluating a String that represents the
rawValue of an enum. But `match` would be more appropriate for evaluating a
single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several
ways "test a value against multiple patterns, executing behavior for each
pattern that matches", but none are as elegant and understandable as the
switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of
pattern matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct,
elegant, and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur.
Would be even better than `switch` for initial introductions to
pattern-matching.
>
> - No need to convert between the strangely slightly different
syntax of `switch` vs. `if case`, such as `case let x where
x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics)
{`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts.
Currently: "An expression pattern represents the value of an expression.
Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than
`controlExpression` being repeated (and possibly changed) in every single
`if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as
typos or changes to the expression being evaluated in a *single* `if case`
from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an
easy "just delete whitespace" conversion from standard multi-line
switch/match syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for
non-exhaustible expressions. Pretty much any non-Enum type being evaluated
is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a
`switch` statement in a loop and constructing each case to match only on a
given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax
non-"switching" pattern-match operator in any other language. If you know
of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to
add one that functioned like switch's default case: run if *no other* cases
were executed. But, conceptually, should a "match any of these patterns"
statement have an else/default clause? I think it should, unless there are
any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

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

In regards to the problem in the root (using if-cases to do membership tests on option sets), the code given there involving thinking of the OptionSet as a sequence and iterating over it makes this far less likely to happen. I think this is a pattern that doesn't need syntax, it needs stdlib support at most. Perhaps an extension to OptionSet when the RawValue is known to be a sized integral type that does just that would clean up here.

~Robert Widmann

2017/11/18 17:25、Benjamin G via swift-evolution <swift-evolution@swift.org>のメール:

···

I think because it's not immediately obvious with multiple if statement, that they all try to compare the same expression to different patterns.

match exp {
case 1
case 2
}
vs
if case 1 = exp
if case 2 = anotherexp

On Sat, Nov 18, 2017 at 10:43 PM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org> wrote:

On Sat, Nov 18, 2017 at 3:12 PM, Peter Kamb <peterkamb@gmail.com> wrote:

A high bar for new syntax is fair and expected, and by posting I was hoping to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns, and execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression, are ok.

But that's definitely not as clear or concise as a switch-like construct with the single control expression at the top. Or perhaps some other alternative such as the mentioned `continue` or somehow enumerating a set of cases.

You're simply restating your proposed new syntax as the thing that's missing. But what is the use case that motivates this construct? In what way are multiple if-case statements "not as clear"?

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
Robert is quite right--I'm not sure what we're designing for here. There's a very high bar for introducing new syntax and a distaste for the existing syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution <swift-evolution@swift.org> wrote:
There have been earlier suggestions for an alternative to `fallthrough` that would continue matching cases; I think that is much more likely to get support than a whole new construct with only a subtle difference from an existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching cases are not executed.
> - `default:` case is required, even for expressions where a default case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.
>
> The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.
>
> - No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

_______________________________________________
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

I think because it's not immediately obvious with multiple if statement, that they all try to compare the same expression to different patterns.

match exp {
case 1
case 2
}
vs
if case 1 = exp
if case 2 = anotherexp

And this is a problem that requires a new syntax because...?

i don't think the reason is that different from creating "switch" : you can very well use "ifs" instead, it's just more convenient . It may provide exhaustive check in the case of optionset one day. But we can live without it ( much like we lived without switch before).

Actually i think there's a nice symetry betwen enum & switch on one side and optionset & match on the other hand. It's interesting but i don't know if it's useful in practice TBH.

···

Le 18 nov. 2017 à 23:47, Xiaodi Wu <xiaodi.wu@gmail.com> a écrit :

On Sat, Nov 18, 2017 at 16:25 Benjamin G <benjamin.garrigues@gmail.com> wrote:

Consider that “exp” can be mutated in case 1; now reflect whether the proposed syntax facilitates or hinders correct code.

On Sat, Nov 18, 2017 at 10:43 PM, Xiaodi Wu via swift-evolution <swift-evolution@swift.org> wrote:

On Sat, Nov 18, 2017 at 3:12 PM, Peter Kamb <peterkamb@gmail.com> wrote:

A high bar for new syntax is fair and expected, and by posting I was hoping to maybe find an alternative in the comments here.

But AFAIK there's currently no ability in Swift to:

"Evaluate a *single* control expression against all of these patterns, and execute any and all cases that match"

Multiple `if-case` statements, each re-stating the control expression, are ok.

But that's definitely not as clear or concise as a switch-like construct with the single control expression at the top. Or perhaps some other alternative such as the mentioned `continue` or somehow enumerating a set of cases.

You're simply restating your proposed new syntax as the thing that's missing. But what is the use case that motivates this construct? In what way are multiple if-case statements "not as clear"?

On Sat, Nov 18, 2017 at 11:18 AM, Xiaodi Wu <xiaodi.wu@gmail.com> wrote:
Robert is quite right--I'm not sure what we're designing for here. There's a very high bar for introducing new syntax and a distaste for the existing syntax is not a motivating use case.

On Sat, Nov 18, 2017 at 12:53 PM, Kevin Nattinger via swift-evolution <swift-evolution@swift.org> wrote:
There have been earlier suggestions for an alternative to `fallthrough` that would continue matching cases; I think that is much more likely to get support than a whole new construct with only a subtle difference from an existing one—would that be an acceptable alternative to you?

> On Nov 17, 2017, at 12:06 PM, Peter Kamb via swift-evolution <swift-evolution@swift.org> wrote:
>
> ## Title
>
> Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
>
> ## Summary:
>
> The syntax of the `switch` statement is familiar, succinct, elegant, and understandable. Swift pattern-matching tutorials use `switch` statements almost exclusively, with small sections at the end for alternatives such as `if case`.
>
> However, the `switch` statement has several unique behaviors unrelated to pattern matching. Namely:
>
> - Only the *first* matching case is executed. Subsequent matching cases are not executed.
> - `default:` case is required, even for expressions where a default case does not make sense.
>
> These behaviors prevent `switch` from being used as a generic match-patterns-against-a-single-expression statement.
>
> Swift should contain an equally-good pattern-matching statement that does not limit itself single-branch switching.
>
> ## Pitch:
>
> Add a `match` statement with the same elegant syntax as the `switch` statement, but without any of the "branch switching" baggage.
>
> ```
> match someValue {
> case patternOne:
> always executed if pattern matches
> case patternTwo:
> always executed if pattern matches
> }
> ```
>
> The match statement would allow a single value to be filtered through *multiple* cases of pattern-matching evaluation.
>
> ## Example:
>
> ```
> struct TextFlags: OptionSet {
> let rawValue: Int
> static let italics = TextFlags(rawValue: 1 << 1)
> static let bold = TextFlags(rawValue: 1 << 2)
> }
>
> let textFlags: TextFlags = [.italics, .bold]
>
>
>
> // SWITCH STATEMENT
> switch textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> default:
> print("forced to include a default case")
> }
> // prints "italics"
> // Does NOT print "bold", despite .bold being set.
>
>
>
> // MATCH STATEMENT
> match textFlags {
> case let x where x.contains(.italics):
> print("italics")
> case let x where x.contains(.bold):
> print("bold")
> }
> // prints "italics"
> // prints "bold"
> ```
>
> ## Enum vs. OptionSet
>
> The basic difference between `switch` and `match` is the same conceptual difference between `Emum` and an `OptionSet` bitmask.
>
> `switch` is essentially designed for enums: switching to a single logical branch based on the single distinct case represented by the enum.
>
> `match` would be designed for OptionSet bitmasks and similar constructs. Executing behavior for *any and all* of the following cases and patterns that match.
>
> The programmer would choose between `switch` or `match` based on the goal of the pattern matching. For example, pattern matching a String. `switch` would be appropriate for evaluating a String that represents the rawValue of an enum. But `match` would be more appropriate for evaluating a single input String against multiple unrelated-to-each-other regexes.
>
> ## Existing Alternatives
>
> `switch` cannot be used to match multiple cases. There are several ways "test a value against multiple patterns, executing behavior for each pattern that matches", but none are as elegant and understandable as the switch statement syntax.
>
> Example using a string of independent `if case` statements:
>
> ```
> if case let x = textFlags, x.contains(.italics) {
> print("italics")
> }
>
> if case let x = textFlags, x.contains(.bold) {
> print("bold")
> }
> ```
>
> ## `match` statement benefits:
>
> - Allow filtering a single object through *multiple* cases of pattern matching, executing *all* cases that match.
>
> - A syntax that exactly aligns with the familiar, succinct, elegant, and understandable `switch` syntax.
>
> - The keyword "match" highlights that pattern matching will occur. Would be even better than `switch` for initial introductions to pattern-matching.
>
> - No need to convert between the strangely slightly different syntax of `switch` vs. `if case`, such as `case let x where x.contains(.italics):` to `if case let x = textFlags, x.contains(.italics) {`
>
> - Bring the "Expression Pattern" to non-branch-switching contexts. Currently: "An expression pattern represents the value of an expression. Expression patterns appear only in switch statement case labels."
>
> - A single `match controlExpression` at the top rather than `controlExpression` being repeated (and possibly changed) in every single `if case` statement.
>
> - Duplicated `controlExpression` is an opportunity for bugs such as typos or changes to the expression being evaluated in a *single* `if case` from the set, rather than all cases.
>
> - Reduces to a pretty elegant single-case. This one-liner is an easy "just delete whitespace" conversion from standard multi-line switch/match syntax, whereas `if case` is not.
>
> ```
> match value { case pattern:
> print("matched")
> }
> ```
>
> - Eliminate the boilerplate `default: break` case line for non-exhaustible expressions. Pretty much any non-Enum type being evaluated is non-exhaustible. (This is not the *main* goal of this proposal.)
>
> ## Prototype
>
> A prototype `match` statement can be created in Swift by wrapping a `switch` statement in a loop and constructing each case to match only on a given iteration of the loop:
>
> ```
> match: for eachCase in 0...1 {
> switch (eachCase, textFlags) {
> case (0, let x) where x.contains(.italics):
> print("italics")
> case (1, let x) where x.contains(.bold):
> print("bold")
> default: break }
> }
>
> // prints "italics"
> // prints "bold"
> ```
>
> ## Notes / Discussion:
>
> - Other Languages - I've been unable to find a switch-syntax non-"switching" pattern-match operator in any other language. If you know of any, please post!
>
> - Should `match` allow a `default:` case? It would be easy enough to add one that functioned like switch's default case: run if *no other* cases were executed. But, conceptually, should a "match any of these patterns" statement have an else/default clause? I think it should, unless there are any strong opinions.
>
> - FizzBuzz using proposed Swift `match` statement:
>
> ```
> for i in 1...100 {
> var output = ""
> match 0 {
> case (i % 3): output += "Fizz"
> case (i % 3): output += "Buzz"
> default: output = String(i)
> }
>
> print(output)
> }
>
> // `15` prints "FizzBuzz"
> ```
> _______________________________________________
> 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

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

1 Like

linking the discourse thread for switch statement continue

1 Like