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

Peter Kamb peterkamb at gmail.com
Sat Nov 18 15:12:03 CST 2017 

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 at 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 at 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 at 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"
>> > ```
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>> > swift-evolution at swift.org
>> > https://lists.swift.org/mailman/listinfo/swift-evolution
>>
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