[swift-evolution] [Idea] [Pitch] Add `match` statement as `switch`-like syntax alternative to `if case` pattern matching
Kevin Nattinger
swift at nattinger.net
Sat Nov 18 13:36:48 CST 2017
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 <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160704/023955.html>
> On Nov 18, 2017, at 11:30 AM, Peter Kamb <peterkamb at 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 at nattinger.net <mailto:swift at 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 at swift.org <mailto: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"
> > ```
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org <mailto:swift-evolution at swift.org>
> > https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20171118/600de9be/attachment.html>
More information about the swift-evolution
mailing list