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

[Robert Widmann]

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 at 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!

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