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"
> ```
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