[swift-evolution] Proposal: Allow #if to guard switch case clauses

[rintaro ishizaki]

Here's proposal link:
https://github.com/rintaro/swift-evolution/blob/conditional-switch-case/proposals/NNNN-conditional-switch-case.md

Fixed some typos, including my name :)

2017-05-10 17:32 GMT+09:00 rintaro ishizaki <fs.output at gmail.com>:

> Hi evolution community,
>
> This proposal allows you to enclose switch cases with #if directive.
> Implementation: https://github.com/apple/swift/pull/9457
> This is one of the oldest SR issue:
> https://bugs.swift.org/browse/SR-2
> https://bugs.swift.org/browse/SR-4196
>
> Thanks!
> Rintaro
>
>
> Allow #if to guard switch case clauses
>
>    - Proposal: SE-NNNN <https://gist.github.com/rintaro/NNNN-filename.md>
>    - Authors: Rintaro Ishziaki <https://github.com/rintaro>
>    - Review Manager: TBD
>    - Status: Awaiting review
>
>
> <https://gist.github.com/rintaro/a5a9d9836027d7df7a5326a3a8cf9d89#introduction>
> Introduction
>
> This proposal adds ability to guard switch case clauses with #if
>  directives.
>
> Swift-evolution thread: Not yet
> <https://lists.swift.org/pipermail/swift-evolution/>
>
> <https://gist.github.com/rintaro/a5a9d9836027d7df7a5326a3a8cf9d89#motivation>
> Motivation
>
> When you want to switch cases only for certain compilation condition, say
> switching #if os(Linux) guarded enum cases, right now you have to write
> switch twice:
>
> enum Operation {
>   case output(String)
> #if os(Linux)
>   case syscall(Syscall)
> #endif
> }
> func execute(operation: Operation) {
> #if !os(Linux)
>    switch operation {
>    case .output(let str):
>        print(str)
>    }
> #else
>    switch operation {
>    case .output(let str):
>        print(str)
>    case .syscall(let call):
>        call.execute()
>    }
> #endif
> }
>
> This is annoying and error prone.
>
> <https://gist.github.com/rintaro/a5a9d9836027d7df7a5326a3a8cf9d89#proposed-solution>Proposed
> solution
>
> This proposal allows #if to guard switch case clauses.
>
> func execute(operation: Operation) {
>     switch operation {
>     case .output(let str):
>         print(str)
> #if os(Linux)
>     case .syscall(let call):
>         call.execute()
> #endif
>     }
> }
>
>
> <https://gist.github.com/rintaro/a5a9d9836027d7df7a5326a3a8cf9d89#detailed-design>Detailed
> design
>
> This change shouldn't affect existing #if directives *within* case clauses.
> This code should works as expected:
>
> func foo(x: MyEnum) {
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if PRINT_SOME
>         print(str)
> #endif
>     case .other:
>         doOther()
>     }
> }
>
> Only if the next token after #if is case or default, the Parser treat it
> as guarding case clauses.
>
> func foo(x: MyEnum) {
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if HAS_OTHER
>     case .other:
>         doOther()
>     }
> #endif
> }
>
> func foo(x: MyEnum) {
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if HAS_OTHER
>     default:
>         break
> #endif
> }
>
> Error cases:
>
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if HAS_OTHER
>     case .other:
>         doOther()
> #else
>         doMore() // error: all statements inside a switch must be covered by a 'case' or 'default'#endif
>     }
>
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if HAS_OTHER
>         doMore()
>     case .other:
>         doOther() // error: 'case' label can only appear inside a 'switch' statement#else
>     }
>
> You can guard multiple cases as long as it is guarding whole clauses:
>
>     switch x {
>     case .some(let str):
>         doSomething(str)
> #if HAS_OTHERS
>     case .other:
>         doOther()
>     case .more:
>         doMore()
> #else
>     }
>
>
>
> <https://gist.github.com/rintaro/a5a9d9836027d7df7a5326a3a8cf9d89#source-compatibility>
>
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