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

[Kevin Nattinger]

I support this proposal 100%. In fact, I just ran into this use case. 

I’d support extending it to allow partial-case coverage (as long as it doesn’t cross a case: boundary, obviously), e.g.

switch result {
case .success(let object):
    doSomething(with: object)
case .error(let error):
    #if debug
    alertUser(error)
    #endif
    handleError(error)
}

I don’t see a reason to prevent that, but if we can just get the current proposal (full cases only) in Swift 4 I’ll be satisfied and just put this on my (large and growing) list of things to bring up for 5.

> On May 10, 2017, at 6:28 AM, rintaro ishizaki via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Here's proposal link:
> https://github.com/rintaro/swift-evolution/blob/conditional-switch-case/proposals/NNNN-conditional-switch-case.md <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 <mailto: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 <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-2>
> https://bugs.swift.org/browse/SR-4196 <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>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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