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:
[SR-2] Build configuration directives can not wrap switch cases · Issue #42628 · apple/swift · GitHub
[SR-4196] Allow #if (conditional compilation) to guard switch cases · Issue #46779 · apple/swift · GitHub
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
<NNNN-conditional-switch-case.md · GitHub;
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/>
<NNNN-conditional-switch-case.md · GitHub;
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.
<NNNN-conditional-switch-case.md · GitHub
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
}
}
<NNNN-conditional-switch-case.md · GitHub
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
}