[swift-evolution] Guaranteed closure execution

[Chris Lattner]

On Feb 1, 2016, at 6:31 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>> func foo() {
>>> 	let bar: Int
>>> 	withNoEscape { bar = 1 }
>>> }
>>> 
>>> func withNoEscape(@autoclosure(once) closure: () -> ()) { /* snip */ }
>> 
>> 
>> Looking back, I do think that there should be a way to exit from `withNoEscape` without calling the closure, so yes, throwing should imply that the closure wasn't executed. If it's possible that `foo` swallowed an error from a throwing `withNoEscape`, the compiler should assume that the variables within haven't been initialized:
> 
> I’m glad to see an @autoclosure func in this thread.  We will want to be able to use this feature with @autoclosure in addition to @noescape.
> 
> As far as exiting without calling the closure, I suggest `@noescape(once?)`.  The `?` indicates the closure may or may not be called, but will not be called more than once.

I don’t see how this is useful.  You wouldn’t be able to initialize a value with this semantic, so it isn’t any more powerful than @noescape on the caller side.

-Chris
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