[swift-evolution] Guaranteed closure execution

Chris Lattner clattner at apple.com
Mon Feb 1 15:25:51 CST 2016

On Feb 1, 2016, at 1:02 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>>> 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.
> Right, you would not be able to use it for initialization.
> It gives a guarantee that the closure will not be executed twice.  This semantic guarantee could be useful, especially if the closure has side-effects.  It adds the clarity of a guarantee  to APIs where the zero-or-one-times semantic is implicit with @noescape alone.

Right, but unless the compiler is going to enforce it somehow, it doesn’t add any value above a comment.  Particularly given that we want to keep the language simple where possible, I think that a comment would be perfectly fine for this.  We don’t want to be in the business of providing a "documentation hook” in the language for every theoretical invariant someone might want to express.


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