[swift-evolution] Guaranteed closure execution
matthew at anandabits.com
Mon Feb 1 16:15:28 CST 2016
> On Feb 1, 2016, at 3:59 PM, Chris Lattner <clattner at apple.com> wrote:
>> On Feb 1, 2016, at 1:43 PM, Matthew Johnson <matthew at anandabits.com <mailto:matthew at anandabits.com>> wrote:
>>> On Feb 1, 2016, at 3:25 PM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>>> On Feb 1, 2016, at 1:02 PM, Matthew Johnson <matthew at anandabits.com <mailto: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.
>> I agree if the compiler isn’t going to enforce it. The value is in the guarantee.
>> Why wouldn’t the compiler enforce it in this case? Is it hard to implement? It doesn’t seem like it should be any harder than guaranteeing exactly once.
> You’re right, the caller could enforce this, so it does provide marginal value beyond a comment. I think the rest of my point stands though.
Sure, I think it’s a somewhat subjective judgement call.
I prefer to get as many guarantees as possible from the compiler. In cases where the semantics are likely to be documented in a comment, assumed or implicit I would prefer to see them expressed directly in the language.
I understand the case that it is simpler to leave it in a comment, I just have a different preference.
There have been other topics where a similar tradeoff / judgement call is relevant, such as the `local` access control proposal.
My sense so far is that Swift tries to strike a middle path, introducing annotations where the perceived benefit passes a test of being significant enough (such as the @noescape annotation and likely the once modifier). It seems like in *most* cases a mere semantic guarantee by the compiler isn’t considered to be benefit enough to pass that test - optimization potential and / or a new capability (such as the ability to initialize values).
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