Sorry, your answer was perfectly fine. I’m the one who did a poor job acknowledging it :)
I totally see how semantic analysis of such lifetimes may represent a complex challenge. In fact, if I’m not mistaking, the “non-escapability” of `f` in `let f = c ? nonEscParam : escParam` isn’t even statically decidable. One simple (but maybe too naïve?) solution might be to consider `f` as non-escaping as soon as there’s one path in the CFG where it’s assigned to an non-escaping closure. That way we could still disallow things like: let f = c ? nonEscParam : escParam self.callback = f // <- assignment to possibly non-escaping closure But even so, the added complexity to the escape analysis is undeniable. Once again, thanks for your answer. Best, Dimitri On 31 May 2017, at 22:20, John McCall <rjmcc...@apple.com<mailto:rjmcc...@apple.com>> wrote: On May 31, 2017, at 1:17 PM, Dimitri Racordon via swift-dev <swift-dev@swift.org<mailto:swift-dev@swift.org>> wrote: Thanks for your answer. I agree that it may not be the most useful feature (although I’m sure we could find not-so-contrived yet useful use-cases). Anyway, I guess that discussion would rather belong to the evolution list :) I was more wondering if there were situations where such local assignments would have to be disallowed. I was trying to answer that question; perhaps I did a poor job of explaining, and some examples would help. This should always be fine, and in principle we could allow it specifically: let function = someParameter This would probably also be okay: let function = { someClosure } This would be a problem: var function = someParameter As would this: let function = (someCondition ? someParameter : someOtherParameter) John. Best, Dimitri On 31 May 2017, at 22:10, John McCall <rjmcc...@apple.com<mailto:rjmcc...@apple.com>> wrote: On May 31, 2017, at 12:21 PM, Dimitri Racordon via swift-dev <swift-dev@swift.org<mailto:swift-dev@swift.org>> wrote: Hi everyone, I failed to find the reason why Swift does not allows a non-escaping parameter to be assigned to a local variable. Here is a minimal example: func f(_ closure: () -> Int) { let a = closure } I do understand that assigning a non-escaping closure to a variable whose lifetime exceeds that of the function would (by definition) violate the non-escaping property. For instance, doing that is understandably illegal: var global = { 0 } func f(_ closure: () -> Int) { global = closure } But in my first example, since `a` is stack allocated, there’s no risk that `closure` escapes the scope of `f`. Is there some use case I’m missing, where such assignment could be problematic? Or is this a limitation of the compiler, which wouldn't go all the way to check whether the lifetime of the assignee is compatible with that of the non-escaping parameter may exceed that of the variable it is assigned to? Thank you very much for your time and your answer. Examples like yours, where a non-escaping closure parameter has a new constant name bound to it, are supportable but rather pointless — as a programmer, why have two names for the same value? Examples that would be more useful, like assigning the closure into a local variable or allowing it to be used in a more complex expression (like ? :), complicate the analysis for non-escaping closures in a way that would significantly subvert their purpose. John. _______________________________________________ swift-dev mailing list swift-dev@swift.org<mailto:swift-dev@swift.org> https://lists.swift.org/mailman/listinfo/swift-dev
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