[swift-dev] Reason for call only non-escaping parameter

[John McCall]

> On May 31, 2017, at 1:17 PM, Dimitri Racordon via swift-dev <swift-dev at 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 <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> 
>>> On May 31, 2017, at 12:21 PM, Dimitri Racordon via swift-dev <swift-dev at swift.org <mailto:swift-dev at 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.
> 
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