[swift-users] withoutActuallyEscaping example question

[Slava Pestov]

Here’s another fun one: the following type checks despite the literal closure; we’ve constrained the return type of filter() to [Int], and not LazyFilterCollection<Int>, so again the type checker picks the overload that takes the non-escaping closure:

func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
  return array.lazy.filter { predicate($0) }
}

Slava

> On Mar 26, 2017, at 1:14 AM, Slava Pestov via swift-users <swift-users at swift.org> wrote:
> 
> Hi Ray,
> 
> There are two overloads of filter() available on ‘array.lazy’; the version that takes an escaping closure and returns a LazyFilterCollection, and the version that takes a non-escaping closure and returns [Int].
> 
> In the first example, we pick the LazyFilterCollection-returning overload, because the literal closure { predicate($0) } can be coerced to both an escaping or a non-escaping closure type, and in the absence of additional constraints we go with the overload from a concrete type over an overload in a protocol extension. After the overload has been picked we validate the body of the closure, and notice that it is invalid because whole the closure is already known to be @escaping, it references the non- at escaping ‘predicate’.
> 
> In the second example, ‘predicate’ is known to be non- at escaping, which rules out the first overload completely, so we go with the second overload and perform a non-lazy filter.
> 
> I would argue this is somewhat confusing, but it might be difficult to change the overload resolution rules in a way where the first overload is always chosen.
> 
> Slava
> 
>> On Mar 26, 2017, at 12:13 AM, Ray Fix via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> 
>> 
>> Hi,
>> 
>> One of the motivating examples for withoutActuallyEscaping looks like the following.  This predictably  doesn’t work because "use of non-escaping parameter 'predicate' may allow it to escape"
>> 
>> func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
>>   return Array(array.lazy.filter { predicate($0) })
>> }
>> 
>> The solution is to use withoutActuallyEscaping.  This works and produces the expected results.
>> 
>> func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
>>   return withoutActuallyEscaping(predicate) { predicate in
>>     Array(array.lazy.filter({predicate($0)}))
>>   }
>> }
>> 
>> What I find puzzling is the below example compiles and runs correctly. It seems like it should be the same compiler error as in the first example.
>> 
>> func myFilter(array: [Int], predicate: (Int) -> Bool) -> [Int] {
>>   return Array(array.lazy.filter(predicate))
>> }
>> 
>> If you understand why this is so, it would be very helpful.
>> 
>> Thank you and best wishes,
>> Ray
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> 
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