[swift-evolution] [Pitch] Improving capturing semantics of local functions

[Robert Widmann]

~Robert Widmann 

2017/11/14 22:02、Matthew Johnson via swift-evolution <swift-evolution at swift.org>のメール:

> 
> 
> Sent from my iPhone
> 
>> On Nov 14, 2017, at 6:56 PM, Howard Lovatt via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Having read all the arguments for what to add to local functions it still strikes me as a poor use of engineering resources to fix them (though I do agree they have problems). A better use of resources would be:
>> 
>>   1. Deprecate local functions.
>>   2. Allow closures when assigned to a function type to be:
>>       2a. Recursive.
>>       2b. Annotatable with:
>>             2bi.  @inline
>>             2bii. @escaping
>>       2c. Generic.
>> 
>> That would be a similar engineering effort and give a better short term result of better closures which would be much more widely applicable as well as addressing the issues with local functions.
> 
> I believe generic closures would require adding higher rank types to Swift.  That would be pretty cool but I suspect the engineering effort is at least an order of magnitude greater than the changes discussed in this thread.

100% correct. Slava raises good points about implementation, I’ll raise one about semantics:

Without sufficient restrictions on this kind of polymorphism, type checking will become significantly more difficult for little corresponding benefit.  Enabling the formation of polymorphic types just because you’re near an arrow means all sorts of fun things now get to happen

let f = { x in { y in y } }

This program is illegal without a type signature, but still typeable in that Swift will try to look for bindings to the tn’s in this signature

f : <T0, T1> (T0) -> (T1) -> T1

This is good - this is the most general unifier for this expression promised to us by the “Hindley-Milner-like” label we have in the type checker docs.

Given even rank-2 types, the most general unifier is (ideally) now 

f : <T0> (T0) -> (<T1> (T1) -> T1)

Which is significant because despite an obvious isomorphism, the former is not a specialization of the latter.  It is a completely separate polytype that would require additional structural rules to recover the correct behavior.  This can also block inference or generate counterintuitive unifiers if we’re forced to e.g. unify nested polytypes against each other.

~Robert Widmann

> 
>> 
>> It also gives a better long term result of not having to maintain local functions.
>>   
>> 
>>   -- Howard.
>> 
>>> On 15 November 2017 at 09:08, Alex Lynch via swift-evolution <swift-evolution at swift.org> wrote:
>>> The inference algebra just suggested was enjoyable to read, but is still a new syntax. Which is interesting and deserving of its own proposal. The purpose of this proposal is simply to introduce the existing capture syntax to local functions. Thanks to everyone's feedback pointing out that the `self` reference analysis is a deeper question than initially realized. 
>>> 
>>> Alex
>>> 
>>>>> On Tue, Nov 14, 2017 at 4:36 PM, Mike Kluev via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> On 14 November 2017 at 21:02, David Hart <david at hartbit.com> wrote:
>>>> 
>>>>> 
>>>>> 
>>>>> I’d be very hesitant to introduce this syntax:
>>>>> 
>>>>> it’s new syntax, so it comes with a complexity tax (it isn’t naturally obvious what it means for a func to be weak)
>>>>> it’s only sugar for the capture of self
>>>> 
>>>> it might cover well over 90% of use cases (by my "pessimistic" estimate)... if someone has a quick way to scan and analyse, say, github swift sources we may even know that current percentage number of real life usage.
>>>>> it doesn’t transpose well to local closures
>>>> 
>>>> the last one - maybe not. follow me:
>>>> 
>>>> let closure = { [weak self, bar] in ... }
>>>> 
>>>> which today can be written as: 
>>>> 
>>>> let closure = { [weak self, bar] () -> Int in ... } // full form
>>>> 
>>>> or as:
>>>> 
>>>> let closure: () -> Int = { [weak self, bar] in ... } // full form
>>>> 
>>>> which allows this change:
>>>> 
>>>> let closure:  [weak self, bar] () -> Int = { ... } // full alt form
>>>> 
>>>> or in alternative form:
>>>> 
>>>> let closure:  weak () -> Int = { [bar] in ... } // short hand form
>>>> 
>>>> same can be with functions:
>>>> 
>>>> func fn() -> Int { [weak self, bar] in ... } // full form
>>>> 
>>>> weak func fn() -> Int { [bar] in ... } // short hand form
>>>> 
>>>> the two capture attributes will in practice be "close" to each other:
>>>> 
>>>> weak func fn() {
>>>>     [bar] in
>>>>     ....
>>>> }
>>>> 
>>>> and in majority of cases there will be only weak self:
>>>> 
>>>> weak func fn() {
>>>>     ....
>>>> }
>>>> 
>>>> Mike
>>>> 
>>>> 
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