[swift-evolution] Revisiting SE-0110

Thu Jun 1 14:46:49 CDT 2017

I, for one, would be willing to accept Xiaodi's suggestion involving
`let`–especially if (pipe dream follows) we could use the same syntax in
functions/methods to destructure parameters.

On Thu, Jun 1, 2017 at 3:32 PM, Vladimir.S via swift-evolution <
swift-evolution at swift.org> wrote:

> On 01.06.2017 19:31, Tommaso Piazza wrote:
>
>> Dear all,
>>
>> I made a comparison of Swift's 4 lack of tuple unsplatting, here is how
>> it stands in comparison with other languages
>>
>> https://gist.github.com/blender/53f9568617654c38a219dd4a8353d935
>>
>>
> Thank you! Very useful information. And also I really like the opinion of
> @AliSoftware in comments for this article.
>
> I'd suggest to add this variant to Swift section in your article:
>
> let eighteenOrMore = ["Tom" : 33, "Rebecca" : 17, "Siri" : 5].filter {
>         (arg: (name: String, age: Int)) in arg.age >= 18 }
>
> (I believe it is better that 2 others Swift variants.)
>
> It seems for me that we need to allow some special syntax for *explicit*
> tuple destructuring in closures to make all happy.
>
> FWIW These suggestions are my favorite:
>
> 1. Just allow type inference for tuple's destructured variables in this
> position:
>
> .filter { (arg: (name, age)) in arg.age >= 18 }
>
>
> 2. (1) + allow underscore for tuple argument name:
>
> .filter { (_: (name, age)) in age >= 18 }
>
>
> 3. (2) + allow to omit parenthesis (probably only in case of just one
> tuple argument)
>
> .filter { _: (name, age) in age >= 18 }
>
>
> 4. Use pattern matching syntax:
>
> .filter { case let (name, age) in age >= 18 }
>
> (looks similar as allowed today: if case let (name, age) = x { print(name,
> age) }  )
>
>
> 5. Use two pairs of parenthesis :
>
> .filter { ((name, age)) in age >= 18 }
>
> Btw, about the 5th variant. If took what is allowed today:
> .filter { (arg: (name: String, age: Int)) in arg.age >= 18 }
> , and allow type inference for tuple part arguments, we'll have this:
> .filter { (arg: (name, age)) in arg.age >= 18 }
> , and if additionally allow skipping of tuple argument declaration we'll
> have:
> .filter { ((name, age)) in arg.age >= 18 }
> I.e. two pairs for parenthesis for tuple destructuring, and such syntax is
> similar to the type this closure should have : ((String, Int)) -> Bool
>
>
>
>>
>>
>> On Thursday, June 1, 2017 12:25 PM, Vladimir.S via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>> On 01.06.2017 0:42, John McCall wrote:
>>  >> On May 31, 2017, at 2:02 PM, Stephen Celis <stephen.celis at gmail.com
>> <mailto:stephen.celis at gmail.com>> wrote:
>>  >>> On May 28, 2017, at 7:04 PM, John McCall via swift-evolution
>>  >>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>
>> wrote:
>>  >>>
>>  >>> Yes, I agree.  We need to add back tuple destructuring in closure
>> parameter
>>  >>> lists because this is a serious usability regression.  If we're
>> reluctant to
>>  >>> just "do the right thing" to handle the ambiguity of (a,b), we
>> should at least
>>  >>> allow it via unambiguous syntax like ((a,b)).  I do think that we
>> should just
>>  >>> "do the right thing", however, with my biggest concern being whether
>> there's
>>  >>> any reasonable way to achieve that in 4.0.
>>  >>
>>  >> Closure parameter lists are unfortunately only half of the equation
>> here. This
>>  >> change also regresses the usability of point-free expression.
>>  >
>>  > The consequences for point-free style were expected and cannot really
>> be
>>  > eliminated without substantially weakening SE-0110.  Closure
>> convenience seems to
>>  > me to be a much more serious regression.
>>
>> John, do you also want to say "and without weakening SE-0066"? Because,
>> if I
>> understand correctly, in this case:
>>
>>    func add(_ x: Int, _ y: Int) -> Int {
>>      return x + y
>>    }
>>
>>    zip([1, 2, 3], [4, 5, 6]).map(add)
>>
>> .. we have a clear function type mismatch situation, when map() expects
>> function of
>> type ((Int, Int))->Int, but function of type (Int,Int)->Int is provided ?
>> So probably
>> the additional 'reason' of the 'problem' in this case is SE-0066, no?
>> Or I don't understand the SE-0066 correctly..
>> Do we want to allow implicit conversions between function type
>> ((Int,Int))->Int and
>> (Int,Int)->Int?
>>
>> Quote from SE-0066:
>> ---
>> (Int, Int) -> Int    // function from Int and Int to Int
>> ((Int, Int)) -> Int  // function from tuple (Int, Int) to Int
>> ---
>>
>> During this discussion I see a wish of some group of developers to just
>> return back
>> tuple splatting for function/closure arguments, so they can freely send
>> tuple to
>> function/closure accepting a list of parameters(and probably vise-versa).
>> Is it worth to follow SE-0066 and SE-0110 as is, i.e. disallow tuple
>> deconstructing
>> and then, as additive change improve the situation with tuple
>> splatting/deconstructing later with separate big proposal?
>>
>> Btw, about the SE-0110 proposal. It was discussed, formally reviewed and
>> accepted. I
>> expect that its revision also should be formally
>> proposed/reviewed/accepted to
>> collect a wide range of opinions and thoughts, and attract the attention
>> of
>> developers in this list to the subject.
>>
>>
>> Also, if we revisit SE-0110, will this code be allowed?:
>>
>> func foo(_ callback: ((Int,Int))->Void) {}
>> let mycallback = {(x:Int, y:Int)->Void in }
>> foo(mycallback)
>>
>> and
>>
>> func foo(_ callback: (Int,Int)->Void) {}
>> let mycallback = {(x: (Int, Int))->Void in }
>> foo(mycallback)
>>
>> If so, what will be result of this for both cases? :
>>
>> print(type(of:mycallback)) // (Int,Int)->Void or ((Int,Int))->Void
>>
>> If allowed, do we want to allow implicit conversion between types
>> (Int,Int)->Void and
>> ((Int,Int))->Void in both directions?  (Hello tuple splatting?)
>>
>>
>>  >
>>  > John.
>>  >
>>  >
>>  >>
>>  >> func add(_ x: Int, _ y: Int) -> Int { return x + y }
>>  >>
>>  >> zip([1, 2, 3], [4, 5, 6]).map(add)
>>  >>
>>  >> // error: nested tuple parameter '(Int, Int)' of function
>> '(((_.Element,
>>  >> _.Element)) throws -> _) throws -> [_]' does not support destructuring
>>  >>
>>  >> This may not be a common pattern in most projects, but we heavily use
>> this style
>>  >> in the Kickstarter app in our functional and FRP code. Definitely not
>> the most
>>  >> common coding pattern, but a very expressive one that we rely on.
>>  >>
>>  >> Our interim solution is a bunch of overloaded helpers, e.g.:
>>  >>
>>  >> func tupleUp<A, B, C>(_ f: (A, B) -> C) -> ((A, B)) -> C { return }
>>  >>
>>  >> zip([1, 2, 3], [4, 5, 6]).map(tupleUp(add))
>>  >>
>>  >> Stephen
>>  >
>>  > .
>>  >
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>>
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