[swift-evolution] Revisiting SE-0110

Thu Jun 1 14:32:42 CDT 2017

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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