[swift-evolution] [Core team] Addressing the SE-0110 usability regression in Swift 4

[Víctor Pimentel Rodríguez]

On Tue, 20 Jun 2017 at 01:41, Douglas Gregor via swift-evolution <
swift-evolution at swift.org> wrote:

> Hello Swift community,
>
> Swift 3’s SE-0110
> <https://github.com/apple/swift-evolution/blob/master/proposals/0110-distingish-single-tuple-arg.md> eliminated
> the equivalence between function types that accept a single type and
> function types that take multiple arguments. However, for various implementation
> reasons
> <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170612/037560.html>,
> the implementation of SE-0110 (as well as the elimination of tuple “splat”
> behavior in SE-0029
> <https://github.com/apple/swift-evolution/blob/master/proposals/0029-remove-implicit-tuple-splat.md>)
> was not fully completed.
>
> Swift 4 implemented more of SE-0110, which caused a fairly serious
> usability regression, particularly with closures. Here are a few simple
> examples involving closures that worked in Swift 3 but do not work in Swift
> 4:
>
> // #1: Works in Swift 3, error in Swift 4
> myDictionary.forEach {
>   print("\($0) -> \($1)")
> }
>
> // #2: Works in Swift 3, error in Swift 4
> myDictionary.forEach { key, value in
>   print("\(key) -> \(value)")
> }
>
> // #3: Works in Swift 3, error in Swift 4
> myDictionary.forEach { (key, value) in
>   print("\(key) -> \(value)")
> }
>
>
> Similar issues occur with passing multi-argument functions where a tuple
> argument is expected:
>
> // #4: Works in Swift 3, error in Swift 4
> _ = zip(array1, array2).map(+)
>
>
> In all of these cases, it is possible to write a closure that achieves the
> desired effect, but the result is more verbose and less intuitive:
>
> // Works in both Swift 3 and Swift 4
> myDictionary.forEach { element in
>   let (key, value) = element
>   print("\(key) -> \(value)")
> }
>
>
> The Swift core team feels that these usability regressions are
> unacceptable for Swift 4. There are a number of promising solutions that
> would provide a better model for closures and address the usability
> regression, but fully designing and implementing those are out of scope for
> Swift 4.  Therefore, we will “back out” the SE-0110 change regarding
> function arguments from Swift 4.
>
> Specifically, when passing an argument value of function type (including
> closures) to a parameter of function type, a multi-parameter argument
> function can be passed to a parameter whose function type accepts a single
> tuple (whose tuple elements match the parameter types of the argument
> function). Practically speaking, all of the examples #1-#4 will be accepted
> in both Swift 3 and Swift 4.
>
> We will revisit the design in this area post-Swift 4.
>
> - Doug
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
Thank you :)

--
Víctor Pimentel
-- 

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*Víctor Pimentel Rodríguez · *Principal iOS Engineer
vpimentel at tuenti.com

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