This issue about `numbers in` was raised during review of SE-0066; if I
recall, the core team considered and rejected disallowing that syntax in
closures. Since we're minimizing source-breaking changes, the issue is
settled in my view, having been proposed, commented upon, reviewed, and
rejected.
On Mon, Dec 19, 2016 at 15:39 Anton Zhilin via swift-evolution <
swift-evolution@swift.org> wrote:
> 2016-12-17 2:55 GMT+03:00 Vip Malixi via swift-evolution <
> swift-evolution@swift.org>:
>
> var oneParameterAndMultipleReturn: ([Int]) -> (even:[Int], odd:[Int]) = {
> numbers -> ([Int], [Int]) in
> var evenNumberArray = [Int]()
> var oddNumberArray = [Int]()
>
> for number in numbers {
> number % 2 == 0 ? evenNumberArray.append(number) :
> oddNumberArray.append(number)
> }
>
> return (evenNumberArray, oddNumberArray)
> }
>
> The above code is valid, because:
>
> 1. Multi-return is just a type of tuple with labeled components.
> 2. Labeled tuples and normal tuples are implicitly convertible.
>
> The only way we could make such code invalid is by removing labeled tuples
> altogether. -1 on that.
> This problem mostly refers to code style, and the above variant can be
> more self-documenting in some cases.
>
> Also, again for consistency and clarity, parameters in Closures should
> always be surrounded by parentheses, even single parameters:
>
> var oneParameterAndMultipleReturn: ([Int]) -> ([Int], [Int]) = { (numbers)
> -> (even:[Int], odd:[Int]) in
> var evenNumberArray = [Int]()
> var oddNumberArray = [Int]()
>
> for number in numbers {
> number % 2 == 0 ? evenNumberArray.append(number) :
> oddNumberArray.append(number)
> }
>
> return (evenNumberArray, oddNumberArray)
> }
>
> This should be considered a bug. numbers in or (numbers) in or (numbers)
> -> (...) in, but not numbers -> (...) in, because the latter forms should
> be correct types.
> Whether or not just numbers in should be allowed, is another talk.
>
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