[swift-evolution] Xcode Fix-It action for returning closures

iCloud mkchoi212 at icloud.com
Thu Jul 13 21:45:05 CDT 2017

Hi Swift community,

I was wondering if we have considered adding support for providing Xcode automatic fix-it’s for closures being returned from a function.

For example, here is an incorrect version of a function that requires two `@escaping` keywords to be inserted.

func mapping <A, B, C> (f: (A) -> (B)) -> ((C, B) -> (C)) -> ((C, A) -> (C))

Despite the complexity, the compiler manages to catch two errors.

1. Closure use of non-escaping parameter `f` may allow it to escape
 a. Automatic Fix-it by doing `f: @escaping (A) - > (B)`

2. Closure use of non-escaping parameter `reducer` may allow it to escape
 b. No Fix-it provided <————

As you see above, the two places where a compile-time error occurred had the same exact problem; they both needed a `@escaping`. However, while the function parameter was offered an automatic fix-it, the nested closure being returned was not.

Here’s the correct version of the function.

func mapping <A, B, C> (f: @escaping (A) -> (B)) -> (@escaping ((C, B) -> (C))) -> ((C, A) -> (C)) {
    return { reducer in
        return { accum, input in
            reducer(accum, f(input))

I know this is a small feature for a very specific use case but I think it could help make Xcode a little smarter :D
Thanks for reading!

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