Xcode requires to put _ before return value name in completion handler

Question

I have a completion handler function with a string in return value.

func hardProcessingWithString(input: String, completion: (result: String) -> Void) {}

However, Xcode requires me to put _ before my return value name so I have to put _ before result

func hardProcessingWithString(input: String, completion: (_ result: String) -> Void) {}

and because of that, when I call my function, my return value doesn't have name but instead it shows this

hardProcessingWithString(input: String, completion: (String) -> Void)

Is there a way that instead of showing

completion: (String) -> Void)

I want it to show

completion: result -> Void)

so that I know exactly what return value means instead of the completion type. Thanks!

This Swift feature has been discussed before in some post, remember it start from Swift 3.2, I find it really bad also, but there's no way to show param names for now. — Tj3n, Jul 26 '18 at 02:49

score 1 · Accepted Answer · answered Jul 26 '18 at 02:46

1

Since some version of Swift, closure types can't have parameter names. However, there is a workaround:

typealias Result = String

And now you can use Result as the closure type:

func f(completion: (Result) -> Void) { ... }

Many methods in Foundation also does this. For example TimeInterval is just an alias for Double, but naming it TimeInterval makes the purpose clearer.

answered Jul 26 '18 at 02:46

Sweeper

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Can't say I'm a fan of this, unless the context makes the data type absolutely clear. – Alexander Jul 26 '18 at 03:04
I tried your suggestion but the return value also includes view Controller name where I declare typealias. Do you have that issue as well? I look like this "ViewControllerName.Result" – KevinVuD Jul 26 '18 at 03:11
You need to declare the typealias in the global scope, not inside any VC. @KevinVuD – Sweeper Jul 26 '18 at 03:24
Yeah. Thank you – KevinVuD Jul 26 '18 at 08:37

score 1 · Answer 2 · answered Jul 26 '18 at 03:14

You shouldn't add labels for closure definition. Just define it like this.

func hardProcessingWithString(input: String, completion: (String) -> Void) {}

When this function called, you should define the label.

hardProcessingWithString(input: "") { (result) in

}

score 0 · Answer 3 · answered Jul 26 '18 at 03:12

This is by design (though, to be sure, not everyone likes it). See

https://bugs.swift.org/browse/SR-2894

As Jordan Rose explains:

This is correct behavior. The argument labels are considered part of a function's name, not its type. A function value / closure has only a base name and no argument labels, so they cannot be specified.

Xcode requires to put _ before return value name in completion handler

3 Answers3