[swift-evolution] Argument labels in callbacks

[Franklin Schrans]

I don’t see why it should be available in function arguments.

The separation of the type and its label makes sense semantically, but the syntax doesn’t look as pretty anymore. 

Writing something like 
  foo(completion(success:error:): (Bool, Error) -> Void) {}
seems a bit convoluted to me. We first have to write the labels, then remember their order to write the type.

Minor: this also adds additional complexity when Xcode tries to generate a stub for the closure, as it needs to find the labels in the function name.

I was thinking of an implementation where closure types could have labels, but these could only be used within the body of that closure and would be erased elsewhere. This however still makes it difficult for the caller to figure out what each argument in the closure is representing when using foo.

- Franklin

> On Feb 22, 2017, at 9:05 AM, Iain Smith via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Would this proposed syntax, using argument labels, also be available in function arguments?
> 
> e.g 
> func items(withCompletion completion(success:error:): ([Item]?, NSError?) -> Void) {
>   ...
>   completion(success: items, error:nil)
>  }
> 
> On 22 Feb 2017, at 08:49, Charlie Monroe via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> This was pointed out during the discussions surrounding this proposal and it was agreed that the type simplification was important.
>> 
>> There were several suggestions how to bring this back using different features - e.g. compound names that would contain the labels. For example:
>> 
>> let callback(success:error:): (Bool, Error?) -> Void = ...
>> callback(success: true, error: nil)
>> 
>> This way the type itself wouldn't contain the label information, but the name of the variable would.
>> 
>>> On Feb 22, 2017, at 9:41 AM, Goffredo Marocchi via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> I am quite interested in this as well, thanks for bringing it up! It was quite disappointing to fall back to multi argument method calls without labels as it was going against the emphasis on the value of labels in the language as well as decreasing readability of what is supposed to be self documenting code.
>>> 
>>> Sent from my iPhone
>>> 
>>> On 22 Feb 2017, at 08:36, Franklin Schrans via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> Hi,
>>>> 
>>>> When SE-0111 <https://github.com/apple/swift-evolution/blob/master/proposals/0111-remove-arg-label-type-significance.md> was approved, I noticed the implication it had when using closures as callbacks:
>>>> 
>>>> Writing
>>>>    func foo(completion: (success: Bool) -> Void) {
>>>>      completion(success: true)
>>>>    }
>>>> 
>>>> is no longer possible, because function types can’t have argument labels anymore, and the function has to be written:
>>>>    func foo(completion: (Bool) -> Void) {
>>>>      completion(true)
>>>>    }
>>>> 
>>>> which doesn’t look very nice, especially as the number of the arguments increases.
>>>> 
>>>> After talking to Chris Lattner about this, he referred me to this <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160711/024331.html> email.
>>>> I was wondering if there's been any further work or plans in restoring the use of argument labels in closures.
>>>> 
>>>> - Franklin
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