[swift-evolution] [Accepted] SE-0111: Remove type system significance of function argument labels

[Mark Lacey]

> On Jul 10, 2016, at 10:53 PM, Austin Zheng via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Jul 10, 2016, at 10:30 PM, David Owens II <david at owensd.io <mailto:david at owensd.io>> wrote:
>>>> 
>>>> I wish the core team or the author of the proposal came to this thread and engaged again with the community. 
>>> 
>>> I'm not inclined to spend time engaging with people who couldn't be bothered to give feedback during the week-long official review period.
>> 
>> Not all people "couldn’t be bothered” but had life events, such as moving across states with four kids, that prevented them from being able to engage during the official review period. 
> 
> I hope your move went smoothly. More generally, there will always be people with good reasons for not being able to participate in the review process, but the procedure is set: one week of formal discussion, followed by a decision by the core team. If a proposal should be re-reviewed or amended, someone should submit (or at least draft) a follow-up proposal; none of the other proposals that have been accepted have been taken up for re-review by the core team based merely on reviews that were submitted after the review period ended (and there have been at least a few whose acceptance was very controversial).
> 
>> 
>> I’ve read through all of the posts that I see in my mailbox regarding this topic and I’ve yet to see any real answer to the concerns of tooling, typealias usage, closures, and code readability and maintainability concerns under this new proposal. This is the closest I’ve seen (from Douglas Gregor a few days ago):
>> 
>>> The core team’s intent is that one can add cosmetic labels to function types, but that those labels are not (cannot be) used at the call site, e.g.,
>> 
>> Do you have specific post in mind that addresses the these concerns? Maybe I’m just missing them, but I really don’t see those addressed and they are not mentioned in the proposal at all.
>> 
>> Let’s say I want to model a problem regarding some library functions that work with resizing some image type. Today, if I did that, the tooling would give me auto-completion for all of the parameter labels and the code is very legible. 
>> 
>> Note that both `original` and `resized` get auto-completed for us here. This provides great code clarity and insights. This is also self-documenting code.
>> 
>> However, under this proposal as accepted (as I understand it), we are left with this:
>> 
>> func doResizeC(image: Image, completed: (Image, Image) -> Void) {
>>     let newData = image.data
>>     let newSize = image.size
> 
> You can still have labels in the type: `completed: (original: Image, resized: Image)`.
> 
>> 
>>     // do some work that's really slow...
>>     
>>     completed(image, Image(data: newData, size: newSize))
> 
> This is definitely a problem. I am considering writing a follow-up proposal that would allow for compound naming of values of function type, which would alleviate this problem: `let foo(x:y:) : (Int, Int) -> Void`, which was brought up a couple of times during the review thread. (This was going to be part of the original proposal, but was removed for various reasons.)

In the meantime one option here would be to define a nested function that invokes the callback. It results in boilerplate, which is unfortunate, but would work:

func doResize
(
  image: Image,
  completed completedParam: (original: Image, resized: Image) -> Void
) {
  func completed(original: Image, resized: Image) {
    completedParam(original, resized)
  }

  // do lots of work

  completed(original: image, resized: Image(data: newData, size: newSize))
}

Mark


> 
>> }
>> 
>> -David
> 
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