[swift-evolution] [Review] SE-0018 Flexible Memberwise Initialization

[Matthew Johnson]

Sent from my iPad

> On Jan 6, 2016, at 7:47 PM, Joe Groff <jgroff at apple.com> wrote:
> 
> 
>>> On Jan 6, 2016, at 5:23 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>>> On Jan 6, 2016, at 6:04 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> 
>>>> On Jan 6, 2016, at 2:47 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Hello Swift community,
>>>> 
>>>> The review of "Flexible Memberwise Initialization" begins now and runs through January 10th. The proposal is available here:
>>>> 
>>>> 	https://github.com/apple/swift-evolution/blob/master/proposals/0018-flexible-memberwise-initialization.md	
>>>> 
>>>> Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at
>>>> 
>>>> 	https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
>>>> or, if you would like to keep your feedback private, directly to the review manager.
>>>> 
>>>> What goes into a review?
>>>> 
>>>> The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:
>>>> 
>>>> 	* What is your evaluation of the proposal?
>>> 
>>> It’s okay.
>>> 
>>>> 	* Is the problem being addressed significant enough to warrant a change to Swift?
>>> 
>>> I’m lukewarm about that.  I have never found writing out the initializers I want to be a significant burden, and I find my code is better when they’re explicit.  Every new feature increases the language's complexity and surface area, and I fear this one is not going to pay its way.
>>> 
>>>> 	* Does this proposal fit well with the feel and direction of Swift?
>>> 
>>> Yes, but I worry that it may be too early to add it.  Other features in this space, like truly generic variadics, may well obsolete anything we do today.  I’m not sure we should be designing convenience features that are likely to overlap with more general features coming down the road unless the inconvenience is very painful… which I personally don’t find it to be.
>> 
>> It isn’t clear to me how generic variadics might obsolete the functionality of this proposal.  Can you elaborate on that?
> 
> Not sure if this is exactly what Dave has in mind, but an idea that comes to mind: we could say that structs and classes have a magic "members" tuple and typealias:
> 
> struct Foo {
>   var x, y: Int
> 
>   // Implicit members
>   typealias Members = (x: Int, y: Int)
>   var members: Members {
>     get { return (x: x, y: y) }
>     set { (x, y) = newValue }
>   }
> }
> 
> With plausible future features for forwarding tuples as arguments, then the memberwise initializer could be implemented like this:
> 
> 	    // Say that a parameter declared 'x...' receives a tuple of arguments labeled according to its type,
> 	    // like '**x' in Python
> init(members...: Members) {
>   self.members = members
> }
> 
> And I think all your other use cases could be covered as well.

Thanks for posting an example.  This makes sense and is definitely an interesting approach.  

Would you be able to use the members property in phase 1 of initialization to initialize those members?  Is part of the magic that the compiler understands enough about that property and tuple to allow for that?

How would use cases involving a subset of members be handled?  This is the primary use case I have had in mind while developing this proposal.

> 
> -Joe
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