[swift-evolution] [Proposal][Discussion] Deprecate Tuple Shuffles

[David Hart]

> On 5 May 2017, at 07:17, Robert Widmann via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On the contrary, this is precisely what it means to deprecate tuple shuffles.  You can’t map common parlance onto this term; the proposal and the Twitter thread weren’t merely about reordering arguments.
> 
>> but it is entirely another ballgame to remove labels from tuple patterns altogether.
> 
> It’s really not.  Let me demonstrate:
> 
>>> To be clear, are you proposing the prohibition of *adding or removing* labels as well? A previous discussion on tuple shuffling on this list saw consensus that assigning a value of type (label1: T, label2: U) to a variable of type (T, U) and vice versa should absolutely be supported, whether or not reordering is permitted.
> 
> I am not proposing any changes to switching parameter labels through well-typed re-assignments.  This is absolutely still going to be allowed:
> 
> var z : (Int, Int) = (0, 0)
> var w : (x : Int, y : Int) = (5, 10)
> z = w
> w = z
> 
> This is modeled internally with a tuple shuffle, but not the kind of shuffle I’m interested in banning.  It’s a far simpler kind of 
> 
>>> And how about *restating* existing labels without any adding or removing? To be clear:
>>> 
>>> ```
>>> let (partialValue: v, overflow: o) = 42.addingReportingOverflow(42)
>>> ```
>>> 
>>> ...involves absolutely no changes in labels whatsoever. The return type is (partialValue: Int, overflow: ArithmeticOverflow).
> 
> That, however, is a kind of shuffle I intend to deprecate here.  This kind of pattern is subject to the “arcane syntax” part of the proposal.
> 
>>> 
>>> Either one of these scenarios is commonly used, and it is astonishing to me that they would be eliminated.
> 
> Do you have proof of that claim? I have never seen the relevant kinds of tuple shuffle used before, and I doubt you have either before today.

For what it's worth, I thought I knew Swift inside out and I had never seen or used the syntax your are proposing to ban, so I'm all for it.

> ~Robert Widmann
> 
>> On May 5, 2017, at 12:53 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>> 
>> Ah, I see from your proposed grammar update: you're proposing to prohibit the use of labels entirely in a tuple pattern.
>> 
>> This is much more than just prohibiting tuple shuffling, and I'm rather disappointed that you described such a dramatic change using a corner case. There are very good reasons why someone finds 'let (y: x, x: y) = (x: 1, y: 2)' confusing and would support its removal, but it is entirely another ballgame to remove labels from tuple patterns altogether.
>> 
>> 
>>> On Thu, May 4, 2017 at 23:47 Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> Now I'm confused. The ordinary meaning of the word "shuffle" is not changing but rather reordering, and all of your examples are of reordering.
>>> 
>>> To be clear, are you proposing the prohibition of *adding or removing* labels as well? A previous discussion on tuple shuffling on this list saw consensus that assigning a value of type (label1: T, label2: U) to a variable of type (T, U) and vice versa should absolutely be supported, whether or not reordering is permitted.
>>> 
>>> And how about *restating* existing labels without any adding or removing? To be clear:
>>> 
>>> ```
>>> let (partialValue: v, overflow: o) = 42.addingReportingOverflow(42)
>>> ```
>>> 
>>> ...involves absolutely no changes in labels whatsoever. The return type is (partialValue: Int, overflow: ArithmeticOverflow).
>>> 
>>> Either one of these scenarios is commonly used, and it is astonishing to me that they would be eliminated.
>>> 
>>>> On Thu, May 4, 2017 at 23:28 Robert Widmann <devteam.codafi at gmail.com> wrote:
>>>> That doesn't involve a parameter reordering, but because it changes argument labels it's a shuffle.
>>>> 
>>>> ~Robert Widmann
>>>> 
>>>> 2017/05/05 0:16、Xiaodi Wu <xiaodi.wu at gmail.com> のメッセージ:
>>>> 
>>>>> Robert,
>>>>> 
>>>>> As I mentioned on Twitter, getting rid of tuple shuffles would not cure your example, which does not involve a shuffle. Unless you're proposing to disallow the use of labels during destructuring entirely, which I would think to be very much unacceptable. Example:
>>>>> 
>>>>> ```
>>>>> let (partialValue: v, overflow: o) = 42.addingReportingOverflow(42)
>>>>> ```
>>>>> 
>>>>> This involves no shuffling and should absolutely remain allowed.
>>>>> 
>>>>> 
>>>>>> On Thu, May 4, 2017 at 21:15 Robert Widmann via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> Hi all,
>>>>>> 
>>>>>> So sorry that this proposal comes so late in the game, but I feel it’s too important not to bring it to the attention of the community now.  Attached is a proposal to deprecate a language feature many of you will probably have never had the chance to use: Tuple Shuffles.  I’ve attached a copy of the first draft of the proposal below, but the latest copy can be read on Github.
>>>>>> 
>>>>>> Thanks!
>>>>>> 
>>>>>> ~Robert Widmann
>>>>>> 
>>>>>> Deprecate Tuple Shuffles
>>>>>> Proposal: SE-NNNN
>>>>>> Authors: Robert Widmann
>>>>>> Review Manager: TBD
>>>>>> Status: Awaiting review
>>>>>> Introduction
>>>>>> 
>>>>>> This proposal seeks the deprecation of a little-known feature of Swift called a "Tuple Shuffle".
>>>>>> 
>>>>>> Motivation
>>>>>> 
>>>>>> A tuple-shuffle is an undocumented feature of Swift in which one can re-order the indices of a tuple by writing a pattern that describes a permutation in a syntax reminiscent of adding type-annotations to a parameter list:
>>>>>> 
>>>>>> let a = (x: 1, y: 2)
>>>>>> var b: (y: Int, x: Int)
>>>>>> b = a
>>>>>> It can be used to simultaneously destructure and reorder a tuple:
>>>>>> 
>>>>>> let tuple = (first: 0, second: (x: 1, y: 2))
>>>>>> let (second: (x: b, y: c), first: a) = tuple
>>>>>> It can also be used to map parameter labels out of order in a call expression:
>>>>>> 
>>>>>> func foo(_ : (x : Int, y : Int)) {}
>>>>>> foo((y: 5, x: 10)) // Valid
>>>>>> Note that a tuple shuffle is distinct from a re-assignment through a tuple pattern. For example, this series of statements will continue to function as before:
>>>>>> 
>>>>>> var x = 5
>>>>>> var y = 10
>>>>>> var z = 15
>>>>>> (z, y, x) = (x, z, y)
>>>>>> Their inclusion in the language complicates every part of the compiler stack, uses a syntax that can be confused for type annotations, contradicts the goals of earlier SE's (see SE-0060), and makes non-sensical patterns possible in surprising places.
>>>>>> 
>>>>>> Take switch-statements, for example:
>>>>>> 
>>>>>> switch ((0, 0), 0){ 
>>>>>> case (_ : let (y, z), _ : let s): () // We are forbidden from giving these patterns names other than "_" 
>>>>>> default: () 
>>>>>> }
>>>>>> This proposal seeks to deprecate them in Swift 3 compatibility mode and enforce that deprecation as a hard error in Swift 4 to facilitate their eventual removal from the language.
>>>>>> 
>>>>>> Proposed solution
>>>>>> 
>>>>>> Construction of Tuple Shuffle Expressions will become a warning in Swift 3 compatibility mode and will be a hard-error in Swift 4.
>>>>>> 
>>>>>> Detailed design
>>>>>> 
>>>>>> In addition to the necessary diagnostics, the grammar will be ammended to simplify the following productions:
>>>>>> 
>>>>>> tuple-pattern → (tuple-pattern-element-list <opt>)
>>>>>> tuple-pattern-element-list → tuple-pattern-element | tuple-pattern-element , tuple-pattern-element-list
>>>>>> - tuple-pattern-element → pattern | identifier:pattern
>>>>>> + tuple-pattern-element → pattern
>>>>>> Impact on Existing Code
>>>>>> 
>>>>>> Because very little code is intentionally using Tuple Shuffles, impact on existing code will be negligible but not non-zero.
>>>>>> 
>>>>>> Alternatives considered
>>>>>> 
>>>>>> Continue to keep the architecture in place to facilitate this feature.
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
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