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

Fri May 5 02:03:11 CDT 2017

That is weird indeed, there is need of more argument labels, like argument labels back in stored closures and callbacks, not even less argument labels all around :/.

-1 as a corner case of the language throws the baby out with the bathwater.

Sent from my iPhone

> On 5 May 2017, at 05:53, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> 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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