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

[Robert Widmann]

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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