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

[Félix Cloutier]

Why?

> Le 4 mai 2017 à 19:14, Robert Widmann via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> 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 <https://github.com/apple/swift-evolution/pull/705/files>.
> 
> Thanks!
> 
> ~Robert Widmann
> 
> Deprecate Tuple Shuffles
> 
> Proposal: SE-NNNN <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-filename.md>
> Authors: Robert Widmann <https://github.com/codafi>
> Review Manager: TBD
> Status: Awaiting review
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#introduction>Introduction
> 
> This proposal seeks the deprecation of a little-known feature of Swift called a "Tuple Shuffle".
> 
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#motivation>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 <https://twitter.com/CodaFi_/status/860246169854894081>, contradicts the goals of earlier SE's (see SE-0060 <https://github.com/apple/swift-evolution/blob/9cf2685293108ea3efcbebb7ee6a8618b83d4a90/proposals/0060-defaulted-parameter-order.md>), 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.
> 
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#proposed-solution>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.
> 
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#detailed-design>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
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#impact-on-existing-code>Impact on Existing Code
> 
> Because very little code is intentionally using Tuple Shuffles, impact on existing code will be negligible but not non-zero.
> 
>  <https://github.com/CodaFi/swift-evolution/blob/8eaf320b3c2a117909fc0269c398e89c033a4b9f/proposals/NNNN-deprecate-tuple-shuffles.md#alternatives-considered>Alternatives considered
> 
> Continue to keep the architecture in place to facilitate this feature.
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