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

[Robert Widmann]

None of those are affected.  See the note in the proposal.

~Robert Widmann

> On May 5, 2017, at 1:16 AM, Adrian Zubarev <adrian.zubarev at devandartist.com> wrote:
> 
> Hi Robert,
> 
> I’m trying to understand the impact of your proposal and the feature you want to remove from the language. Could you describe a little more on how if ever this affects tuple destructuring? 
> 
> var (a, b, c) = (0, "", 0)
> 
> (a, b) = (42, "Hello Swift")
> 
> (a = 42, b = "Hello Swift")
> 
> var (x, y) = (0, 42)
> 
> (x, y) = (y, x) // swap without a temporary value
> 
> 
> 
> -- 
> Adrian Zubarev
> Sent with Airmail
> 
> Am 5. Mai 2017 um 04:15:12, Robert Widmann via swift-evolution (swift-evolution at swift.org <mailto:swift-evolution at swift.org>) schrieb:
> 
>> 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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> 

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