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

[Xiaodi Wu]

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 <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 = 5var y = 10var 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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> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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