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

[Robert Widmann]

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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