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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