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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