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

[Adrian Zubarev]

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
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Am 5. Mai 2017 um 04:15:12, Robert Widmann via swift-evolution (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.

Thanks!

~Robert Widmann

Deprecate Tuple Shuffles
Proposal: SE-NNNN
Authors: Robert Widmann
Review Manager: TBD
Status: Awaiting review
Introduction

This proposal seeks the deprecation of a little-known feature of Swift called a "Tuple Shuffle".

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, contradicts the goals of earlier SE's (see SE-0060), 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.

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.

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
Impact on Existing Code

Because very little code is intentionally using Tuple Shuffles, impact on existing code will be negligible but not non-zero.

Alternatives considered

Continue to keep the architecture in place to facilitate this feature.
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