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

[Anders Ha]

I don’t think overshadowing types defined in another module is an issue at all, in which case `Int` and `Void` belong to the Swift module.

Regards,
Anders

> On 5 May 2017, at 11:34 PM, Vladimir.S via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On 05.05.2017 6:12, Jose Cheyo Jimenez via swift-evolution wrote:
>> I can't think of a single time when I would need a tuple shuffle but I don't really work with tuples much. It would be great to hear other people's usage of this feature.
>> I did not know that types could be overloaded with values. That was very surprising to me. Honestly I think that is the issue here. Not sure what can be done about it.
> 
>> let Int = 5 // works
>> let Void = "what?" // works.
> 
> Very interesting. Could some one clarify, why this is allowed? Isn't this is a bug we want to fix? At least by requiring a tilda like `Int`
> And of course, you can't "overload" user defined type:
> struct S {}
> let S = 10 // Error: invalid redeclaration of 'S'
> 
>> On May 4, 2017, at 7:14 PM, Robert Widmann via swift-evolution <swift-evolution at swift.org <mailto: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=  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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