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

Thu May 4 23:53:08 CDT 2017

Ah, I see from your proposed grammar update: you're proposing to prohibit
the use of labels entirely in a tuple pattern.

This is much more than just prohibiting tuple shuffling, and I'm rather
disappointed that you described such a dramatic change using a corner case.
There are very good reasons why someone finds 'let (y: x, x: y) = (x: 1, y:
2)' confusing and would support its removal, but it is entirely another
ballgame to remove labels from tuple patterns altogether.

On Thu, May 4, 2017 at 23:47 Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> Now I'm confused. The ordinary meaning of the word "shuffle" is not
> changing but rather reordering, and all of your examples are of reordering.
>
> To be clear, are you proposing the prohibition of *adding or removing*
> labels as well? A previous discussion on tuple shuffling on this list saw
> consensus that assigning a value of type (label1: T, label2: U) to a
> variable of type (T, U) and vice versa should absolutely be supported,
> whether or not reordering is permitted.
>
> And how about *restating* existing labels without any adding or removing?
> To be clear:
>
> ```
> let (partialValue: v, overflow: o) = 42.addingReportingOverflow(42)
> ```
>
> ...involves absolutely no changes in labels whatsoever. The return type is
> (partialValue: Int, overflow: ArithmeticOverflow).
>
> Either one of these scenarios is commonly used, and it is astonishing to
> me that they would be eliminated.
>
> On Thu, May 4, 2017 at 23:28 Robert Widmann <devteam.codafi at gmail.com>
> wrote:
>
>> That doesn't involve a parameter *reordering*, but because it changes
>> argument labels it's a shuffle.
>>
>> ~Robert Widmann
>>
>> 2017/05/05 0:16、Xiaodi Wu <xiaodi.wu at gmail.com> のメッセージ:
>>
>> Robert,
>>
>> As I mentioned on Twitter, getting rid of tuple shuffles would not cure
>> your example, which does not involve a shuffle. Unless you're proposing to
>> disallow the use of labels during destructuring entirely, which I would
>> think to be very much unacceptable. Example:
>>
>> ```
>> let (partialValue: v, overflow: o) = 42.addingReportingOverflow(42)
>> ```
>>
>> This involves no shuffling and should absolutely remain allowed.
>>
>>
>> On Thu, May 4, 2017 at 21:15 Robert Widmann via swift-evolution <
>> 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 = 5var y = 10var 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.
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
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