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

Adrian Zubarev adrian.zubarev at devandartist.com
Mon May 8 01:30:07 CDT 2017

Exactly, everything else is simply an excuse and nothing more. Tuple destructuring is not even aligned to enum case patterns at all, because it only uses the shorthand version and does not allow us to write the full explicit version.

enum Foo { case a(b: Int, c: Int) }

switch Foo.a(b: 42, c: 0) {
    case let .a(b: num1, c: num2): …
    // Or the explicit longer version:
    case .a(b: let num1, c: let: num2): …
Tuple destructuring do not support the latter, which in our previous example would really be unambiguous.

let tuple: (x: Int, y: Int) = (3, 1)

// Short version:
let (x: Int, y: Double): (x: Int, y: Int) = tuple

// Not supported explicit version:
(x: let Int, y: let Double) = tuple
Why is that the case? Because tuple destructuring creates another tuple on the left side of the same tuple type as the tuple on the right side of the assignment operator.

Erica Sudan has a proposal, which could change that for guards and ifs (second design): https://github.com/erica/swift-evolution/blob/783fdf8d3723d51f350b917af23c207cebdd1ad7/proposals/9999-ifcaseguardcase.md

Adrian Zubarev
Sent with Airmail

Am 8. Mai 2017 um 07:16:50, David Hart via swift-evolution (swift-evolution at swift.org) schrieb:

On 7 May 2017, at 00:21, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:

To which human would it be misleading?

To the writer? No, because the compiler will warn you right away. By the time you're done with writing the first line, it'll warn you that Int and Double are unused variables. And if you try to use x and y, you get an error.

To the reader? Only if the writer knowingly wrote this misleading code. In other words, it's a nice puzzle, but no reader will encounter this in real-world code, unless they're being tormented by the writer on purpose.

IMHO, the fact that the compiler warns you does no change the fact that it's a very confusing part of the language. It should not be an excuse for fixing it. Consistency teaches us to expect a type after a colon.

On Sat, May 6, 2017 at 16:28 Brent Royal-Gordon <brent at architechies.com> wrote:
> On May 5, 2017, at 11:06 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> The identifier after a colon is *never* a type in any pattern matching, and there's no need of which I'm aware to support type annotations in pattern matching. We put colons after labels, and the current syntax is perfectly consistent here. What is the defect you're trying to cure?

The defect underlying this proposal: `let (x: Int, y: Double)` looks like it's declaring `x` and `y` of types `Int` and `Double`, but it's actually declaring `Int` and `Double` and binding them to `x` and `y`. Your code's meaning is perfectly unambiguous to the compiler, of course, but it's misleading to the human.

Brent Royal-Gordon

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