[swift-evolution] [Pitch] Require tuple conversions to be explicit when labels don't match

[Jacob Bandes-Storch]

... and one might also want to require labels when passing values *to* a
labeled tuple:

    func foo() -> (left: Int, right: Int) {
        return (3, 4)  // error: conversion between tuple types '(Int,
Int)' and '(left: Int, right: Int)' requires explicit 'as' operator
    }

I've personally been bitten by a typo of this sort (mistakenly swapping the
values) before.
Jacob

On Mon, May 9, 2016 at 12:23 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> A sensible solution, IMO. Error with Fix-It when attempting to convert
> implicitly between tuples with mismatched labels.
>
> On Mon, May 9, 2016 at 01:47 Jacob Bandes-Storch via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> There was some previous discussion under "[Discussion] Enforce argument
>> labels on tuples
>> <http://thread.gmane.org/gmane.comp.lang.swift.evolution/14910>".
>>
>> Halfway through the thread, Haravikk clearly stated the key point:
>>
>> On Thu, Apr 21, 2016 at 12:14 AM, Haravikk via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> I think the important thing to remember is that the label check is
>>> intended to prevent cases like this:
>>> let a:(left:Int, right:Int) = (1, 2)
>>> var b:(right:Int, left:Int) = a
>>> While the two tuples are compatible by type, the meaning of the values
>>> may differ due to the different labels; in this case the values are
>>> represented in a different order that a developer should have to explicitly
>>> reverse to ensure they aren’t making a mistake, or they could represent
>>> radically different concepts altogether.
>>
>>
>>
>> I agree there's a potential for confusion here, and I suggest we should
>> add an error (or warning) imploring the user to make the conversion
>> explicit, when the source tuple is labeled:
>>
>>     func foo() -> (left: Int, right: Int) { return (3, 4) }
>>
>>     let (left: a, right: b) = foo()  // ok, labels match
>>
>>     var x = 0, y = 0
>>     (left: x, right: y) = (1, 3)  // ok, source is unlabeled
>>
>>     let (c, d) = foo()
>>     // error: conversion between tuple types '(left: Int, right: Int)'
>> and '(Int, Int)' requires explicit 'as' operator
>>     // suggested fix: "let (c, d) = foo() as (Int, Int)"
>>
>>     let (right: e, left: f) = foo()
>>     // error: conversion between tuple types '(left: Int, right: Int)'
>> and '(right: Int, left: Int)' requires explicit 'as' operator
>>     // suggested fix: "let (right: e, left: f) = foo() as (right: Int,
>> left: Int)"
>>
>>
>> Thoughts?
>>
>> Jacob
>> _______________________________________________
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>> swift-evolution at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
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