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

[Xiaodi Wu]

Hmm, not as sure about that one. To my mind it's a clear expression of
intent there. You're saying you know what the labels are and you're
choosing not to repeat them. I think it should be on you if you express
that intent and you're just plain wrong.


On Mon, May 9, 2016 at 2:32 AM, Jacob Bandes-Storch <jtbandes at gmail.com>
wrote:

> ... 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
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>
>
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