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

[Xiaodi Wu]

Also--I didn't read this carefully enough in your initial example--I
disagree that either (left: Int, right: Int) to (Int, Int) or vice versa
should require explicit casting, and I think previous conversations on the
topic showed that at least some on the list felt the same way.

Mismatched labels, yes, because IMO it's alarming without a more explicit
indication of intent to assume that a user intends to swap labels going
from (left: Int, right: Int) to (right: Int, left: Int). But to work around
that restriction, I should be able to erase labels and affix new ones
without using "as", since no unintentional label swapping can occur in
either direction.


On Mon, May 9, 2016 at 2:57 AM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> 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
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160509/d20e96a1/attachment.html>