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

[Jacob Bandes-Storch]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20160508/2cb3cfa0/attachment.html>