[swift-evolution] [pitch] Variadic Arguments should accept Arrays

[Dimitri Racordon]

I was also in favour of using an ellipsis operator to “unsplat” (to borrow the term in some other languages) an array into variadic parameters. However, after taking a look at Haravikk’s proposal [1] I think I would oppose using ellipsis for that purpose.

The main advantage I see is that this proposal enables future extensions to other kind of iterable collections (as shown in the “Proposed Extension” section). It'd break with “legacy” variadic features and would give Swift a more powerful feature to design APIs.

[1] https://github.com/Haravikk/swift-evolution/blob/a13dc03d6a8c76b25a30710d70cbadc1eb31b3cd/proposals/nnnn-variadics-as-attribute.md


On 27 Feb 2017, at 04:00, Derrick Ho via swift-evolution <swift-evolution at swift.org<mailto:swift-evolution at swift.org>> wrote:

Well, I have found these discussions...

It was marked as a bug<https://bugs.swift.org/browse/SR-128> and then directed to swift evolution<https://github.com/ArtSabintsev/swift-evolution/blob/5ef676ec2616465c1cb854f82e3fd4e90b4b09c6/proposals/array-variadic-function.md>.

@jose, the prefix operator you mention looks good, but in the past discussions they wanted a postfix operator so that it would match the declaration.  I think I like the postfix operator.

func foo(_ a: String...) {}

foo(["a", "b", "c"]...) //<- This looks clear.  It is turning the array into a variadic argument.

However, that thread was closed because it wasn't "discussed" enough prior making the pull request.

therefore, if people want it, they got to cast their vote in here by either supporting it or opposing it.

I like my original suggestion since it is more explicit.

foo(["a", "b", "c"] as String...)



On Sun, Feb 26, 2017 at 7:59 PM Jose Cheyo Jimenez <cheyo at masters3d.com<mailto:cheyo at masters3d.com>> wrote:
On Feb 26, 2017, at 8:26 AM, Derrick Ho via swift-evolution <swift-evolution at swift.org<mailto:swift-evolution at swift.org>> wrote:

In swift, a variadic argument can become an array without too much effort.

func foo(_ va: String...) {
   let a: [String] = va
}

However, it seems odd to me that an array can not be converted into a variadic argument

foo(["a", "b", "c"]) // <-error
foo("a", "b", "c") // no error

Other people have wondered about this too.<http://stackoverflow.com/questions/24024376/passing-an-array-to-a-function-with-variable-number-of-args-in-swift>

According to this thread<https://devforums.apple.com/message/970958#970958> Doug Gregor says it is due to some type ambiguity. with Generics.

If type ambiguity is the issue, Do we have the option to cast it to the correct type?

foo(["a", "b", "c"] as String...) // <- error.  doesn't consider String... to be a type.

I think this needs to be done with a spread operator in order to disambiguate.

foo(...["a", "b", "c”]

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

I like the idea. Its syntactic sugar so I am not sure how open the core team would be to adding it.


What does the community think? Should we be granted some mechanism to turn an array into a variadic argument?
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