[swift-evolution] [Idea] Passing an Array to Variadic Functions

[Keith Smiley]

We've been dealing with this as well. We've chosen to go with your option 1 for
most of our cases, sometimes dropping varargs all together and just using the
array signature.

It would be great if you could have a safe apply function for this.

--
Keith Smiley

On 04/17, Justin Jia via swift-evolution wrote:
> Hi!
>
> Currently, we can’t call a variadic function with an array of arguments.
>
> Reference:
> 1. http://stackoverflow.com/questions/24024376/passing-an-array-to-a-function-with-variable-number-of-args-in-swift <http://stackoverflow.com/questions/24024376/passing-an-array-to-a-function-with-variable-number-of-args-in-swift>
> 2. https://www.drivenbycode.com/the-missing-apply-function-in-swift/ <https://www.drivenbycode.com/the-missing-apply-function-in-swift/>
>
> Consider the following use case:
>
> ```
> func average(numbers: Double…) -> Double {
>    return sum(numbers) / numbers.count // Error: Cannot convert value of type ‘[Double]’ to expected argument type ‘Double'
> }
>
> func sum(numbers: Double...) -> Double { … }
> ```
>
> Right now, there are two ways to fix it:
>
> 1. Add another function that accept `[Double]` as input.
>
> ```
> func average(numbers: Double…) -> Double {
>    return sum(numbers) / numbers.count
> }
>
> func sum(numbers: Double...) -> Double {
>    return sum(numbers)
> }
>
> func sum(numbers: [Double]) -> Double { … }
> ```
>
> 2. Implement an `apply()` function using `unsafeBitCast`.
>
> ```
> func average(numbers: Double…) -> Double {
>    return sum(apply(numbers)) / numbers.count
> }
>
> func sum(numbers: [Double]) -> Double { … }
>
> func apply<T, U>(fn: (T...) -> U, args: [T]) -> U {
>    typealias FunctionType = [T] -> U
>    return unsafeBitCast(fn, FunctionType.self)(args)
> }
> ```
>
> However, both solutions are not very elegant. The first solution requires the library author to implement both functions, and the second solution breaks the guarantees of Swift’s type system.
>
> Swift should allow passing an array to variadic functions, or we should somehow implement a type-safe `apply()` function in the standard library.
>
> Justin

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> swift-evolution at swift.org
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