[swift-evolution] [Proposal] Allow passing a variadic parameter to another variadic function

[Christopher Boyd]

Currently, Swift doesn't have a way to easily pass a variadic parameter to
another variadic function because the variadic parameter itself has already
been collected into an array.

There was a prior posting a few weeks back about allowing arrays to be
passed to a variadic parameter which would resolve this very specific issue:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160411/015185.html

Unfortunately, that thread devolved into an argument on whether or not
variadic parameters should be removed entirely.

I, certainly, see merit in having functions that can accept a variable
number of arguments and variadic functions should seem intuitive to anyone
else that might be reading the code.

Swift's documentation presents a very intuitive example:

   1. arithmeticMean(1, 2, 3, 4, 5)

It's obvious what this function is doing, and any intelligent person would
expect 3 as the result.  Even WolframAlpha knows how to interpret this:
http://www.wolframalpha.com/input/?i=arithmeticMean(1,+2,+3,+4,+5)

Moreover, a function can only have ONE variadic parameter, which I think
reduces any possible confusion.

Regardless, if variadic parameters are going to stay in Swift, then I think
they should at least support chaining to another function.

The most obvious way of allowing this would be to allow variadic parameters
to accept an array.

However, I think that if an array (of the parameter type) is specified for
a variadic parameter, then no other values should be allowed.  This
eliminates any unnecessary complexity of potentially having to add values
to the array.

- Christopher Boyd
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