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

Slava Pestov spestov at apple.com
Wed Apr 20 15:27:54 CDT 2016

> On Apr 19, 2016, at 10:54 AM, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
> Possibly optimisations unavailable to Array passing?

Indeed, it should be possible to implement varargs by allocating the argument array on the stack (and having the compiler implicitly copy it to the heap inside the callee if it escapes). Although you can imagine this type of optimization being performed for arbitrary arrays, it would be more difficult and less predictable.


> Cons:
> Doesn’t do anything that passing an array directly can’t.
> Passing an array is actually slightly more flexible (can dynamically pass more or less arguments as required at the call site).
> Less explicit type at call site; gives the appearance of passing instance(s) of Foo, rather than one instance of [Foo], can lead to ambiguity with overloaded functions.
> Extra syntax to support (enabling array passing would be more code required to support this feature)
> I’d also argue that variadic functions increase the learning curve, as the first time you’re presented with one it isn’t necessarily clear what it does unless you’ve encountered them before. Like I say it can be ambiguous at the call-site in particular, as it doesn’t show that an array of [Foo] is being passed as opposed to N instances of Foo (however many are in the call).
> While I’ve used them in the past, I’ve never really felt that they simplify anything enough to justify them, as it’s just two square brackets extra to pass an array; this is extra noise sure, but clarifies what is actually happening.
> So if variadic functions don’t have any other advantages, then it’s really just a very minor piece of syntactic sugar that can lead to more confusion, less explicit types in function calls, an extra piece of syntax to parse and handle and possibly other features required to support it better (like the one being inquired about). The only other argument I can think of for them is that many other languages have them, but that’s not important to me vs cutting out cruft.
> Short version; removing variadic functions would solve the problem of defining overloads for both variadic and array types by only ever requiring the latter.
> P.S- I’d also like to note that where possible people should be accepting Sequences or Collections in their methods anyway rather than arrays specifically, as it’s more flexible that way ;)
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