[Pitch] Parameter Packs

[Jumhyn (Frederick Kellison-Linn)]

@John_McCall

There's an innate conflict here between the declaration of a pack, where the name stands for a sequence and it feels right to pluralize it (e.g. Widgets), and the uses of a pack, where the name is always part of an expansion and stands for one component and so feels like it ought to be singular (e.g. Widget).

We could always introduce a language model into name lookup so that Widgets... would implicitly introduce the Widget name as well, and Indices... would properly singularize to Index.

@John_McCall

Now, this conflict would be resolved if we allowed the use site to bind a different name to the pack element. I can think of a couple different syntaxes for that:

• for A in Arguments { A? }
• each A in Arguments { A? }
• Arguments.map { A in A? }

I don't love any of them, though.

Yeah, I was thinking about something similar in an early variadic generics thread, though only as a tool for writing constraints:

@Jumhyn

I could imagine us introducing, for instance, a new type of generic constraint that would allow us to supply a name for each element in a type sequence. Something like:

func allPresent<Haystacks..., Needles...>(haystacks: Haystacks..., needles: Elements...) -> Bool
  where forEach(Haystack in Haystacks, Needle in Needles, Haystack == Set<Needle>)

I'm not totally sold either.

@John_McCall

Maybe the biggest problem is that this feels really heavyweight when you're just trying to expand the pack directly, which is very common.

Yep, this is my feeling as well, and I wouldn't be opposed the type of regime you're proposing where we have a shorthand syntax for the simplest case and then fall back to a heavier syntax as soon as you want to do anything more complex. I think I'd still prefer something that makes it clear in elements... that we're working with a pack rather than a range bound, but at least it doesn't have the issue of the pack reference being buried deep inside a complex expression.

@hborla

I have only used variadic generics in other languages for educational purposes, but when writing out the examples in Swift, I found my self almost always using the singular name for type parameter packs, and the plural name for value parameter packs.

I feel like we can only really resolve this with one of two things:

• Explicit pack syntax to make it visually obvious when you're referring to the full pack
• Explicit destructuring syntax that allows you to introduce a singular name

As you (and John) noted in your previous post(s), there's a fundamental conflict between different usages of a pack that make a single choice for singular or plural look silly in some contexts.