Matching function parameter to Type in parameter pack

Using Swift
, ,
1

I'm trying to remove a type from a parameter pack by "division" if you will. But the compiler tells me function/operator cannot be applied to the given operands. Is this expected behaviour or can I achieve this in a different way perhaps?

struct V<each U> { }

func /<each U1, each U2, U>(lhs: V<repeat each U1, U, repeat each U2>, rhs: V<U>) -> V<repeat each U1, repeat each U2> {
    .init()
}

let v1 = V<Int, Int, String, Int>()
let v2 = V<String>()

let v3 = v1 / v2 // Binary operator '/' cannot be applied to operands of type 'V<Int, Int, String, Int>' and 'V<String>'
// I would have expected v3 to be of type V<Int, Int, Int>
1 Like
3

Would be really interested to know why this isn't currently possible and if it's expected.

4

This is not supported. The rules for unification of pack types are documented in the “type matching” section of the proposal—they’re probably a bit simpler than what you’re imagining: swift-evolution/proposals/0393-parameter-packs.md at main · apple/swift-evolution · GitHub

5

Ah thanks for pointing me to those! Could you see any other way of achieving this? I'm trying to achieve a way to have type safe unit measurements and freely converting from one to the other without explicitly defining every combination.
I have that partly working with nested generics, but it could be greatly simplified by using a variadic operation like I was trying above.