SE-0348: buildPartialBlock for Result Builders

[ensan-hcl (ensan)]

+1.

I'm very surprised at this proposal, in terms of its strong ability.

Variadic Generics

Just gave the latest version of the document a read. Looks great! I love the idea of this being a STATIC feature. I hope we are all on the same page how this limits expressiveness. For instance, you can't express infinte lists like (0...). Also, one can't exchange the underyling type of an element of the list (with dynamic existential types, you could). But these are small prices to pay for the optimizations that the compiler can do if this is a static feature. There's one thing I would like to…

When I read previous discussion about variadic generics, I found this comment. It cannot be expressed by simple variadic generics, because it's hard to express the exact type of result. While variadic generics struggle to do that, buildPartialBlock can easily do that.

static func buildPartialBlock<A, B>(first: (A) -> B) -> (A) -> B {
    return first   
}
static func buildPartialBlock<A, B, C>(accumulated: (A) -> B, next: (B) -> C) -> (A) -> C {
    return { (x: A) in next(accumulated(x)) }
}

func f(_: A) -> B
func g(_: B) -> C
func h(_: C) -> D
let hgf: (A) -> D = Chain {
    f
    g
    h
}

Considering this, I feel we should put more thought on how result builders interact with variadic generics. I don't have solid vision of them, but they definitely correlate with each other.