Completion type instead of (a → Void)

Completion.swift

struct Completion<A> {
    let f: (A -> Void)
    init(f: A -> Void) {
        self.f = f
    }
    func run(a: A) {
        f(a)
    }
}

func proc(completion: (Either<NSError, Int> -> Void)) {
    completion(Either(right: 0))
}

func proc(c: Completion(Either<NSError, Int>)) {
    c.run(Either(right: 0))
}