Swift：将通用类型限制为闭包

Question

How can I restrict generic type to closures? Like this:

struct Closure<T where T:closure> {
  var closure:T
  init(_ c:T) { closure = c }
}

Answer 1

I don't think you can – instead, use generic placeholders to constrain the input and return arguments of the closure, which amounts to the same thing:

struct Closure<T,U> {
    var closure: T->U
    init(_ c: T->U) { closure = c }
}

let c = Closure { $0 % 2 == 0 }
// c will be a Closure<Int,Bool>