Swift:将泛型类型限制为闭包

Question

如何将泛型类型限制为闭包？像这样:

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

Answer 1

我不认为你可以 - 相反，使用通用占位符来约束闭包的输入和返回参数，这相当于同一件事:

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>