swift - How to handle closure recursivity

Question

Here's a very simple recursive function:

func lap (n: Int) -> Int {
    if n == 0 { return 0 }
   return lap (n - 1)
}

If I want to convert it as closure:

let lap = {
    (n: Int) -> Int in
    if n == 0 { return 0 }
    return lap (n - 1)
}

I got a compiler error: "Variable used within its own initial value"

Answer 1

you can workaround it with two step assignment

var lap : (Int) -> Int!
lap = {
    (n: Int) -> Int in
    if n == 0 { return 0 }
    return lap(n - 1)
}

or you can use Y combinator

func Y<T, R>( f: (T -> R) -> (T -> R) ) -> (T -> R) {
    return { t in f(Y(f))(t) }
}

let lap = Y {
    (f : Int -> Int) -> (Int -> Int) in
    return { (n : Int) -> Int in return n == 0 ? 0 : f(n - 1) }
}

// with type inference 
let lap2 = Y {
    f in { n in n == 0 ? 0 : f(n - 1) }
}

---

This is a workaround of the memory leak problem that @zneak found (It doesn't have memory leak but captured the wrong value)

func f(n: Int) {
    var f = Foo()
    var lap: @objc_block (Int)->Int = { $0 }
    var obj: NSObject = reinterpretCast(lap)
    lap = {
        [weak obj] (n: Int) -> Int in // unowned will cause crush
        if n == 0 { return 0 }
        println(f)
        var lap2 : @objc_block (Int)->Int = reinterpretCast(obj)
        return lap2 (n - 1)
    }
    lap(n)
}

for i in 0..<5 {
    f(i)
}

class Foo {
    init() {
        println("init");
    }

    deinit {
        println("deinit")
    }
}