foldr in Swift

foldr.swift

func foldr<A : SequenceType, B>(xs: A, y: B, f: (A.Generator.Element, () -> B) -> B) -> B { 
    var g = xs.generate() 
    var next: () -> B = {y} 
    next = { return g.next().map {x in f(x, next)} ?? y } 
    return next() 
} 

foldr([1, 2, 3, -4, -5, 6], "") {a, b in
    println(a)
    return a < 0 ? toString(a) : b()
}
// Prints the lines 1, 2, 3, -4, and then returns "-4".

Your implementation cause a leak, here fixed swift 2 foldr

extension CollectionType {

public func foldr<B>(zero: B, f: (Generator.Element, () -> B) -> B) -> B {

    var g = generate()
    var next: (() -> B)! = {zero}

    next = { return g.next().map {x in f(x, next)} ?? zero }
    let result = next()
    next = nil
    return result
}

}