Cont.swift

struct Cont<A, B> {
    var run: ((A) -> B) -> B
    var flatMap: (A) -> Cont<A, B> {
        get {
            fatalError()
        }
        _modify {
            let that = self
            var f: (A) -> Cont<A, B> = { (a: A) -> Cont<A, B> in that }
            yield &f
            run = { (k: (A) -> B) -> B in that.run({ (a: A) -> B in f(a).run(k) }) }
       }
   }
}

var a = Cont<String, String>(
    run:     { (k) in k(    "a↓ ") + "f!" }
)

a.flatMap = { (x) in
    Cont(
        run: { (k) in k(x + "b↓ ") + "e↑ " }
    )
}

a.flatMap = { (x) in
    Cont(
        run: { (k) in k(x + "c→ ") + "d↑ " }
    )
}

func identity<A>(_ a: A) -> A {
    return a
}

print(a.run(identity))