// The factoral function in continuation-passing style, in Java

// sestoft@dina.kvl.dk 2002-04-03, 2003-03-19

// A Cont object is a continuation: it has a method k that given the

// result of a subcomputation will return the final result:

interface Cont {

int k(int v);

}

// Calling the two versions of factorial:

class Factorial {

public static void main(String[] args) {

int n = Integer.parseInt(args[0]);

System.out.println(facc(n,

new Cont() {

public int k(int v) {

return v;

}

}));

System.out.println(faci(n,

new Cont() {

public int k(int v) {

return v;

}

}));

}

// A continuation-passing version of factorial. This is a straight

// translation of the Standard ML version:

static int facc(final int n, final Cont cont) {

if (n == 0)

return cont.k(1);

else

return facc(n-1,

new Cont() {

public int k(int v) {

return cont.k(n * v);

}

});

}

// Another continuation-passing version of factorial. Since facc

// above is tail-recursive, it can be rewritten as a loop. This

// shows how it builds up a continuation and finally applies it:

static int faci(int n, Cont cont) {

while (n != 0) {

cont = new FacCont(n, cont);

n--;

}

return cont.k(1);

}

}

// Auxiliary class to implement faci. Needed for Java-technical

// reasons; a local inner class can refer only to `final' variables in

// the enclosing method, but faci must update the cont variable in the

// loop.

class FacCont implements Cont {

private final int n;

private final Cont cont;

public FacCont(int n, Cont cont) {

this.n = n; this.cont = cont;

}

public int k(int v) {

return cont.k(n * v);

}

}