[lisper]

Right. The way it was explained to me years ago that really stuck with me is that one way to implement call/cc is to copy the stack onto the heap.

(And another way to do it is to simply allocate all your function call frames on the heap to begin with and not have a stack. Then call/cc is essentially free, but you need a really good incremental garbage collector because now every function call conses.)

[ssp]

Does copying the stack really work? What if some local variable changes in between the call/cc and calling the continuation? If the stack is a copy, that change will not be reflected when the continuation runs.

[lisper]

That's a very interesting point. AFAICT, the Scheme standard doesn't actually say what should happen in this case. But here's what e.g. Chibi scheme does:

    > (let ((x 0)) (list (call/cc (lambda (f) (set! x 1) (f 2))) x))
    (2 0)

[ssp]

The equivalent in Oort produces:

    localhost:~/oort/examples% ../oort callcc2.nl 
    2
    1

as expected since Oort uses heap allocated activation records.

The Oort program:

    typedef continuation_t = fn (p: int32) -> void;
    
    call_cc (fun: fn(k: continuation_t) -> void) -> int32
    {
        retval: int32;
    
        fun (fn (v: int32) -> void {
            retval = v;
            goto current_continuation;
        });
        
        // If fun returns without calling the continuation,
        // then just return 0
        return 0;
    
    @current_continuation:
        return retval;
    }
    
    x: int32 = 0;
    
    print call_cc (fn (k: continuation_t) {
        x = 1;
        k (2);
        });
    
    print x;