Returning continuations from call/cc

Question

Matt Might, in his posting on continuations by example, defines the helper function

(define (current-continuation)
  (call/cc (lambda (cc) (cc cc))))

to grab and return the current continuation. Why use (cc cc) to return the continuation? Why not use cc:

(define (current-continuation)
  (call/cc (lambda (cc) cc)))

Might's examples work identically with either version of current-continuation. Is this just a matter of style, or is there a deeper issue at work?

Answer 1

So, just to remember call/cc in CPS is this:

(define (call/cc& f k)
  (f (lambda (v ignored-cont) (k v)) k))

So the first one does this:

(define (current-continuation k1)
  (call/cc& (lambda (cc k2) (cc cc k2)) k1))

The second one does this:

(define (current-continuation k1)
  (call/cc& (lambda (cc k2) (k2 cc)) k1))

Since call/cc will pass k1 as k2 and cc will use k1 both will end up doing (k1 cc). Thus there is no difference between the two.