Teach Yourself Scheme: 13.2 Escaping continuations

Escaping continuations are the simplest use of call/cc and are very useful for programming procedure or loop exits. Consider a procedure list-product that takes a list of numbers and multiplies them. A straightforward recursive definition for list-product is:

(define list-product
  (lambda (s)
    (let recur ((s s))
      (if (null? s) 1
          (* (car s) (recur (cdr s)))))))

There is a problem with this solution. If one of the elements in the list is 0, and if there are many elements after 0 in the list, then the answer is a foregone conclusion. Yet, the code will have us go through many fruitless recursive calls to recur before producing the answer. This is where an escape continuation comes in handy. Using call/cc, we can rewrite the procedure as:

(define list-product
  (lambda (s)
    (call/cc
      (lambda (exit)
        (let recur ((s s))
          (if (null? s) 1
              (if (= (car s) 0) (exit 0)
                  (* (car s) (recur (cdr s))))))))))

If a 0 element is encountered, the continuation exit is called with 0, thereby avoiding further calls to recur.

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