Chapter: Call by value-result

Maybe we can get the best of both worlds. Let's do the following.

• Pass VARs to the called procedure,
• Have the called procedure copy the values into local variables,
• Work with the local variables
• Just before returning, copy the values of the local variables into the caller's VARs.

We can implement this in Scheme via a macro:

(extend-syntax (proc-by-value-result)
  [(proc-by-value-result (id ...) body ...)
   (with ([(newvar ...) (map (lambda (x) (gensym)) '(id ...))])
     (lambda (newvar ...)
       (let ([id (box (^ newvar))] ...)
	 (let ([result (begin body ...)])
	   (:= newvar (^ id)) ...
	   result))))])

This differs from the macro for proc-by-value only in the assignments to the input parameters at the end. Try a few calls to expand to be sure you understand what is going on. Here's an example.

(expand '(proc-by-value-result (x y) (+ (^ foo) (* (^ x) (^ y)))))

Here is the factorial program done by value-result.

(define fac-by-val-result
  (proc-by-value-result (x acc)
     (while (> (^ x) 0)
	    (:= acc (* (^ x) (^ acc)))
	    (:= x (1- (^ x))))))