#lang racket

; all functions can be written in continuation passing style in which
; the next thing to be done is passed into the call along with the 
; arguments.  The next thing to be done, cc, is called a continuation.
; Programming in this way is called continuation passing style (CPS).

(define C+ (lambda (x y cc) (cc (+ x y))))
(define C* (lambda (x y cc) (cc (* x y))))

(C+ 3 5 (lambda (r) r))  ; return result 3 + 5
                         ; same as ((lambda (r) r) (+ 3 5))

; to chain a sequence of cps functions computing (3 + 5) * 10
(C+ 3 5 (lambda (r) 
           (C* r 10 (lambda (rr) rr))))

; this is the same as
    

; and even for this: (3 + 5) * (8 + 2) i.e. (* (+ 3 5) (+ 8 2) )
; Note how it reads left to right.
(C+ 3 5 
        (lambda (r1) ( C+ 8 2
             (lambda (r2) (C* r1 r2 (lambda (r3) r3) )
               )
             )
          )
        )

; this means that we can even write recursive functions that make
; multiple recursive calls in continuation passing style.  The
; stack is replaced by bindings in lambdas to heap values