ex-1.9.scm

(define (inc n)
  (+ n 1))

(define (dec n)
  (- n 1))

(define (rec+ a b)
  (if (= a 0)
    b
    (inc (rec+ (dec a) b))))
; recursive process
; (rec+ 4 5)
; (inc (rec+ 3 5))
; (inc (inc (rec+ 2 5)))
; (inc (inc (inc (rec+ 1 5))))
; (inc (inc (inc (inc (rec+ 0 5)))))
; (inc (inc (inc (inc 5))))
; (inc (inc (inc 6)))
; (inc (inc 7))
; (inc 8)
; 9

(define (iter+ a b)
  (if (= a 0)
    b
    (iter+ (dec a) (inc b))))
; iterative process
; (iter+ 4 5)
; (iter+ 3 6)
; (iter+ 2 7)
; (iter+ 1 8)
; (iter+ 0 9)
; 9

(define (main args)
  (print (rec+ 4 5))
  ; => 9

  (print (iter+ 4 5))
  ; => 9
  )