sicp-4-2-1-normal-order-and-applicative-order

#lang racket

;;;;;;;;;;
;; 4.25 ;;
;;;;;;;;;;

(define (my-unless condition usual-value exceptional-value)
  (if condition exceptional-value usual-value))

(define (factorial n)
  (my-unless (= n 1)
             (* n (factorial (sub1 n)))
             1))

;; When evaluating (factorial 5), the final recursive call will be
;; (my-unless 1 (* 1 (factorial 0)) 1).  Because of eager evaluation, Scheme will try
;; to evaluate (factorial 0) and fall into an infinite loop.

;; But lazy evaluation would not evaluate the unneeded usual-value and terminate as
;; expected, returning 120.

(define (lazy-my-unless condition delayed-usual-value exceptional-value)
  (if condition
    exceptional-value
    (force delayed-usual-value)))

(define (factorial2 n)
  (lazy-my-unless (= n 1)
             (delay (* n (factorial2 (sub1 n))))
             1))

(factorial2 5)
;; 120

;;;;;;;;;;
;; 4.26 ;;
;;;;;;;;;;

;; Unless would be implemented exactly like if but with branches reversed.

;; If you implemented unless as a special form, you could not pass it to map like you
;; could a procedure.  For example, you cannot pass if to map:

(define bools (list false true false))
(define xs '(a b c))
(define ys '(1 2 3))

;; (map if bools xs ys)
;; ;; . if: bad syntax in: if