15 hours ago
my attempt to do the exercises in sicp.
Saturday, July 5, 2008
sicp exercise 1.6
; Exercise 1.6. Alyssa P. Hacker doesn't see why if needs to be provided as a special form. ``Why can't I
; just define it as an ordinary procedure in terms of cond?'' she asks. Alyssa's friend Eva Lu Ator claims
; this can indeed be done, and she defines a new version of if:
; (define (new-if predicate then-clause else-clause)
; (cond (predicate then-clause)
; (else else-clause)))
; Eva demonstrates the program for Alyssa:
; (new-if (= 2 3) 0 5)
; (new-if (= 1 1) 0 5)
; Delighted, Alyssa uses new-if to rewrite the square-root program:
; (define (sqrt-iter guess x)
; (new-if (good-enough? guess x)
; (sqrt-iter (improve guess x)
; What happens when Alyssa attempts to use this to compute square roots? Explain.
; since the new-if is a procedure, the if-clause and then-clause are evaluated at the time of procedure call
; so the sqrt-iter will be called infinite times in applicative order evaluation.