From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Sun, 27 May 2012 16:25:57 +0000 (+0530)
Subject: Better explanation of the unless procesure if it is not defined
X-Git-Url: https://git.rkrishnan.org/pf/content/en/seg/COPYING.TGPPL.html?a=commitdiff_plain;h=d8388032e6e749f7cab7a18d840bc6462826dccc;p=sicp.git

Better explanation of the unless procesure if it is not defined
in a lazy way.
---

diff --git a/src/sicp/ex4_25.rkt b/src/sicp/ex4_25.rkt
index 20c4136..7a01051 100644
--- a/src/sicp/ex4_25.rkt
+++ b/src/sicp/ex4_25.rkt
@@ -2,9 +2,21 @@
 
 #|
 
-The call (factorial 5) will never call 'unless' and will be recursively call 'factorial' for ever.
+The call (factorial 5) will never call 'unless' and will be recursively call 'factorial' for ever. 'n' goes
+negative and the factorial won't notice it, as it will never get to evaluate the function body of `unless'.
 
 It will work in a normal order language because none of the arguments of 'unless' are evaluated until
 they are used inside.
 
+#lang lazy
+(define (unless predicate if-exp else-exp)
+  (if (not predicate)
+      if-exp
+      else-exp))
+
+(define (factorial n)
+  (unless (= n 1)
+          (* n (factorial (- n 1)))
+          1))
+
 |#
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