From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Sat, 19 Nov 2011 05:05:38 +0000 (+0530)
Subject: bug fixes
X-Git-Url: https://git.rkrishnan.org/vdrive/%22file:/frontends/FOOURL?a=commitdiff_plain;h=018629b42d482b2825b75c0622b04212197e81af;p=sicp.git

bug fixes
---

diff --git a/src/sicp/ex4_11.rkt b/src/sicp/ex4_11.rkt
index 00f79a2..0b60119 100644
--- a/src/sicp/ex4_11.rkt
+++ b/src/sicp/ex4_11.rkt
@@ -1,4 +1,8 @@
-#lang racket
+#lang r5rs
+
+(define (error x)
+  (display x)
+  (newline))
 
 (define (make-frame variables values)
   (if (not (= (length variables)
@@ -13,4 +17,49 @@
   (map cdr frame))
 
 (define (add-binding-to-frame! var val frame)
-  (set! frame (cons (cons var val) frame)))
+  (define (add-to-end-of-frame! b f)
+    (if (null? (cdr f))
+        (set-cdr! f b)
+        (add-to-end-of-frame! b (cdr f))))
+  (add-to-end-of-frame! (cons var val) frame))
+
+(define (lookup-variable-value var env)
+  (define (env-loop env)
+    (define (scan frame)
+      (if (null? frame)
+          (env-loop (enclosing-environment env))
+          (let ((p (car frame)))
+            (if (eq? var (car p))
+                (cdr p)
+                (scan (cdr frame))))))
+    (if (eq? env the-empty-environment)
+        (error "variable not defined")
+        (let ((f (first-frame env)))
+          (scan f))))
+  (env-loop env))
+
+(define (set-variable-value! var val env)
+  (define (env-loop env)
+    (define (scan frame)
+      (if (null? frame)
+          (env-loop (enclosing-environment env))
+          (let ((p (car frame)))
+            (if (eq? var (car p))
+                (set-cdr! p val)
+                (scan (cdr frame))))))
+    (if (eq? env the-empty-environment)
+        (error "variable not defined")
+        (let ((f (first-frame env)))
+          (scan f))))
+  (env-loop env))
+
+(define (define-variable! var val env)
+  (let ((frame (first-frame env)))
+    (define (scan frame)
+      (if (null? frame)
+          (add-binding-to-frame var val frame)
+          (let ((p (car frame)))
+            (if (eq? (car p) var)
+                (set-cdr! p val)
+                (scan (cdr frame))))))
+    (scan frame)))
diff --git a/src/sicp/ex4_7.rkt b/src/sicp/ex4_7.rkt
index 956095d..8bb4aa0 100644
--- a/src/sicp/ex4_7.rkt
+++ b/src/sicp/ex4_7.rkt
@@ -1,5 +1,7 @@
 #lang racket
 
+(provide let*? let*->nested-lets)
+
 (define (tagged-list? exp tag)
   (if (pair? exp)
       (eq? (car exp) tag)
diff --git a/src/sicp/ex4_8.rkt b/src/sicp/ex4_8.rkt
index d58eb4d..1647aea 100644
--- a/src/sicp/ex4_8.rkt
+++ b/src/sicp/ex4_8.rkt
@@ -1,6 +1,6 @@
 #lang racket
 
-(provide let->combination)
+(provide let->combination let?)
 
 (define (tagged-list? exp tag)
   (if (pair? exp)