From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Sat, 10 Dec 2011 19:02:52 +0000 (+0530)
Subject: solution to 4.16
X-Git-Url: https://git.rkrishnan.org/frontends/pb.xhtml?a=commitdiff_plain;h=4b6730d361677a72c737f82eb7da1a748580eacd;p=sicp.git

solution to 4.16
---

diff --git a/src/sicp/ex4_16.rkt b/src/sicp/ex4_16.rkt
new file mode 100644
index 0000000..5a717b5
--- /dev/null
+++ b/src/sicp/ex4_16.rkt
@@ -0,0 +1,93 @@
+#lang racket
+
+;; a
+(define (lookup-variable-value var env)
+  (define (env-loop env)
+    (define (scan vars vals)
+      (cond [(null? vars)
+             (env-loop (enclosing-environment env))]
+            [(eq? var (car vars))
+             (let ([value (car vals)])
+               (if (eq? value '*unassigned*)
+                   (error "evaluating a variable which has not been assigned -- LOOKUP-VARIABLE-VALUE:" value)
+                   value))]
+            [else (scan (cdr vars) (cdr vals))]))
+    (if (eq? env the-empty-environment)
+        (error "unbound variable" var)
+        (let ([frame (first-frame env)])
+          (scan (frame-variables frame)
+                (frame-values frame)))))
+  (env-loop env))
+
+;; b
+(define (tagged-list? expr tag)
+  (and (pair? expr) (eq? (car expr) tag)))
+
+(define (definition? expr)
+  (tagged-list? expr 'define))
+
+(define (lambda? expr)
+  (tagged-list? expr 'lambda))
+
+(define (lambda-parameters expr)
+  (car (cdr expr)))
+
+(define (make-lambda params body)
+  (cons 'lambda (cons params body)))
+
+;;(lambda (x) (..)(..)...(..))
+(define (lambda-body expr)
+  (cdr (cdr expr)))
+
+(define (scan-out-defines body)
+  (define (internal-definition-names defs)
+    (cond
+      [(empty? defs) '()]
+      [else (cons (cadr (car defs)) (internal-definition-names (cdr defs)))]))
+  (define (internal-definition-values defs)
+    (cond
+      [(empty? defs) '()]
+      [else (cons (caddr (car defs)) (internal-definition-values (cdr defs)))]))
+  
+  ;; assumes that definitions come first in the body
+  (letrec ([get-body-internal (lambda (b)
+                                (cond
+                                  [(empty? b) '()]
+                                  [(definition? (car b)) (get-body-internal (cdr b))]
+                                  [else (cons (car b)
+                                              (get-body-internal (cdr b)))]))]
+           [get-definitions (lambda (b)
+                              (cond
+                                [(empty? b) '()]
+                                [(definition? (car b)) (cons (car b) (get-definitions (cdr b)))]
+                                [else '()]))])
+    (let ([internal-definitions (get-definitions body)])
+      (let ([def-names (internal-definition-names internal-definitions)]
+            [def-vals (internal-definition-values internal-definitions)]
+            [internal-body (get-body-internal body)])
+        (letrec ([p (lambda (name-list)
+                      (cond
+                        [(empty? name-list) '()]
+                        [else (cons (list (car name-list) ''*unassigned*)
+                                    (p (cdr name-list)))]))]
+                 [s (lambda (names vals)
+                      (cond
+                        [(empty? names) '()]
+                        [else (cons (list 'set! (car names) (car vals))
+                                    (s (cdr names) (cdr vals)))]))])
+          (cons 'let 
+                (cons (p def-names)
+                      (append (s def-names def-vals)
+                              internal-body))))))))
+
+;; c
+
+#|
+It is better to do this change in the procedure-body so that the procedure abstractions
+are maintained as it is. The change we are introducing is related to Scheme and hence
+should be done at the lowest level. We should be able to replace procedure-body for another
+language and still be able to use the same eval function.
+
+|#
+
+(define (procedure-body p) (scan-out-defines (caddr p)))
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