X-Git-Url: https://git.rkrishnan.org/?p=sicp.git;a=blobdiff_plain;f=src%2Fsicp%2Fex4_51.rkt;fp=src%2Fsicp%2Fex4_51.rkt;h=ccb5caa25fa031a2b82443cf1276b93fba6e52da;hp=0000000000000000000000000000000000000000;hb=5158bd64ca487b858c9d95e8a2bccb81fe2113a7;hpb=273f2bd230187a3164803ac32d9b5734d0123ad0

diff --git a/src/sicp/ex4_51.rkt b/src/sicp/ex4_51.rkt
new file mode 100644
index 0000000..ccb5caa
--- /dev/null
+++ b/src/sicp/ex4_51.rkt
@@ -0,0 +1,28 @@
+#lang racket
+
+;;; assignment
+(define (analyze-assignment expr)
+  (let ([var (assignment-var expr)]
+        [vproc (analyze (assignment-value expr))])
+    (lambda (env succeed fail)
+      (vproc env
+             (lambda (val fail2)
+               (let ([old-val (lookup-variable-value var env)])
+                 (set-variable-value! var val)
+                 (succeed 'ok
+                          (lambda ()
+                            (set-variable-value! var old-val env)
+                            (fail2)))))
+             fail))))
+
+;; permenent-set!
+;;; assignment
+(define (analyze-permanent-assignment expr)
+  (let ([var (perm-assignment-var expr)]
+        [vproc (analyze (perm-assignment-value expr))])
+    (lambda (env succeed fail)
+      (vproc env
+             (lambda (val fail2)
+               (set-variable-value! var val)
+               (succeed 'ok fail2))
+             fail))))