From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Wed, 7 Sep 2011 16:26:13 +0000 (+0200)
Subject: solutions for 4.1 - 4.4
X-Git-Url: https://git.rkrishnan.org/pf/content/quickstart.html?a=commitdiff_plain;h=70cd85563327ac622029847fb75349375a14dfe4;p=sicp.git

solutions for 4.1 - 4.4
---

diff --git a/src/sicp/ex4_1.rkt b/src/sicp/ex4_1.rkt
new file mode 100644
index 0000000..626ebd4
--- /dev/null
+++ b/src/sicp/ex4_1.rkt
@@ -0,0 +1,19 @@
+#lang racket
+
+;; left to right
+(define (list-of-values exps env)
+  (define (iter valuelist expressions)
+    (if (no-operands? exps)
+        valuelist
+        (let ((value (eval (first-operand expressions) env)))
+          (iter (append valuelist (list value))
+                (rest-operands expressions)))))
+  (iter '() exps))
+
+;; right to left
+(define (list-of-values exps env)
+  (if (no-operands? exps)
+      '()
+      (let ((right (list-of-values (rest-exps exps) env)))
+        (cons (eval (first-exp exps) env)
+              right))))
diff --git a/src/sicp/ex4_2.rkt b/src/sicp/ex4_2.rkt
new file mode 100644
index 0000000..91daa83
--- /dev/null
+++ b/src/sicp/ex4_2.rkt
@@ -0,0 +1,28 @@
+#lang racket
+
+#|
+
+a. application? only checks whether the given expression is a pair. It assumes that the rest of
+   the built in language clauses are already handled. So this will upset all other classes like
+   define, let where Louis's eval would think define is a procedure and would proceed to evaluate
+   the operands, which is wrong.
+
+|#
+
+#|
+
+b.
+
+We only change the selectors and the predicate procedures, everything else remain the same to
+implement the new proposal.
+
+|#
+
+(define (application? exp)
+  (if (pair? exp)
+      (tagged-list? expr 'call)
+      #f))
+
+(define (operator exp) (car (cdr exp)))
+(define (operands exp) (cdr (cdr exp)))
+
diff --git a/src/sicp/ex4_3.rkt b/src/sicp/ex4_3.rkt
new file mode 100644
index 0000000..bbbf9a8
--- /dev/null
+++ b/src/sicp/ex4_3.rkt
@@ -0,0 +1,20 @@
+#lang racket
+
+(define (eval exp env)
+  (cond ((self-evaluating? exp) exp)
+        ((variable? exp) (lookup-variable-value exp env))
+        ((get (type exp)) ((get (type exp)) exp env))))
+
+(define (type expr) (car expr))
+
+(put 'quote 
+     (lambda (expr env)
+       (text-of-quotation expr)))
+
+(put 'set!
+     (lambda (expr env)
+       (eval-assignment expr env)))
+
+
+
+     
\ No newline at end of file
diff --git a/src/sicp/ex4_4.rkt b/src/sicp/ex4_4.rkt
new file mode 100644
index 0000000..aea7173
--- /dev/null
+++ b/src/sicp/ex4_4.rkt
@@ -0,0 +1,46 @@
+#lang racket
+
+(define (tagged-list? exp tag)
+  (if (pair? exp)
+      (eq? (car exp) tag)
+      #f))
+
+(define (make-if predicate consequent alternative)
+  (list 'if predicate consequent alternative))
+
+;; and
+(define (and? expr) (tagged-list? expr 'and))
+(define (eval-and expr env) (eval-and-predicates (cdr expr) env))
+
+(define (eval-and-predicates expr env)
+  (cond ((null? expr) #t)
+        ((eval (car expr) env) (eval-and-predicates (cdr expr) env))
+        (else #f)))
+
+;; or
+(define (or? expr) (tagged-list? expr 'or))
+(define (eval-or expr env) (eval-or-predicates (cdr expr) env))
+
+(define (eval-or-predicates expr env)
+  (cond ((null? expr) #f)
+        ((eval (car expr) env) #t)
+        (else (eval-or-predicates (cdr expr) env))))
+
+;; derived expressions
+(define (and->if expr)
+  (expand-and-predicates (cdr expr)))
+
+(define (expand-and-predicates expr)
+  (cond ((null? expr) '#t)
+        (else (make-if (car expr)
+                       (expand-and-predicates (cdr expr))
+                       '#f))))
+
+(define (or->if expr)
+  (expand-or-predicates expr))
+
+(define (expand-or-predicates expr)
+  (cond ((null? expr) '#f)
+        (else (make-if (car expr)
+                       '#t
+                       (expand-or-predicates (cdr expr))))))
\ No newline at end of file