ch4

ch4/ex4-2.scm

@@ -2,4 +2,34 @@
 
 ;; Example 4.2
 
-;; trying to apply (define) func. But define is the special case.
+;; a. trying to apply (define) func. But define is the special case.
+
+;; b.
+
+(define (eval exp env)
+  (cond ((self-evaluating? exp) exp)
+        ((variable? exp) (lookup-variable-value exp env))
+        ((quoted? exp) (text-of-quotation exp))
+        ((application? exp)
+         (apply (eval (operator exp) env)                 ;; apply procedure (proc, args)
+                (list-of-values (operands exp) env)))
+        ((assignment? exp) (eval-assignment exp env))
+        ((definition? exp) (eval-definition exp env))
+        ((if? exp) (eval-if exp env))
+        ((lambda? exp)
+         (make-procedure (lambda-parameters exp)
+                         (lambda-body exp)
+                         env))
+        ((begin? exp)
+         (eval-sequence (begin-actions exp) env))
+        ((cond? exp) (eval (cond->if exp) env))
+        (else
+         (error "Unknown expression type -- EVAL" exp))))
+
+;; procedure invocation
+(define (application? exp) (tagged-list? exp 'call))
+(define (operator exp) (cadr exp))
+(define (operands exp) (cddr exp))
+(define (no-operands? ops) (null? ops))
+(define (first-operand ops) (car ops))
+(define (rest-operands ops) (cdr ops))

ch4/ex4-3.scm

@@ -0,0 +1 @@
+#lang racket

ch4/ex4-4.scm

@@ -0,0 +1,47 @@
+#lang racket
+
+;; Example 4.4
+
+;; not sure need testing
+
+(define (eval exp env)
+  (cond ((self-evaluating? exp) exp)
+        ((variable? exp) (lookup-variable-value exp env))
+        ((quoted? exp) (text-of-quotation exp))
+        ((assignment? exp) (eval-assignment exp env))
+        ((definition? exp) (eval-definition exp env))
+        ((if? exp) (eval-if exp env))
+
+        ((and? exp) (eval-and (and-operands exp) env))
+        ((or? exp) (eval-or (or-operands exp) env))
+
+        ((lambda? exp)
+         (make-procedure (lambda-parameters exp)
+                         (lambda-body exp)
+                         env))
+        ((begin? exp)
+         (eval-sequence (begin-actions exp) env))
+        ((cond? exp) (eval (cond->if exp) env))
+        ((application? exp)
+         (apply (eval (operator exp) env)                 ;; apply procedure (proc, args)
+                (list-of-values (operands exp) env)))
+        (else
+         (error "Unknown expression type -- EVAL" exp))))
+
+;; and
+
+(define (and? exp) (tagged-list? exp 'and))
+(define (and-operands exp) (cdr exp))
+(define (eval-and exp env)
+  (cond ((null? exp) 'true)
+        ((false? (eval (car exp))) 'false)
+        (else (eval-and (cdr exp) env))))
+
+;; or
+
+(define (or? exp) (tagged-list? exp 'or))
+(define (or-operands exp) (cdr exp))
+(define (eval-or exp env)
+  (cond ((true? (eval (car exp))) 'true)
+        ((null? exp) 'false)
+        (else (eval-or (cdr exp) env))))

ch4/ex4-5.scm

@@ -0,0 +1,55 @@
+#lang racket
+
+;; Example 4.5
+
+;; TODO
+
+(define (eval exp env)
+  (cond ((self-evaluating? exp) exp)
+        ((variable? exp) (lookup-variable-value exp env))
+        ((quoted? exp) (text-of-quotation exp))
+        ((assignment? exp) (eval-assignment exp env))
+        ((definition? exp) (eval-definition exp env))
+        ((if? exp) (eval-if exp env))
+        ((lambda? exp)
+         (make-procedure (lambda-parameters exp)
+                         (lambda-body exp)
+                         env))
+        ((begin? exp)
+         (eval-sequence (begin-actions exp) env))
+        ((cond? exp) (eval (cond->if exp) env))
+        ((application? exp)
+         (apply (eval (operator exp) env)                 ;; apply procedure (proc, args)
+                (list-of-values (operands exp) env)))
+        (else
+         (error "Unknown expression type -- EVAL" exp))))
+
+(define (cond? exp) (tagged-list? exp 'cond))
+(define (cond-clauses exp) (cdr exp))
+(define (cond-else-clause? clause)
+  (eq? (cond-predicate clause) 'else))
+(define (cond-predicate clause) (car clause))
+(define (cond-actions clause) (cdr clause))
+(define (cond->if exp)
+  (expand-clauses (cond-clauses exp)))
+
+(define (expand-clauses clauses)
+  (if (null? clauses)
+      'false
+      (let ((first (car clauses))
+            (rest (cdr clauses)))
+        (if (cond-else-clause? first)
+            (if (null? rest)
+                (sequence->exp (cond-actions first))
+                (error "Clause else not the last" clauses))
+            (make-if (cond-predicate first)
+                     (sequence->exp (cond-actions first))
+                     (expand-clauses rest))))))
+
+(define (last-exp? seq) (null? (cdr seq)))
+(define (first-exp seq) (car seq))
+(define (make-begin seq) (cons 'begin seq))
+(define (sequence->exp seq)
+  (cond ((null? seq) seq)
+        ((last-exp? seq) (first-exp seq))
+        (else (make-begin seq))))