3 ;; metacircular evaluator
5 (cond ((self-evaluating? exp) exp)
6 ((variable? exp) (lookup-variable-value exp env))
7 ((quoted? exp) (text-of-quotation exp))
8 ((assignment? exp) (eval-assignment exp env))
9 ((definition? exp) (eval-definition exp env))
10 ((if? exp) (eval-if exp env))
12 (make-procedure (lambda-parameters exp)
15 ((begin? exp) (eval-sequence (begin-actions exp) env))
16 ((cond? exp) (eval (cond->if exp) env))
18 (apply (eval (operator exp) env)
19 (list-of-values (operands exp) env)))
21 (error "unknown expression type -- EVAL" exp))))
23 (define (apply procedure arguments)
24 (cond ((primitive-procedure? procedure)
25 (apply-primitive-procedure procedure arguments))
26 ((compound-procedure? procedure)
28 (procedure-body procedure)
30 (procedure-parameters procedure)
32 (procedure-environment procedure))))
34 (error "unknown procedure type -- APPLY" procedure))))
36 (define (list-of-values exps env)
37 (if (no-operands? exps)
39 (cons (eval (first-operand exps) env)
40 (list-of-values (rest-operands exps) env))))
42 (define (eval-if exp env)
43 (if (true? (eval (if-predicate exp) env))
44 (eval (if-consequent exp) env)
45 (eval (if-alternative exp) env)))
47 (define (eval-sequence exps env)
48 (cond ((last-exp? exps)
49 (eval (first-exp exps) env))
51 (eval (first-exp exps) env)
52 (eval-sequence (rest-exps exps) env))))
54 (define (eval-assignment exp env)
55 (set-variable-value! (assignment-variable exp)
56 (eval (assignment-value exp) env)
60 (define (eval-definition exp env)
61 (define-variable! (definition-variable exp)
62 (eval (definition-value exp) env)
67 (define (self-evaluating? expr)
68 (cond [(number? expr) #t]
72 (define (variable? expr)
76 (define (quoted? expr)
77 (tagged-list? expr 'quote))
79 (define (tagged-list? expr tag)
80 (or (pair? expr) (eq? (car expr) tag)))
82 (define (text-of-quotation expr)
86 (define (assignment? expr)
87 (tagged-list? expr 'set!))
89 (define (assignment-variable expr)
92 (define (assignment-value expr)
93 (car (cdr (cdr expr))))
96 (define (definition? expr)
97 (tagged-list? expr 'define))
99 (define (definition-variable expr)
100 (if (symbol? (car (cdr expr))) ;; (define foo 42)
102 ;; (define (foo x) (....))
103 (car (car (cdr expr)))))
105 (define (definition-value expr)
107 (if (symbol? (car (cdr expr)))
108 (car (cdr (cdr expr)))
109 ;; (define (foo x) (....))
110 (make-lambda (cdr (car (cdr expr))) ;; gives (x)
111 (cdr (cdr expr))))) ;; body
114 (define (lambda? expr)
115 (tagged-list? expr 'lambda))
117 (define (lambda-parameters expr)
120 ;;(lambda (x) (..)(..)...(..))
121 (define (lambda-body expr)
124 (define (make-lambda params body)
125 (cons 'lambda (cons params body)))
128 (define (if? exp) (tagged-list? exp 'if))
129 (define (if-predicate expr) (car (cdr expr)))
130 (define (if-consequent expr) (car (cdr (cdr expr))))
131 (define (if-alternative expr)
132 (if (not (null? (cdr (cdr (cdr expr)))))
133 (car (cdr (cdr (cdr expr))))
136 (define (make-if predicate consequent alternative)
137 (list 'if predicate consequent alternative))
140 (define (begin? exp) (tagged-list? exp 'begin))
141 (define (begin-actions exp) (cdr exp))
142 (define (last-exp? seq) (null? (cdr seq)))
143 (define (first-exp seq) (car seq))
144 (define (rest-exp seq) (cdr seq))
146 (define (sequence->exp seq)
147 (cond ((null? seq) seq)
148 ((last-exp? seq) (first-exp seq))
149 (else (make-begin seq))))
150 (define (make-begin seq) (cons 'begin seq))
152 ;; procedure application
153 (define (application? exp) (pair? exp))
154 (define (operator exp) (car exp))
155 (define (operands exp) (cdr exp))
156 (define (no-operands? ops) (null? ops))
157 (define (first-operand ops) (car ops))
158 (define (rest-operands ops) (cdr ops))
160 ;; expander for `cond' expressions to be transformed into `if' expressions
161 (define (cond? exp) (tagged-list? exp 'cond))
162 (define (cond-clauses exp) (cdr exp))
163 (define (cond-else-clause? clause)
164 (eq? (cond-predicate clause) 'else))
165 (define (cond-predicate clause) (car clause))
166 (define (cond-actions clause) (cdr clause))
167 (define (cond->if exp)
168 (expand-clauses (cond-clauses exp)))
170 (define (expand-clauses clauses)
172 'false ; no else clause
173 (let ((first (car clauses))
174 (rest (cdr clauses)))
175 (if (cond-else-clause? first)
177 (sequence->exp (cond-actions first))
178 (error "ELSE clause isn't last -- COND->IF"
180 (make-if (cond-predicate first)
181 (sequence->exp (cond-actions first))
182 (expand-clauses rest))))))