3 (require (rename-in racket/base
4 (apply apply-in-underlying-scheme)
5 (eval eval-in-underlying-scheme)))
7 (define (self-evaluating? expr)
14 (define (variable? expr) (symbol? expr))
18 (define (eval-sequence exps env)
19 (cond ((last-exp? exps) (eval (first-exp exps) env))
20 (else (eval (first-exp exps) env)
21 (eval-sequence (rest-exps exps) env))))
24 (define (last-exp? seq) (null? (cdr seq)))
25 (define (first-exp seq) (car seq))
26 (define (rest-exps seq) (cdr seq))
29 (define (make-lambda parameters body)
30 (cons 'lambda (cons parameters body)))
32 ;; environment data structures
33 (define (enclosing-environment env) (cdr env))
34 (define (first-frame env) (car env))
35 (define the-empty-environment '())
37 (define (make-frame variables values)
38 (let ([ht (make-hash)])
39 (for-each (lambda (var val)
40 (hash-set! ht var val))
45 (define (frame-variables frame)
48 (define (frame-values frame)
51 (define (add-binding-to-frame! var val frame)
52 (hash-set! frame var val))
54 ;; environment is a list of frames, most recent being the last
55 ;; one consed into the list
56 (define (extend-environment vars vals base-env)
57 (let ([frame (make-frame vars vals)])
58 (cons frame base-env)))
60 (define (lookup-variable-value var env)
61 (if (eq? env the-empty-environment)
62 (error "unbound variable:" var)
63 (let ([frame (first-frame env)])
64 (hash-ref frame var (lambda () (lookup-variable-value var (enclosing-environment env)))))))
66 (define (set-variable-value! var val env)
67 (if (eq? env the-empty-environment)
68 (error "unbound variable")
69 (let ([frame (first-frame env)])
70 (if (hash-has-key? frame var)
71 (hash-set! frame var val)
72 (set-variable-value! var val (enclosing-environment env))))))
74 (define (define-variable! var val env)
75 (let ([frame (first-frame env)])
76 (if (hash-has-key? frame var)
77 (hash-set! frame var val)
78 (add-binding-to-frame! var val frame))))
80 ;; primitive procedure
81 (define (primitive-implementation proc) (cadr proc))
83 (define primitive-procedures
95 (define (primitive-procedure-names)
96 (map car primitive-procedures))
98 (define (primitive-procedure-objects)
99 (map (lambda (proc) (list 'primitive (cadr proc)))
100 primitive-procedures))
102 (define (apply-primitive-procedure proc args)
103 (apply-in-underlying-scheme
104 (primitive-implementation proc) args))
107 (define (setup-environment)
109 (extend-environment (primitive-procedure-names)
110 (primitive-procedure-objects)
111 the-empty-environment)))
112 (define-variable! 'true true initial-env)
113 (define-variable! 'false false initial-env)
116 (define the-global-environment (setup-environment))
119 (define (list-of-values exps env)
120 (if (no-operands? exps)
122 (cons (eval (first-operand exps) env)
123 (list-of-values (rest-operands exps) env))))
125 (define (no-operands? ops) (null? ops))
126 (define (first-operand ops) (car ops))
127 (define (rest-operands ops) (cdr ops))
129 ;; compound procedure
130 (define (make-procedure params body env)
131 (list 'procedure params (scan-out-definitions body) env))
133 (define (procedure-parameters p) (cadr p))
134 (define (procedure-body p) (scan-out-definitions (caddr p)))
135 (define (procedure-environment p) (cadddr p))
137 (define (apply procedure arguments)
139 [`(primitive ,f ...) (apply-primitive-procedure procedure arguments)]
140 [`(procedure ,f ...) (eval-sequence
141 (procedure-body procedure)
143 (procedure-parameters procedure)
145 (procedure-environment procedure)))]
146 [_ (error "Unknown procedure type -- APPLY" procedure)]))
156 (define (cond->if clauses)
157 (define (seq->exp actions)
158 (if (empty? (cdr actions))
163 (let ([clause (car clauses)])
165 [`(else ,action ...) (seq->exp action)]
166 [`(,pred ,action ...) `(if ,pred
168 ,(cond->if (cdr clauses)))]))))
171 (define (let->combination lexpr)
172 (match-let* ([`(let ,bindings ,body ..1) lexpr]
173 [`((,var ,val) ...) bindings])
174 `((lambda ,var ,@body) ,@val)))
177 (define (let*->nested-lets lexpr)
179 [`(let* (,first-binding ,rest-bindings ...) ,body ..1)
180 `(let (,first-binding) ,(let*->nested-lets `(let* ,rest-bindings ,@body)))]
181 [`(let* () ,body ..1) `(let () ,@body)]))
183 ;; internal definitions
184 (define (scan-out-definitions body)
186 [`((define ,var ,e) ..1 ,rest)
187 `((let ,(map (lambda (v) (list (car v) ''*unassigned*)) var)
188 ,@(map (lambda (v e) `(set! ,(car v) (lambda ,(cdr v) ,e))) var e)
193 (define (eval exp env)
194 (display (format "~s~%" exp))
196 [(? self-evaluating? exp) exp]
197 [(? variable? exp) (lookup-variable-value exp env)]
199 [`(set! ,var ,val) (set-variable-value! var (eval val env) env)]
200 [`(define ,(? (lambda (x) (not (pair? x))) var) ,b) (define-variable! var (eval b env) env)]
201 [`(define ,(? pair? var) ,b ..1) (define-variable! (car var) (eval (make-lambda (cdr var) b) env) env)]
202 [`(if ,pred ,consequent ,alternative) (if (true? (eval pred env)) (eval consequent env) (eval alternative env))]
203 [`(lambda ,parameters ,body ..1) (make-procedure parameters body env)]
204 [`(begin ,exp ...) (eval-sequence exp env)]
205 [`(cond ,clauses ...) (eval (cond->if clauses) env)]
206 [`(let ,bindings ,body ..1) (eval (let->combination exp) env)]
207 [`(let* ,bindings ,body ..1) (eval (let*->nested-lets exp) env)]
208 [(list f x ...) (apply (eval f env) (list-of-values x env))]
209 [_ (error "unable to evaluate expression -- EVAL " exp)]))
213 (let loop ([input (read)]
214 [env the-global-environment])
215 (let ([output (eval input env)])
220 (let ([t1 (current-inexact-milliseconds)])
221 (eval expr the-global-environment)
222 (displayln (- (current-inexact-milliseconds) t1))))