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 (let ([value (hash-ref frame var (lambda () (lookup-variable-value var (enclosing-environment env))))])
65 (if (eq? value '*unassigned*)
66 (error "evaluating a variable that is not assigned a value -- " var)
69 (define (set-variable-value! var val env)
70 (if (eq? env the-empty-environment)
71 (error "unbound variable")
72 (let ([frame (first-frame env)])
73 (if (hash-has-key? frame var)
74 (hash-set! frame var val)
75 (set-variable-value! var val (enclosing-environment env))))))
77 (define (define-variable! var val env)
78 (let ([frame (first-frame env)])
79 (if (hash-has-key? frame var)
80 (hash-set! frame var val)
81 (add-binding-to-frame! var val frame))))
83 ;; primitive procedure
84 (define (primitive-implementation proc) (cadr proc))
86 (define primitive-procedures
98 (define (primitive-procedure-names)
99 (map car primitive-procedures))
101 (define (primitive-procedure-objects)
102 (map (lambda (proc) (list 'primitive (cadr proc)))
103 primitive-procedures))
105 (define (apply-primitive-procedure proc args)
106 (apply-in-underlying-scheme
107 (primitive-implementation proc) args))
110 (define (setup-environment)
112 (extend-environment (primitive-procedure-names)
113 (primitive-procedure-objects)
114 the-empty-environment)))
115 (define-variable! 'true true initial-env)
116 (define-variable! 'false false initial-env)
119 (define the-global-environment (setup-environment))
122 (define (list-of-values exps env)
123 (if (no-operands? exps)
125 (cons (eval (first-operand exps) env)
126 (list-of-values (rest-operands exps) env))))
128 (define (no-operands? ops) (null? ops))
129 (define (first-operand ops) (car ops))
130 (define (rest-operands ops) (cdr ops))
132 ;; compound procedure
133 (define (make-procedure params body env)
134 (list 'procedure params (scan-out-definitions body) env))
136 (define (procedure-parameters p) (cadr p))
137 (define (procedure-body p) (scan-out-definitions (caddr p)))
138 (define (procedure-environment p) (cadddr p))
140 (define (apply procedure arguments)
142 [`(primitive ,f ...) (apply-primitive-procedure procedure arguments)]
143 [`(procedure ,f ...) (eval-sequence
144 (procedure-body procedure)
146 (procedure-parameters procedure)
148 (procedure-environment procedure)))]
149 [_ (error "Unknown procedure type -- APPLY" procedure)]))
159 (define (cond->if clauses)
160 (define (seq->exp actions)
161 (if (empty? (cdr actions))
166 (let ([clause (car clauses)])
168 [`(else ,action ...) (seq->exp action)]
169 [`(,pred ,action ...) `(if ,pred
171 ,(cond->if (cdr clauses)))]))))
174 (define (let->combination lexpr)
175 (match-let* ([`(let ,bindings ,body ..1) lexpr]
176 [`((,var ,val) ...) bindings])
177 `((lambda ,var ,@body) ,@val)))
180 (define (let*->nested-lets lexpr)
182 [`(let* (,first-binding ,rest-bindings ...) ,body ..1)
183 `(let (,first-binding) ,(let*->nested-lets `(let* ,rest-bindings ,@body)))]
184 [`(let* () ,body ..1) `(let () ,@body)]))
186 ;; internal definitions
187 (define (scan-out-definitions body)
189 [`((define ,var ,e) ..1 ,rest)
190 `((let ,(map (lambda (v) (list (car v) ''*unassigned*)) var)
191 ,@(map (lambda (v e) `(set! ,(car v) (lambda ,(cdr v) ,e))) var e)
196 (define (letrec->combination lexpr)
198 [`(letrec (,bindings ...) ,body ..1)
199 `(let ,(map (lambda (v) (list (car v) ''*unassigned*)) bindings)
200 ,@(map (lambda (binding)
201 (let ([name (car binding)]
202 [value (cadr binding)])
203 `(set! ,name ,value)))
208 (define (eval exp env)
209 ;; use this display statement to visualize the recursive evaluation process
210 ;;(display (format "~s~%" exp))
212 [(? self-evaluating? exp) exp]
213 [(? variable? exp) (lookup-variable-value exp env)]
215 [`(set! ,var ,val) (set-variable-value! var (eval val env) env)]
216 [`(define ,(? (lambda (x) (not (pair? x))) var) ,b) (define-variable! var (eval b env) env)]
217 [`(define ,(? pair? var) ,b ..1) (define-variable! (car var) (eval (make-lambda (cdr var) b) env) env)]
218 [`(if ,pred ,consequent ,alternative) (if (true? (eval pred env)) (eval consequent env) (eval alternative env))]
219 [`(lambda ,parameters ,body ..1) (make-procedure parameters body env)]
220 [`(begin ,exp ...) (eval-sequence exp env)]
221 [`(cond ,clauses ...) (eval (cond->if clauses) env)]
222 [`(let ,bindings ,body ..1) (eval (let->combination exp) env)]
223 [`(let* ,bindings ,body ..1) (eval (let*->nested-lets exp) env)]
224 [`(letrec ,bindings ,body ..1) (eval (letrec->combination exp) env)]
225 [(list f x ...) (apply (eval f env) (list-of-values x env))]
226 [_ (error "unable to evaluate expression -- EVAL " exp)]))
230 (let loop ([input (read)]
231 [env the-global-environment])
232 (let ([output (eval input env)])
237 (let ([t1 (current-inexact-milliseconds)])
238 (eval expr the-global-environment)
239 (displayln (- (current-inexact-milliseconds) t1))))