1 ;; (define apply-in-underlying-scheme apply)
4 (cond ((self-evaluating? exp) exp)
5 ((variable? exp) (lookup-variable-value exp env))
6 ((quoted? exp) (text-of-quotation exp))
7 ((assignment? exp) (eval-assignment exp env))
8 ((definition? exp) (eval-definition exp env))
9 ;; ((unbound? exp) (eval-unbound exp env))
10 ((if? exp) (eval-if exp env))
11 ((and? exp) (eval-and exp env))
12 ((or? exp) (eval-or exp env))
14 (make-procedure (lambda-parameters exp)
18 (eval-sequence (begin-actions exp) env))
19 ((cond? exp) (eval (cond->if exp) env))
20 ((let? exp) (eval (let->combination exp) env))
21 ((let*? exp) (eval (let*->nested-lets exp) env))
22 ((named-let? exp) (eval (named-let->combination exp) env))
23 ((do? exp) (eval (do->combination exp) env))
25 (apply (eval (operator exp) env)
26 (list-of-values (operands exp) env)))
28 (error "Unknown expression type -- EVAL" exp))))
29 (define (apply procedure arguments)
30 (cond ((primitive-procedure? procedure)
31 (apply-primitive-procedure procedure arguments))
32 ((compound-procedure? procedure)
34 (procedure-body procedure)
36 (procedure-parameters procedure)
38 (procedure-environment procedure))))
41 "Unknown procedure type -- APPLY" procedure))))
43 (define (list-of-values exps env)
44 (if (no-operands? exps)
46 (cons (eval (first-operand exps) env)
47 (list-of-values (rest-operands exps) env))))
49 (define (tagged-list? exp tag)
54 ;; self-evaluating/variable/quoted
55 (define (self-evaluating? exp)
56 (cond ((number? exp) true)
59 (define (variable? exp) (symbol? exp))
61 (tagged-list? exp 'quote))
62 (define (text-of-quotation exp) (cadr exp))
64 ;; assignment/definition
65 (define (assignment? exp)
66 (tagged-list? exp 'set!))
67 (define (assignment-variable exp) (cadr exp))
68 (define (assignment-value exp) (caddr exp))
69 (define (make-assignment var val)
71 (define (definition? exp)
72 (tagged-list? exp 'define))
73 (define (definition-variable exp)
74 (if (symbol? (cadr exp))
77 (define (definition-value exp)
78 (if (symbol? (cadr exp))
80 (make-lambda (cdadr exp) ; formal parameters
82 (define (eval-assignment exp env)
83 (set-variable-value! (assignment-variable exp)
84 (eval (assignment-value exp) env)
87 (define (eval-definition exp env)
88 (define-variable! (definition-variable exp)
89 (eval (definition-value exp) env)
92 (define (make-definition var val)
97 ;; (define (unbound? exp)
98 ;; (tagged-list? exp 'make-unbound!))
99 ;; (define (unbound-var exp)
101 ;; (define (eval-unbound exp env)
102 ;; (remove-binding-from-frame! (unbound-var exp) (first-frame env)))
107 (define (if? exp) (tagged-list? exp 'if))
108 (define (if-predicate exp) (cadr exp))
109 (define (if-consequent exp) (caddr exp))
110 (define (if-alternative exp)
111 (if (not (null? (cdddr exp)))
114 (define (make-if predicate consequent alternative)
115 (list 'if predicate consequent alternative))
116 (define (eval-if exp env)
117 (if (true? (eval (if-predicate exp) env))
118 (eval (if-consequent exp) env)
119 (eval (if-alternative exp) env)))
122 (tagged-list? exp 'and))
123 (define (and-clauses exp)
126 (tagged-list? exp 'or))
127 (define (or-clauses exp)
129 (define (eval-and exp env)
130 (define (eval-clauses clauses)
131 (cond ((null? clauses) true)
132 ((null? (cdr clauses)) (eval (car clauses) env))
133 (else (and (eval (car clauses) env)
134 (eval-clauses (cdr clauses))))))
135 (eval-clauses (and-clauses exp)))
136 (define (eval-or exp env)
137 (define (eval-clauses clauses)
140 (or (eval (car clauses) env)
141 (eval-clauses (cdr clauses)))))
142 (eval-clauses (or-clauses exp)))
146 (define (lambda? exp) (tagged-list? exp 'lambda))
147 (define (lambda-parameters exp) (cadr exp))
148 (define (lambda-body exp) (cddr exp))
149 (define (make-lambda parameters body)
150 (cons 'lambda (cons parameters body)))
152 (define (make-let vars vals body)
154 (cons (map list vars vals)
157 (and (tagged-list? exp 'let)
158 (not (symbol? (cadr exp)))))
159 (define (let-vars exp)
160 (map car (cadr exp)))
161 (define (let-vals exp)
162 (map cadr (cadr exp)))
163 (define (let-body exp)
165 (define (let->combination exp)
166 (make-application (make-lambda (let-vars exp) (let-body exp))
168 (define (named-let? exp)
169 (and (tagged-list? exp 'let)
170 (symbol? (cadr exp))))
171 (define (named-let-name exp)
173 (define (named-let-vars exp)
174 (map car (caddr exp)))
175 (define (named-let-vals exp)
176 (map cadr (caddr exp)))
177 (define (named-let-body exp)
179 (define (named-let->combination exp)
181 (list (make-definition (named-let-name exp)
182 (make-lambda (named-let-vars exp)
183 (named-let-body exp)))
184 (make-application (named-let-name exp)
185 (named-let-vals exp)))))
186 (define (make-named-let name vars vals body)
189 (cons (map list vars vals)
192 (define (make-application op args)
196 (tagged-list? exp 'let*))
197 (define let*-vars let-vars)
198 (define let*-vals let-vals)
199 (define let*-body let-body)
200 (define (let*->nested-lets exp)
201 (define (expand-lets vars vals)
202 (if (null? (cdr vars))
203 (make-let (list (car vars))
206 (make-let (list (car vars))
208 (list (expand-lets (cdr vars) (cdr vals))))))
209 (let ((vars (let*-vars exp))
210 (vals (let*-vals exp)))
212 (sequence->exp (let*-body exp))
213 (expand-lets vars vals))))
217 (tagged-list? exp 'do))
218 (define (do-vars exp)
219 (map car (cadr exp)))
220 (define (do-inits exp)
221 (map cadr (cadr exp)))
222 (define (do-steps exp)
223 (map (lambda (var-init-step)
224 (if (null? (cddr var-init-step))
226 (caddr var-init-step)))
228 (define (do-test exp)
230 (define (do-expressions exp)
231 (if (null? (cdaddr exp))
234 (define (do-commands exp)
236 (define (do->combination exp)
244 (sequence->exp (do-expressions exp))
246 (append (do-commands exp)
247 (list (make-application
249 (do-steps exp)))))))))
253 (define (begin? exp) (tagged-list? exp 'begin))
254 (define (begin-actions exp) (cdr exp))
255 (define (last-exp? seq) (null? (cdr seq)))
256 (define (first-exp seq) (car seq))
257 (define (rest-exps seq) (cdr seq))
258 (define (sequence->exp seq)
259 (cond ((null? seq) seq)
260 ((last-exp? seq) (first-exp seq))
261 (else (make-begin seq))))
262 (define (make-begin seq) (cons 'begin seq))
263 (define (eval-sequence exps env)
264 (cond ((last-exp? exps) (eval (first-exp exps) env))
265 (else (eval (first-exp exps) env)
266 (eval-sequence (rest-exps exps) env))))
269 (define (application? exp) (pair? exp))
270 (define (operator exp) (car exp))
271 (define (operands exp) (cdr exp))
272 (define (no-operands? ops) (null? ops))
273 (define (first-operand ops) (car ops))
274 (define (rest-operands ops) (cdr ops))
277 (define (cond? exp) (tagged-list? exp 'cond))
278 (define (cond-clauses exp) (cdr exp))
279 (define (cond-else-clause? clause)
280 (eq? (cond-predicate clause) 'else))
281 (define (cond-predicate clause) (car clause))
282 (define (cond-actions clause) (cdr clause))
283 (define (cond-extended-clause? clause)
284 (and (not (null? (cdr clause))) (eq? (cadr clause) '=>)))
285 (define (cond-extended-proc clause)
287 (define (cond->if exp)
288 (expand-clauses (cond-clauses exp)))
289 (define (expand-clauses clauses)
291 'false ; no else clause
292 (let ((first (car clauses))
293 (rest (cdr clauses)))
294 (if (cond-else-clause? first)
296 (sequence->exp (cond-actions first))
297 (error "ELSE clause isn't last -- COND->IF"
299 (if (cond-extended-clause? first)
300 (make-if (cond-predicate first)
302 (cond-extended-proc first)
303 (list (cond-predicate first)))
304 (expand-clauses rest))
305 (make-if (cond-predicate first)
306 (sequence->exp (cond-actions first))
307 (expand-clauses rest)))))))
314 (define (make-procedure parameters body env)
315 (list 'procedure parameters (scan-out-defines body) env))
316 (define (scan-out-defines body)
317 (let* ((definitions (filter definition? body))
318 (vars (map definition-variable definitions))
319 (unassigneds (map (lambda (var) ''*unassigned*)
321 (vals (map definition-value definitions))
323 (map (lambda (var val)
324 (make-assignment var val))
326 (exps (remove definition? body)))
327 (if (null? definitions)
332 (append assignments exps))))))
333 (define (compound-procedure? p)
334 (tagged-list? p 'procedure))
335 (define (procedure-parameters p) (cadr p))
336 (define (procedure-body p) (caddr p))
337 (define (procedure-environment p) (cadddr p))
340 (define (enclosing-environment env) (cdr env))
341 (define (first-frame env) (car env))
342 (define the-empty-environment '())
343 (define (make-frame variables values)
344 (cons variables values))
345 (define (frame-variables frame) (car frame))
346 (define (frame-values frame) (cdr frame))
347 (define (add-binding-to-frame! var val frame)
348 (set-car! frame (cons var (car frame)))
349 (set-cdr! frame (cons val (cdr frame))))
350 (define (extend-environment vars vals base-env)
351 (if (= (length vars) (length vals))
352 (cons (make-frame vars vals) base-env)
353 (if (< (length vars) (length vals))
354 (error "Too many arguments supplied" vars vals)
355 (error "Too few arguments supplied" vars vals))))
356 (define (lookup-variable-value var env)
357 (define (env-loop env)
358 (define (scan vars vals)
360 (env-loop (enclosing-environment env)))
361 ((eq? var (car vars))
362 (let ((val (car vals)))
363 (if (eq? val '*unassigned*)
364 (error "Var not yet defined -- LOOKUP-VARIABLE-VALUE" var)
366 (else (scan (cdr vars) (cdr vals)))))
367 (if (eq? env the-empty-environment)
368 (error "Unbound variable" var)
369 (let ((frame (first-frame env)))
370 (scan (frame-variables frame)
371 (frame-values frame)))))
373 (define (set-variable-value! var val env)
374 (define (env-loop env)
375 (define (scan vars vals)
377 (env-loop (enclosing-environment env)))
378 ((eq? var (car vars))
380 (else (scan (cdr vars) (cdr vals)))))
381 (if (eq? env the-empty-environment)
382 (error "Unbound variable -- SET!" var)
383 (let ((frame (first-frame env)))
384 (scan (frame-variables frame)
385 (frame-values frame)))))
387 (define (define-variable! var val env)
388 (let ((frame (first-frame env)))
389 (define (scan vars vals)
391 (add-binding-to-frame! var val frame))
392 ((eq? var (car vars))
394 (else (scan (cdr vars) (cdr vals)))))
395 (scan (frame-variables frame)
396 (frame-values frame))))
398 (define (remove-binding-from-frame! var frame)
399 (define (scan vars vals)
400 (cond ((null? (cdr vars))
401 (error "Binding not found -- REMOVE-BINDING-FROM-FRAME!" var))
402 ((eq? var (cadr vars))
403 (set-cdr! vars (cddr vars))
404 (set-cdr! vals (cddr vals)))
405 (else (scan (cdr vars) (cdr vals)))))
406 (let ((vars (frame-variables frame))
407 (vals (frame-values frame)))
408 (if (eq? var (car vars))
409 (begin (set-car! frame (cdr vars))
410 (set-cdr! frame (cdr vals)))
414 (define (primitive-procedure? proc)
415 (tagged-list? proc 'primitive))
416 (define (primitive-implementation proc) (cadr proc))
417 (define primitive-procedures
418 (list (list 'car car)
434 (list 'remainder remainder)
436 (list 'equal? equal?)
437 (list 'display display)))
438 (define (primitive-procedure-names)
440 primitive-procedures))
441 (define (primitive-procedure-objects)
442 (map (lambda (proc) (list 'primitive (cadr proc)))
443 primitive-procedures))
444 (define (apply-primitive-procedure proc args)
445 (apply-in-underlying-scheme
446 (primitive-implementation proc) args))
449 (define input-prompt ";;; M-Eval input:")
450 (define output-prompt ";;; M-Eval value:")
451 (define (driver-loop)
452 (prompt-for-input input-prompt)
453 (let ((input (read)))
454 (let ((output (eval input the-global-environment)))
455 (announce-output output-prompt)
456 (user-print output)))
458 (define (prompt-for-input string)
459 (newline) (newline) (display string) (newline))
461 (define (announce-output string)
462 (newline) (display string) (newline))
463 (define (user-print object)
464 (if (compound-procedure? object)
465 (display (list 'compound-procedure
466 (procedure-parameters object)
467 (procedure-body object)
470 (define (setup-environment)
472 (extend-environment (primitive-procedure-names)
473 (primitive-procedure-objects)
474 the-empty-environment)))
475 (define-variable! 'true true initial-env)
476 (define-variable! 'false false initial-env)
478 (define the-global-environment (setup-environment))
481 (define (test-case actual expected)
486 (display "Expected: ")
489 (define (geval exp) ;; eval globally
490 (eval exp the-global-environment))
491 (define (test-eval exp expected)
492 (test-case (geval exp) expected))
495 ;; Exercise 4.17. Draw diagrams of the environment in effect when evaluating the expression <e3> in the procedure in the text, comparing how this will be structured when definitions are interpreted sequentially with how it will be structured if definitions are scanned out as described. Why is there an extra frame in the transformed program? Explain why this difference in environment structure can never make a difference in the behavior of a correct program. Design a way to make the interpreter implement the ``simultaneous'' scope rule for internal definitions without constructing the extra frame.
497 ;; there is one extra frame due to the let because:
499 ;; (let ((<var1> <exp1>)
504 ;; is syntactic sugar for
506 ;; ((lambda (<var1> <var2> ...)
508 ;; <exp1> <exp2> ...)
510 ;; we could move all the internal definitions to the top by scanning them out and then moving them to the top
516 ;; procedure definitions
519 '(define (assoc key records)
520 (cond ((null? records) false)
521 ((equal? key (caar records)) (car records))
522 (else (assoc key (cdr records))))))
525 '(define (map proc list)
528 (cons (proc (car list))
529 (map proc (cdr list))))))
532 '(define (accumulate op initial sequence)
536 (accumulate op initial (cdr sequence))))))
539 (test-eval '(begin 5 6) 6)
541 (geval '(define x 3))
543 (test-eval '(set! x -25) 'ok)
545 (geval '(define z (lambda (x y) (+ x (* x y)))))
546 (test-eval '(z 3 4) 15)
547 (test-eval '(cond ((= x -2) 'x=-2)
551 (test-eval '(if true false true) false)
559 (geval '(define x (+ 3 8)))
560 (test-eval '(and 0 true x) 11)
561 (test-eval '(and 0 true x false) false)
562 (test-eval '(and 0 true x (set! x -2) false) false)
564 (test-eval '(and 0 true x false (set! x -5)) false)
566 (test-eval '(or false (set! x 25)) 'ok)
568 (test-eval '(or (set! x 2) (set! x 4)) 'ok)
570 (test-eval '(or false (set! x 25) true false) 'ok)
572 (test-eval '(or ((lambda (x) x) 5)) 5)
573 (test-eval '(or (begin (set! x (+ x 1)) x)) 26)
579 '(cond ((assoc 'b '((a 1) (b 2))) => cadr)
584 '(cond ((= 3 4) 'not-true)
585 ((= (* 2 4) 3) 'also-false)
597 ;; procedure definition and application
599 '(define (factorial n)
602 (* n (factorial (- n 1))))))
603 (test-eval '(factorial 5) 120)
615 '(accumulate + 0 '(1 2 3 4 5))
620 (make-let '(x y) '(3 5) '((+ x y)))
647 (z (let ((a (* 3 2)))
675 (+ (let* ((x (+ y 2))
683 (a (let* ((x (* 3 x)))
699 '(let loop ((count 0))
706 (let prime-iter ((i 2))
707 (cond ((> (* i i) x) true)
708 ((= (remainder x i) 0) false)
709 (else (prime-iter (+ i 1)))))))
716 (primes (+ x 1) (- n 1))))
717 (else (primes (+ x 1) n))))
718 '(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71))
727 (fib-iter (+ a b) a (- count 1))))))
728 (test-eval '(fib 19) 4181)
753 '(do ((y '(1 2 3 4)))
764 '(let ((x '(1 3 5 7 9)))
766 (sum 0 (+ sum (car x))))
771 (do ((x '(1 2 3 4) (cdr x))
772 (y '(1 2 3 4 5 6 7 8) (cddr y)))
774 (set! z (cons (car x) z))))
780 ;; broken now due to scan-out-defines
798 ;; '(let ((y -1) (x 3))
799 ;; (let ((y 0.5) (x 5))
808 ;; '(let ((x 3) (y 4))
815 ;; '(let ((a 1) (b 2) (c 3) (d 4))
821 ;; '(let ((x 4) (y 5))
822 ;; (let ((a 1) (b 2) (c 3))
823 ;; (let ((x (+ a b)) (y (+ c a)))
825 ;; (let ((a x) (b (+ x y)))
851 (test-eval '(f 5) false)
852 (test-eval '(f 10) true)
859 ;; signal an error because x is undefined if variables are scanned out