1 ;; (define apply-in-underlying-scheme apply)
2 
3 (define (eval exp env)
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))
13         ((lambda? exp)
14          (make-procedure (lambda-parameters exp)
15                          (lambda-body exp)
16                          env))
17         ((begin? 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 	((letrec? exp) (eval (letrec->let exp) env))
24 	((do? exp) (eval (do->combination exp) env))
25         ((application? exp)
26          (apply (actual-value (operator exp) env)
27                 (operands exp)
28 		env))
29         (else
30          (error "Unknown expression type -- EVAL" exp))))
31 (define (apply procedure arguments env)
32   (cond ((primitive-procedure? procedure)
33          (apply-primitive-procedure 
34 	  procedure 
35 	  (list-of-arg-values arguments env)))
36         ((compound-procedure? procedure)
37          (eval-sequence
38            (procedure-body procedure)
39            (extend-environment
40              (procedure-parameters procedure)
41              (list-of-delayed-args arguments env)
42              (procedure-environment procedure))))
43         (else
44          (error
45           "Unknown procedure type -- APPLY" procedure))))
46 
47 (define (thunk? obj)
48   (tagged-list? obj 'thunk))
49 (define (thunk-exp thunk)
50   (cadr thunk))
51 (define (thunk-env thunk)
52   (caddr thunk))
53 (define (evaluated-thunk? obj)
54   (tagged-list? obj 'evaluated-thunk))
55 (define (thunk-value evaluated-thunk)
56   (cadr evaluated-thunk))
57 (define (delay-it exp env)
58   `(thunk ,exp ,env))
59 (define (actual-value exp env)
60   (force-it (eval exp env)))
61 (define (force-it obj)
62   (cond ((thunk? obj)
63 	 (let ((result (actual-value 
64 			(thunk-exp obj)
65 			(thunk-env obj))))
66 	   (set-car! obj 'evaluated-thunk)
67 	   (set-car! (cdr obj) result)
68 	   (set-cdr! (cdr obj) '())
69 	   result))
70 	((evaluated-thunk? obj)
71 	 (thunk-value obj))
72 	(else obj)))
73 
74 (define (list-of-arg-values exps env)
75   (if (no-operands? exps)
76       '()
77       (cons (actual-value (first-operand exps) env)
78             (list-of-arg-values (rest-operands exps) env))))
79 (define (list-of-delayed-args exps env)
80   (if (no-operands? exps)
81       '()
82       (cons (delay-it (first-operand exps) env)
83 	    (list-of-delayed-args (rest-operands exps) env))))
84 
85 (define (tagged-list? exp tag)
86   (if (pair? exp)
87       (eq? (car exp) tag)
88       false))
89 
90 ;; self-evaluating/variable/quoted
91 (define (self-evaluating? exp)
92   (cond ((number? exp) true)
93         ((string? exp) true)
94         (else false)))
95 (define (variable? exp) (symbol? exp))
96 (define (quoted? exp)
97   (tagged-list? exp 'quote))
98 (define (text-of-quotation exp) (cadr exp))
99 
100 ;; assignment/definition
101 (define (assignment? exp)
102   (tagged-list? exp 'set!))
103 (define (assignment-variable exp) (cadr exp))
104 (define (assignment-value exp) (caddr exp))
105 (define (make-assignment var val)
106   (list 'set! var val))
107 (define (definition? exp)
108   (tagged-list? exp 'define))
109 (define (definition-variable exp)
110   (if (symbol? (cadr exp))
111       (cadr exp)
112       (caadr exp)))
113 (define (definition-value exp)
114   (if (symbol? (cadr exp))
115       (caddr exp)
116       (make-lambda (cdadr exp)   ; formal parameters
117                    (cddr exp)))) ; body
118 (define (eval-assignment exp env)
119   (set-variable-value! (assignment-variable exp)
120                        (eval (assignment-value exp) env)
121                        env)
122   'ok)
123 (define (eval-definition exp env)
124   (define-variable! (definition-variable exp)
125                     (eval (definition-value exp) env)
126                     env)
127   'ok)
128 (define (make-definition var val)
129   `(define ,var ,val))
130 
131 ;; make-unbound!
132 
133 ;; (define (unbound? exp)
134 ;;   (tagged-list? exp 'make-unbound!))
135 ;; (define (unbound-var exp)
136 ;;   (cadr exp))
137 ;; (define (eval-unbound exp env)
138 ;;   (remove-binding-from-frame! (unbound-var exp) (first-frame env)))
139 
140 
141 
142 ;; if/and/or
143 (define (if? exp) (tagged-list? exp 'if))
144 (define (if-predicate exp) (cadr exp))
145 (define (if-consequent exp) (caddr exp))
146 (define (if-alternative exp)
147   (if (not (null? (cdddr exp)))
148       (cadddr exp)
149       'false))
150 (define (make-if predicate consequent alternative)
151   (list 'if predicate consequent alternative))
152 (define (eval-if exp env)
153   (if (true? (actual-value (if-predicate exp) env))
154       (eval (if-consequent exp) env)
155       (eval (if-alternative exp) env)))
156 
157 (define (and? exp)
158   (tagged-list? exp 'and))
159 (define (and-clauses exp)
160   (cdr exp))
161 (define (or? exp)
162   (tagged-list? exp 'or))
163 (define (or-clauses exp)
164   (cdr exp))
165 (define (eval-and exp env)
166   (define (eval-clauses clauses)
167     (cond ((null? clauses) true)
168 	  ((null? (cdr clauses)) (eval (car clauses) env))
169 	  (else (and (eval (car clauses) env)
170 		     (eval-clauses (cdr clauses))))))
171   (eval-clauses (and-clauses exp)))
172 (define (eval-or exp env)
173   (define (eval-clauses clauses)
174     (if (null? clauses)
175 	false
176 	(or (eval (car clauses) env)
177 	    (eval-clauses (cdr clauses)))))
178   (eval-clauses (or-clauses exp)))
179 
180 
181 ;; lambda/let/let*/letrec
182 (define (lambda? exp) (tagged-list? exp 'lambda))
183 (define (lambda-parameters exp) (cadr exp))
184 (define (lambda-body exp) (cddr exp))
185 (define (make-lambda parameters body)
186   (cons 'lambda (cons parameters body)))
187 
188 (define (make-let vars vals body)
189   (cons 'let
190 	(cons (map list vars vals)
191 	      body)))
192 (define (let? exp)
193   (and (tagged-list? exp 'let)
194        (not (symbol? (cadr exp)))))
195 (define (let-vars exp)
196   (map car (cadr exp)))
197 (define (let-vals exp)
198   (map cadr (cadr exp)))
199 (define (let-body exp)
200   (cddr exp))
201 (define (let->combination exp)
202   (make-application (make-lambda (let-vars exp) (let-body exp))
203 		    (let-vals exp)))
204 (define (named-let? exp)
205   (and (tagged-list? exp 'let)
206        (symbol? (cadr exp))))
207 (define (named-let-name exp)
208   (cadr exp))
209 (define (named-let-vars exp)
210   (map car (caddr exp)))
211 (define (named-let-vals exp)
212   (map cadr (caddr exp)))
213 (define (named-let-body exp)
214   (cdddr exp))
215 (define (named-let->combination exp)
216   (sequence->exp
217    (list (make-definition (named-let-name exp)
218 			  (make-lambda (named-let-vars exp)
219 				       (named-let-body exp)))
220 	 (make-application (named-let-name exp)
221 			   (named-let-vals exp)))))
222 (define (make-named-let name vars vals body)
223   (cons 'let 
224 	(cons name
225 	      (cons (map list vars vals)
226 		    body))))
227 
228 (define (letrec? exp)
229   (tagged-list? exp 'letrec))
230 
231 (define (letrec-vars exp)
232   (map car (cadr exp)))
233 (define (letrec-vals exp)
234   (map cadr (cadr exp)))
235 (define (letrec-body exp)
236   (cddr exp))
237 (define (letrec->let exp)
238   (let* ((vars (letrec-vars exp))
239 	 (unassigneds (map (lambda (var) ''*unassigned*)
240 			   vars))
241 	 (vals (letrec-vals exp))
242 	 (assignments (map (lambda (var val)
243 			     (make-assignment var val))
244 			   vars
245 			   vals))
246 	 (body (letrec-body exp)))
247   (make-let vars 
248 	    unassigneds
249 	    (append assignments body))))
250 
251 
252 
253 
254 (define (let*? exp)
255   (tagged-list? exp 'let*))
256 (define let*-vars let-vars)
257 (define let*-vals let-vals)
258 (define let*-body let-body)
259 (define (let*->nested-lets exp)
260   (define (expand-lets vars vals)
261     (if (null? (cdr vars))
262 	(make-let (list (car vars)) 
263 		  (list (car vals))
264 		  (let*-body exp))
265 	(make-let (list (car vars))
266 		  (list (car vals))
267 		  (list (expand-lets (cdr vars) (cdr vals))))))
268   (let ((vars (let*-vars exp))
269 	(vals (let*-vals exp)))
270     (if (null? vars)
271 	(sequence->exp (let*-body exp))
272 	(expand-lets vars vals))))
273 
274 ;; do loop
275 (define (do? exp)
276   (tagged-list? exp 'do))
277 (define (do-vars exp)
278   (map car (cadr exp)))
279 (define (do-inits exp)
280   (map cadr (cadr exp)))
281 (define (do-steps exp)
282   (map (lambda (var-init-step)
283 	 (if (null? (cddr var-init-step))
284 	     (car var-init-step)
285 	     (caddr var-init-step)))
286        (cadr exp)))
287 (define (do-test exp)
288   (caaddr exp))
289 (define (do-expressions exp)
290   (if (null? (cdaddr exp))
291       (caddr exp)
292       (cdaddr exp)))
293 (define (do-commands exp)
294   (cdddr exp))
295 (define (do->combination exp)
296   (make-named-let
297    'do-iter
298    (do-vars exp)
299    (do-inits exp)
300    (list
301     (make-if
302      (do-test exp)
303      (sequence->exp (do-expressions exp))
304      (sequence->exp
305       (append (do-commands exp)
306 	      (list (make-application
307 		     'do-iter
308 		     (do-steps exp)))))))))
309 
310 
311 ;; begin/sequence
312 (define (begin? exp) (tagged-list? exp 'begin))
313 (define (begin-actions exp) (cdr exp))
314 (define (last-exp? seq) (null? (cdr seq)))
315 (define (first-exp seq) (car seq))
316 (define (rest-exps seq) (cdr seq))
317 (define (sequence->exp seq)
318   (cond ((null? seq) seq)
319         ((last-exp? seq) (first-exp seq))
320         (else (make-begin seq))))
321 (define (make-begin seq) (cons 'begin seq))
322 (define (eval-sequence exps env)
323   (cond ((last-exp? exps) (eval (first-exp exps) env))
324         (else (eval (first-exp exps) env)
325               (eval-sequence (rest-exps exps) env))))
326 
327 ;; application
328 (define (make-application op args)
329   (cons op args))
330 (define (application? exp) (pair? exp))
331 (define (operator exp) (car exp))
332 (define (operands exp) (cdr exp))
333 (define (no-operands? ops) (null? ops))
334 (define (first-operand ops) (car ops))
335 (define (rest-operands ops) (cdr ops))
336 
337 ;; cond
338 (define (cond? exp) (tagged-list? exp 'cond))
339 (define (cond-clauses exp) (cdr exp))
340 (define (cond-else-clause? clause)
341   (eq? (cond-predicate clause) 'else))
342 (define (cond-predicate clause) (car clause))
343 (define (cond-actions clause) (cdr clause))
344 (define (cond-extended-clause? clause)
345   (and (not (null? (cdr clause))) (eq? (cadr clause) '=>)))
346 (define (cond-extended-proc clause)
347   (caddr clause))
348 (define (cond->if exp)
349   (expand-clauses (cond-clauses exp)))
350 (define (expand-clauses clauses)
351   (if (null? clauses)
352       'false                          ; no else clause
353       (let ((first (car clauses))
354             (rest (cdr clauses)))
355         (if (cond-else-clause? first)
356             (if (null? rest)
357                 (sequence->exp (cond-actions first))
358                 (error "ELSE clause isn't last -- COND->IF"
359                        clauses))
360 	    (if (cond-extended-clause? first)
361 		(make-if (cond-predicate first)
362 			 (make-application
363 			  (cond-extended-proc first)
364 			  (list (cond-predicate first)))
365 			 (expand-clauses rest))
366 		(make-if (cond-predicate first)
367 			 (sequence->exp (cond-actions first))
368 			 (expand-clauses rest)))))))
369 (define (true? x)
370   (not (eq? x false)))
371 (define (false? x)
372   (eq? x false))
373 
374 ;; procedure
375 (define (make-procedure parameters body env)
376   (list 'procedure parameters body env))
377 ;; (define (scan-out-defines body)
378 ;;   (let* ((definitions (filter definition? body))
379 ;; 	 (vars (map definition-variable definitions))
380 ;; 	 (unassigneds (map (lambda (var) ''*unassigned*) 
381 ;; 			   vars))
382 ;; 	 (vals (map definition-value definitions))
383 ;; 	 (assignments 
384 ;; 	  (map (lambda (var val)
385 ;; 		 (make-assignment var val))
386 ;; 	       vars vals))
387 ;; 	 (exps (remove definition? body)))
388 ;;     (if (null? definitions)
389 ;; 	body
390 ;; 	(list
391 ;; 	 (make-let vars
392 ;; 		   unassigneds
393 ;; 		   (append assignments exps))))))
394 (define (compound-procedure? p)
395   (tagged-list? p 'procedure))
396 (define (procedure-parameters p) (cadr p))
397 (define (procedure-body p) (caddr p))
398 (define (procedure-environment p) (cadddr p))
399 
400 ;; environment
401 (define (enclosing-environment env) (cdr env))
402 (define (first-frame env) (car env))
403 (define the-empty-environment '())
404 (define (make-frame variables values)
405   (cons variables values))
406 (define (frame-variables frame) (car frame))
407 (define (frame-values frame) (cdr frame))
408 (define (add-binding-to-frame! var val frame)
409   (set-car! frame (cons var (car frame)))
410   (set-cdr! frame (cons val (cdr frame))))
411 (define (extend-environment vars vals base-env)
412   (if (= (length vars) (length vals))
413       (cons (make-frame vars vals) base-env)
414       (if (< (length vars) (length vals))
415           (error "Too many arguments supplied" vars vals)
416           (error "Too few arguments supplied" vars vals))))
417 (define (lookup-variable-value var env)
418   (define (env-loop env)
419     (define (scan vars vals)
420       (cond ((null? vars)
421              (env-loop (enclosing-environment env)))
422             ((eq? var (car vars))
423 	     (let ((val (car vals)))
424 	       (if (eq? val '*unassigned*)
425 		   (error "Var not yet defined -- LOOKUP-VARIABLE-VALUE" var)
426 		   val)))
427             (else (scan (cdr vars) (cdr vals)))))
428     (if (eq? env the-empty-environment)
429         (error "Unbound variable" var)
430         (let ((frame (first-frame env)))
431           (scan (frame-variables frame)
432                 (frame-values frame)))))
433   (env-loop env))
434 (define (set-variable-value! var val env)
435   (define (env-loop env)
436     (define (scan vars vals)
437       (cond ((null? vars)
438              (env-loop (enclosing-environment env)))
439             ((eq? var (car vars))
440              (set-car! vals val))
441             (else (scan (cdr vars) (cdr vals)))))
442     (if (eq? env the-empty-environment)
443         (error "Unbound variable -- SET!" var)
444         (let ((frame (first-frame env)))
445           (scan (frame-variables frame)
446                 (frame-values frame)))))
447   (env-loop env))
448 (define (define-variable! var val env)
449   (let ((frame (first-frame env)))
450     (define (scan vars vals)
451       (cond ((null? vars)
452              (add-binding-to-frame! var val frame))
453             ((eq? var (car vars))
454              (set-car! vals val))
455             (else (scan (cdr vars) (cdr vals)))))
456     (scan (frame-variables frame)
457           (frame-values frame))))
458 
459 (define (remove-binding-from-frame! var frame)
460   (define (scan vars vals)
461     (cond ((null? (cdr vars))
462 	   (error "Binding not found -- REMOVE-BINDING-FROM-FRAME!" var))
463 	  ((eq? var (cadr vars))
464 	   (set-cdr! vars (cddr vars))
465 	   (set-cdr! vals (cddr vals)))
466 	  (else (scan (cdr vars) (cdr vals)))))
467   (let ((vars (frame-variables frame))
468 	(vals (frame-values frame)))
469     (if (eq? var (car vars))
470 	(begin (set-car! frame (cdr vars))
471 	       (set-cdr! frame (cdr vals)))
472 	(scan vars vals))))
473 
474 ;; primitives
475 (define (primitive-procedure? proc)
476   (tagged-list? proc 'primitive))
477 (define (primitive-implementation proc) (cadr proc))
478 (define primitive-procedures
479   (list (list 'car car)
480         (list 'cdr cdr)
481 	(list 'caar caar)
482 	(list 'cadr cadr)
483 	(list 'cddr cddr)
484         (list 'cons cons)
485         (list 'null? null?)
486 	(list '* *)
487 	(list '/ /)
488 	(list '+ +)
489 	(list '- -)
490 	(list '= =)
491 	(list '< <)
492 	(list '> >)
493 	(list '<= <=)
494 	(list '>= >=)
495 	(list 'remainder remainder)
496 	(list 'eq? eq?)
497 	(list 'equal? equal?)
498 	(list 'display display)))
499 (define (primitive-procedure-names)
500   (map car
501        primitive-procedures))
502 (define (primitive-procedure-objects)
503   (map (lambda (proc) (list 'primitive (cadr proc)))
504        primitive-procedures))
505 (define (apply-primitive-procedure proc args)
506   (apply-in-underlying-scheme
507    (primitive-implementation proc) args))
508 
509 ;; driver-loop
510 (define input-prompt ";;; M-Eval input:")
511 (define output-prompt ";;; M-Eval value:")
512 (define (driver-loop)
513   (prompt-for-input input-prompt)
514   (let ((input (read)))
515     (let ((output (actual-value input the-global-environment)))
516       (announce-output output-prompt)
517       (user-print output)))
518   (driver-loop))
519 (define (prompt-for-input string)
520   (newline) (newline) (display string) (newline))
521 
522 (define (announce-output string)
523   (newline) (display string) (newline))
524 (define (user-print object)
525   (if (compound-procedure? object)
526       (display (list 'compound-procedure
527                      (procedure-parameters object)
528                      (procedure-body object)
529                      '<procedure-env>))
530       (display object)))
531 (define (setup-environment)
532   (let ((initial-env
533          (extend-environment (primitive-procedure-names)
534                              (primitive-procedure-objects)
535                              the-empty-environment)))
536     (define-variable! 'true true initial-env)
537     (define-variable! 'false false initial-env)
538     initial-env))
539 (define the-global-environment (setup-environment))
540 
541 ;; auxiliary
542 (define (test-case actual expected)
543   (newline)
544   (display "Actual:   ")
545   (display actual)
546   (newline)
547   (display "Expected: ")
548   (display expected)
549   (newline))
550 (define (geval exp) ;; eval globally
551   (eval exp the-global-environment))
552 (define (test-eval exp expected)
553   (test-case (force-it (geval exp)) expected))
554 
555 
556 ;; delayed-evaluation
557 
558 (geval
559  '(define (try a b)
560     (if (= a 0) 1 b)))
561 (test-eval '(try 0 (/ 1 0)) 1)
562 
563 (geval 
564  '(define (unless condition usual-value exceptional-value)
565     (if condition exceptional-value usual-value)))
566 (test-eval
567  '(let ((a 4) (b 0))
568     (unless (= b 0)
569             (/ a b)
570 	    (begin (display "exception: returning 0")
571 		   0)))
572  0)
573 (test-eval
574   '(let ((a 4) (b 2))
575     (unless (= b 0)
576             (/ a b)
577 	    (begin (display "exception: returning 0")
578 		   0)))
579   2)
580 
581 (geval
582  '(define (factorial n)
583     (unless (= n 1)
584             (* n (factorial (- n 1)))
585 	    1)))
586 (test-eval
587  '(factorial 8)
588  40320)
589 
590 (geval '(define count 0))
591 (geval '(define (id x)
592 	  (set! count (+ count 1))
593 	  x))
594 
595 ;; Give the missing values in the following sequence of interactions, and explain your answers.38
596 
597 (geval '(define w (id (id 10))))
598 (test-eval 'count 1)
599 (test-eval 'w 10)
600 (test-eval 'count 2)
601 
602 ;; test-suite
603 
604 ;; procedure definitions
605 
606 (geval 
607  '(define (assoc key records)
608     (cond ((null? records) false)
609 	  ((equal? key (caar records)) (car records))
610 	  (else (assoc key (cdr records))))))
611 
612 (geval
613  '(define (map proc list)
614     (if (null? list)
615 	'()
616 	(cons (proc (car list))
617 	      (map proc (cdr list))))))
618 
619 (geval 
620  '(define (accumulate op initial sequence)
621     (if (null? sequence)
622 	initial
623 	(op (car sequence)
624 	    (accumulate op initial (cdr sequence))))))
625 
626 ;; all special forms
627 (test-eval '(begin 5 6) 6)
628 (test-eval '10 10)
629 (geval '(define x 3))
630 (test-eval 'x 3)
631 (test-eval '(set! x -25) 'ok)
632 (test-eval 'x -25)
633 (geval '(define z (lambda (x y) (+ x (* x y)))))
634 (test-eval '(z 3 4) 15)
635 (test-eval '(cond ((= x -2) 'x=-2)
636 		  ((= x -25) 'x=-25)
637 		  (else 'failed))
638 	   'x=-25)
639 (test-eval '(if true false true) false)
640 
641 (test-eval 
642  '(let ((x 4) (y 7))
643     (+ x y (* x y)))
644  (+ 4 7 (* 4 7)))
645 
646 
647 ;; and/or
648 (geval '(define x (+ 3 8)))
649 (test-eval '(and 0 true x) 11)
650 (test-eval '(and 0 true x false) false)
651 (test-eval '(and 0 true x (set! x -2) false) false)
652 (test-eval 'x -2)
653 (test-eval '(and 0 true x false (set! x -5)) false)
654 (test-eval 'x -2)
655 (test-eval '(or false (set! x 25)) 'ok)
656 (test-eval 'x 25)
657 (test-eval '(or (set! x 2) (set! x 4)) 'ok)
658 (test-eval 'x 2)
659 (test-eval '(or false (set! x 25) true false) 'ok)
660 (test-eval 'x 25)
661 (test-eval '(or ((lambda (x) x) 5)) 5)
662 (test-eval '(or (begin (set! x (+ x 1)) x)) 26)
663 
664 
665 ;; cond
666 
667 (test-eval 
668  '(cond ((assoc 'b '((a 1) (b 2))) => cadr)
669 	(else false))
670  2)
671 
672 (test-eval
673  '(cond ((= 3 4) 'not-true)
674 	((= (* 2 4) 3) 'also-false)
675 	((map (lambda (x)
676 		(* x (+ x 1)))
677 	      '(2 4 1 9))
678 	 =>
679 	 (lambda (x)
680 	   (accumulate + 0 x)))
681 	(else 'never-reach))
682  118)
683 ;; '(6 20 2 90)
684 
685 
686 ;; procedure definition and application
687 (geval
688  '(define (factorial n)
689     (if (= n 0)
690 	1
691 	(* n (factorial (- n 1))))))
692 (test-eval '(factorial 5) 120)
693 
694 ;; map
695 
696 (test-eval
697  '(map (lambda (x)
698 	 (* x (+ x 1)))
699        '(2 1 4 2 8 3))
700  '(6 2 20 6 72 12))
701 ;; accumulate
702 
703 (test-eval
704  '(accumulate + 0 '(1 2 3 4 5))
705  15)
706 
707 ;; make-let
708 (test-eval 
709  (make-let '(x y) '(3 5) '((+ x y)))
710  8)
711 (test-eval 
712  '(let ()
713     5)
714  5)
715 (test-eval
716  '(let ((x 3))
717     x)
718  3)
719 (test-eval
720  '(let ((x 3)
721 	(y 5))
722     (+ x y))
723  8)
724 (test-eval 
725  '(let ((x 3)
726 	(y 2))
727     (+ (let ((x (+ y 2))
728 	     (y x))
729 	 (* x y))
730        x y))
731  (+ (* 4 3) 3 2))
732 (test-eval
733  '(let ((x 6)
734 	(y (let ((x 2))
735 	     (+ x 3)))
736 	(z (let ((a (* 3 2)))
737 	     (+ a 3))))
738     (+ x y z))
739  (+ 6 5 9))
740     
741 
742 ;; let*
743 
744 (test-eval
745  '(let* ((x 3)
746 	 (y (+ x 2))
747 	 (z (+ x y 5)))
748     (* x z))
749  39)
750 
751 (test-eval
752  '(let* ()
753     5)
754  5)
755 (test-eval
756  '(let* ((x 3))
757     (let* ((y 5))
758       (+ x y)))
759  8)
760 
761 (test-eval 
762  '(let* ((x 3)
763 	 (y (+ x 1)))
764     (+ (let* ((x (+ y 2))
765 	      (y x))
766 	 (* x y))
767        x y))
768  (+ (* 6 6) 3 4))
769 (test-eval
770  '(let* ((x 6)
771 	 (y (let* ((x 2)
772 		   (a (let* ((x (* 3 x)))
773 			(+ x 2))))       
774 	      (+ x a)))                  
775 	 (z (+ x y)))                    
776     (+ x y z))
777  32)
778 
779 ;; named-let
780 
781 (test-eval
782  '(let eight ()
783     5
784     7
785     8)
786  8)
787 (test-eval
788  '(let loop ((count 0))
789     (if (= 100 count)
790 	count
791 	(loop (+ count 1))))
792  100)
793 (geval
794  '(define (prime? x)
795     (let prime-iter ((i 2))
796       (cond ((> (* i i) x) true)
797 	    ((= (remainder x i) 0) false)
798 	    (else (prime-iter (+ i 1)))))))
799 (test-eval
800  '(let primes ((x 2)
801 	       (n 20))
802     (cond ((= n 0) '())
803 	  ((prime? x) 
804 	   (cons x
805 		 (primes (+ x 1) (- n 1))))
806 	  (else (primes (+ x 1) n))))
807  '(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71))
808 
809 (geval
810  '(define (fib n)
811     (let fib-iter ((a 1)
812 		   (b 0)
813 		   (count n))
814       (if (= count 0)
815 	  b
816 	  (fib-iter (+ a b) a (- count 1))))))
817 (test-eval '(fib 19) 4181)
818 
819 ;; do-loop	  
820 (test-eval
821  '(let ((y 0))
822     (do ((x 0 (+ x 1)))
823 	((= x 5) y)
824       (set! y (+ y 1))))
825  5)
826 (test-eval
827  '(do ()
828       (true))
829  true)
830 (test-eval
831  '(do ()
832       (true 5))
833  5)
834 (test-eval
835  '(let ((y 0))
836     (do ()
837 	((= y 5) y)
838       (set! y (+ y 1))))
839  5)
840 
841 (test-eval
842  '(do ((y '(1 2 3 4)))
843       ((null? y))
844     (set! y (cdr y)))
845  true)
846 (test-eval
847  '(let ((y 0))
848     (do ((x 0 (+ x 1)))
849 	((= x 5) y)
850       (set! y (+ y 1))))
851  5)
852 (test-eval
853  '(let ((x '(1 3 5 7 9)))
854     (do ((x x (cdr x))
855 	 (sum 0 (+ sum (car x))))
856 	((null? x) sum)))
857  25)
858 (test-eval 
859  '(let ((z '()))
860     (do ((x '(1 2 3 4) (cdr x))
861 	 (y '(1 2 3 4 5 6 7 8) (cddr y)))
862 	((null? x) y x z)
863       (set! z (cons (car x) z))))
864  '(4 3 2 1))
865 
866 
867 
868 ;; make-unbound!
869 ;; broken now due to scan-out-defines
870 
871 ;; (test-eval
872 ;;  '(let ((x 3))
873 ;;     (let ((x 5))
874 ;;       (make-unbound! x)
875 ;;       (* x x)))
876 ;;  9)
877 
878 ;; (test-eval
879 ;;  '(let ((x 3))
880 ;;     (let ((x 5))
881 ;;       (define y x)
882 ;;       (make-unbound! x)
883 ;;       (* y x)))
884 ;;  15)
885 
886 ;; (test-eval
887 ;;  '(let ((y -1) (x 3))
888 ;;     (let ((y 0.5) (x 5))
889 ;;       (define a x)
890 ;;       (define b y)
891 ;;       (make-unbound! x)
892 ;;       (make-unbound! y)
893 ;;       (* a b x y)))
894 ;;  (* 5 3 -1 0.5))
895 
896 ;; (test-eval
897 ;;  '(let ((x 3) (y 4))
898 ;;     (let ((x 5))
899 ;;       (make-unbound! x)
900 ;;       (+ x 4)))
901 ;;  7)
902 
903 ;; (test-eval 
904 ;;  '(let ((a 1) (b 2) (c 3) (d 4))
905 ;;     (make-unbound! b)
906 ;;     (+ a c d))
907 ;;  (+ 1 3 4))
908 
909 ;; (test-eval
910 ;;  '(let ((x 4) (y 5))
911 ;;     (let ((a 1) (b 2) (c 3))
912 ;;       (let ((x (+ a b)) (y (+ c a)))
913 ;; 	(make-unbound! x)
914 ;; 	(let ((a x) (b (+ x y)))
915 ;; 	  (define z b)
916 ;; 	  (make-unbound! b)
917 ;; 	  (* (+ a z)
918 ;; 	     (+ a b y))))))
919 ;;  (* (+ 4 8)
920 ;;     (+ 4 2 4)))
921  
922 ;; x 3 -- y 4
923 ;; x 4 -- y 4
924 ;; a 4 -- b 4
925 ;; a 4 -- b 2
926 
927 ;; scan-out-defines
928 
929 (geval
930  '(define (f x)
931     (define (even? n)
932       (if (= n 0)
933 	  true
934 	  (odd? (- n 1))))
935     (define (odd? n)
936       (if (= n 0)
937 	  false
938 	  (even? (- n 1))))
939     (even? x)))
940 (test-eval '(f 5) false)
941 (test-eval '(f 10) true)
942 
943 ;; (geval
944 ;;  '(let ((x 5))
945 ;;     (define y x)
946 ;;     (define x 3)
947 ;;     (+ x y)))
948 ;; signal an error because x is undefined if variables are scanned out
949 
950 ;; letrec
951 
952 (geval
953  '(define (f x)
954     (letrec ((even?
955 	      (lambda (n)
956 		(if (= n 0)
957 		    true
958 		    (odd? (- n 1)))))
959 	     (odd?
960 	      (lambda (n)
961 		(if (= n 0)
962 		    false
963 		    (even? (- n 1))))))
964       (even? x))))
965 (test-eval '(f 11) false)
966 (test-eval '(f 16) true)
967 
968 (test-eval 
969  '(letrec ((fact
970 	    (lambda (n)
971 	      (if (= n 1)
972 		  1
973 		  (* n (fact (- n 1)))))))
974     (fact 10))
975  3628800)