exercise-7.scm (20486B)
1 (define-library (sicp solutions chapter-4 exercise-7) 2 (export 3 eval 4 main 5 6 run-internal-tests 7 8 make-environment 9 setup-environment 10 extend-environment 11 ) 12 (import 13 (rename (scheme base) (apply scheme:base:apply)) 14 (scheme cxr) 15 (scheme read) 16 (scheme write) 17 18 (srfi srfi-1) 19 (srfi srfi-9) 20 (srfi srfi-64) 21 22 (system vm trace) 23 24 (ice-9 match) 25 (ice-9 pretty-print) 26 27 (sicp utils) 28 ) 29 30 (begin 31 ;; XXX: In 4.1.3 we see that the procedure `true?` is defined 32 ;; using the host-variable `false` whose value is 'false, but we 33 ;; don't see it defined anywhere in the good book, so we need to 34 ;; define the host-variable false ourselves. 35 ;; 36 ;; In MIT Scheme, the one the SICP people seem to love to use for 37 ;; SICP, the values of `false` and `true` are #f and #t, #f being 38 ;; the only value that is falsituous and #t there for having a 39 ;; truthious value that has no other meaning other than being 40 ;; truthitiousic. 41 ;; 42 ;; ``` 43 ;; # `scheme` is the MIT Scheme executable: 44 ;; $ guix shell mit-scheme -- scheme 45 ;; MIT/GNU Scheme running under GNU/Linux 46 ;; Type `^C' (control-C) followed by `H' to obtain information about interrupts. 47 ;; 48 ;; Copyright (C) 2020 Massachusetts Institute of Technology 49 ;; This is free software; see the source for copying conditions. There is NO warranty; not even for 50 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 51 ;; 52 ;; Image saved on Sunday March 7, 2021 at 3:24:56 PM 53 ;; Release 11.2 || SF || LIAR/x86-64 54 ;; 55 ;; 1 ]=> false 56 ;; 57 ;; ;Value: #f 58 ;; 59 ;; 1 ]=> true 60 ;; 61 ;; ;Value: #t 62 ;; 63 ;; 1 ]=> 64 ;; ``` 65 (define false #f) 66 (define true #t) 67 68 (define (make-environment vars vals) 69 "A comfort procedure because calling extend on the-empty-environment is a PITA." 70 (extend-environment vars 71 vals 72 the-empty-environment)) 73 74 ;; XXX: As described in chapter 4.1.4 note 2, we need to name the 75 ;; host `apply`, and the guest `apply` differently. I define the 76 ;; host's `apply` to be the name used in the book. 77 (define apply-in-underlying-scheme scheme:base:apply) 78 79 ;;; Start: 80 81 ;;; 4.1.1 82 83 (define *form-table* '()) 84 85 (define (add-form-handler! form? handler) 86 (set! *form-table* 87 (cons (cons form? 88 handler) 89 *form-table*))) 90 91 (define (get-form-handler exp) 92 (let loop ((table *form-table*)) 93 (match table 94 ('() 95 (error "Unknown form" exp)) 96 (`((,form? . ,handler) . ,rest) 97 (if (form? exp) 98 handler 99 (loop rest)))))) 100 101 (define (eval exp env) 102 (cond ((self-evaluating? exp) 103 exp) 104 ((variable? exp) 105 (lookup-variable-value exp env)) 106 ((pair? exp) 107 ((get-form-handler exp) exp env)) 108 (else 109 (error "Unknown expression type -- EVAL" exp)))) 110 111 (define (apply procedure arguments) 112 (cond ((primitive-procedure? procedure) 113 (apply-primitive-procedure 114 procedure 115 arguments)) 116 ((compound-procedure? procedure) 117 (eval-sequence 118 (procedure-body procedure) 119 (extend-environment 120 (procedure-parameters 121 procedure) 122 arguments 123 (procedure-environment 124 procedure)))) 125 (else 126 (error "Unknown procedure type -- APPLY" 127 procedure)))) 128 129 ;;; Procedure arguments: 130 131 (define (list-of-values exps env) 132 (if (no-operands? exps) 133 '() 134 (cons (eval (first-operand exps) env) 135 (list-of-values (rest-operands exps) env)))) 136 137 ;;; Conditionals: 138 139 (define (eval-if exp env) 140 (if (true? (eval (if-predicate exp) env)) 141 (eval (if-consequent exp) env) 142 (eval (if-alternative exp) env))) 143 144 ;;; Sequences: 145 146 (define (eval-sequence exps env) 147 (cond ((last-exp? exps) 148 (eval (first-exp exps) env)) 149 (else 150 (eval (first-exp exps) env) 151 (eval-sequence (rest-exps exps) 152 env)))) 153 154 ;;; Assignments and definitions 155 156 (define (eval-assignment exp env) 157 (set-variable-value! 158 (assignment-variable exp) 159 (eval (assignment-value exp) env) 160 env) 161 'ok) 162 163 (define (eval-definition exp env) 164 (define-variable! 165 (definition-variable exp) 166 (eval (definition-value exp) env) 167 env) 168 'ok) 169 170 ;;; end 4.1.1 171 172 ;;; 4.1.2 173 174 175 (define (self-evaluating? exp) 176 (cond ((number? exp) true) 177 ((string? exp) true) 178 (else false))) 179 180 (define (variable? exp) (symbol? exp)) 181 182 (define (quoted? exp) 183 (tagged-list? exp 'quote)) 184 185 (define (text-of-quotation exp) 186 (cadr exp)) 187 188 (define (tagged-list? exp tag) 189 (if (pair? exp) 190 (eq? (car exp) tag) 191 false)) 192 193 (define (assignment? exp) 194 (tagged-list? exp 'set!)) 195 196 (define (assignment-variable exp) 197 (cadr exp)) 198 199 (define (assignment-value exp) (caddr exp)) 200 201 (define (definition? exp) 202 (tagged-list? exp 'define)) 203 204 (define (definition-variable exp) 205 (if (symbol? (cadr exp)) 206 (cadr exp) 207 (caadr exp))) 208 209 (define (definition-value exp) 210 (if (symbol? (cadr exp)) 211 (caddr exp) 212 (make-lambda 213 (cdadr exp) ; formal-parameters 214 (cddr exp)))) ; body 215 216 (define (lambda? exp) 217 (tagged-list? exp 'lambda)) 218 (define (lambda-parameters exp) (cadr exp)) 219 (define (lambda-body exp) (cddr exp)) 220 221 (define (make-lambda parameters body) 222 (cons 'lambda (cons parameters body))) 223 224 (define (if? exp) (tagged-list? exp 'if)) 225 (define (if-predicate exp) (cadr exp)) 226 (define (if-consequent exp) (caddr exp)) 227 (define (if-alternative exp) 228 (if (not (null? (cdddr exp))) 229 (cadddr exp) 230 'false)) 231 232 (define (make-if predicate 233 consequent 234 alternative) 235 (list 'if 236 predicate 237 consequent 238 alternative)) 239 240 (define (begin? exp) 241 (tagged-list? exp 'begin)) 242 (define (begin-actions exp) (cdr exp)) 243 (define (last-exp? seq) (null? (cdr seq))) 244 (define (first-exp seq) (car seq)) 245 (define (rest-exps seq) (cdr seq)) 246 247 (define (sequence->exp seq) 248 (cond ((null? seq) seq) 249 ((last-exp? seq) (first-exp seq)) 250 (else (make-begin seq)))) 251 252 (define (make-begin seq) (cons 'begin seq)) 253 254 (define (application? exp) (pair? exp)) 255 (define (operator exp) (car exp)) 256 (define (operands exp) (cdr exp)) 257 (define (no-operands? ops) (null? ops)) 258 (define (first-operand ops) (car ops)) 259 (define (rest-operands ops) (cdr ops)) 260 261 (define (cond? exp) 262 (tagged-list? exp 'cond)) 263 (define (cond-clauses exp) (cdr exp)) 264 (define (cond-else-clause? clause) 265 (eq? (cond-predicate clause) 'else)) 266 (define (cond-predicate clause) 267 (car clause)) 268 (define (cond-actions clause) 269 (cdr clause)) 270 (define (cond->if exp) 271 (expand-clauses (cond-clauses exp))) 272 (define (expand-clauses clauses) 273 (match clauses 274 ('() 'false) 275 ((('else actions ...)) 276 (sequence->exp actions)) 277 ((('else actions ...) rest ...) 278 (error "ELSE clause isn't last: COND->IF" 279 clauses)) 280 (((predicate '=> recipient) rest ...) 281 `((lambda (value) 282 (if value 283 (,recipient value) 284 ,(expand-clauses rest))) 285 ,predicate)) 286 (((predicate actions ...) rest ...) 287 (make-if predicate 288 (sequence->exp actions) 289 (expand-clauses rest))) 290 (otherwise 291 (error "-- EXPAND-CLAUSES" clauses)))) 292 293 (define (let->combination exp) 294 (match exp 295 (('let (var-exps ...) 296 body ...) 297 (let ((vars (map car var-exps)) 298 (exps (map cadr var-exps))) 299 `((lambda ,vars 300 ,@body) 301 ,@exps))))) 302 303 (define (let*->nested-lets exp) 304 (match exp 305 (('let* () 306 body ...) 307 (make-begin body)) 308 (`(let* ((,var ,exp) . ,rest-of-var-exps) 309 . ,body) 310 `(let ((,var ,exp)) 311 ,(let*->nested-lets 312 `(let* ,rest-of-var-exps 313 ,@body)))))) 314 315 ;;; end 4.1.2 316 317 ;;; 4.1.3 318 319 ;; XXX: By default, false's value is #f, which is the only true 320 ;; false. All others are fake and would have the truthitousness 321 ;; property. Consult `info r7rs-small` or `C-h 322 (define (true? x) 323 (not (eq? x false))) 324 325 (define (false? x) 326 (eq? x false)) 327 328 (define (make-procedure parameters body env) 329 (list 'procedure parameters body env)) 330 (define (compound-procedure? p) 331 (tagged-list? p 'procedure)) 332 (define (procedure-parameters p) (cadr p)) 333 (define (procedure-body p) (caddr p)) 334 (define (procedure-environment p) (cadddr p)) 335 336 (define (enclosing-environment env) (cdr env)) 337 (define (first-frame env) (car env)) 338 (define the-empty-environment '()) 339 340 (define (make-frame variables values) 341 (cons variables values)) 342 (define (frame-variables frame) (car frame)) 343 (define (frame-values frame) (cdr frame)) 344 (define (add-binding-to-frame! var val frame) 345 (set-car! frame (cons var (car frame))) 346 (set-cdr! frame (cons val (cdr frame)))) 347 348 (define (extend-environment vars vals base-env) 349 (if (= (length vars) (length vals)) 350 (cons (make-frame vars vals) base-env) 351 (if (< (length vars) (length vals)) 352 (error "Too many arguments supplied" 353 vars 354 vals) 355 (error "Too few arguments supplied" 356 vars 357 vals)))) 358 359 (define (lookup-variable-value var env) 360 (define (env-loop env) 361 (define (scan vars vals) 362 (cond ((null? vars) 363 (env-loop 364 (enclosing-environment env))) 365 ((eq? var (car vars)) 366 (car vals)) 367 (else (scan (cdr vars) 368 (cdr vals))))) 369 (if (eq? env the-empty-environment) 370 (error "Unbound variable -- LOOKUP-VARIABLE-VALUE" var) 371 (let ((frame (first-frame env))) 372 (scan (frame-variables frame) 373 (frame-values frame))))) 374 (env-loop env)) 375 376 (define (set-variable-value! var val env) 377 ;; XXX: Just trying to see what it would look like if I just used named lets. 378 ;; 379 ;; (let loop-over-environments ((env env)) 380 ;; (cond 381 ;; ((eq? env the-empty-enviornment) 382 ;; (error "Unbound variable: SET!" var)) 383 ;; (else 384 ;; (let ((frame (first-frame env))) 385 ;; (let loop-over-frame ((vars (frame-variables frame)) 386 ;; (vals (frame-values frame))) 387 ;; (cond 388 ;; ((null? vars) 389 ;; (loop-over-environments (enclosing-environment env))) 390 ;; ((eq? var (car vars)) 391 ;; (set-car! vals val)) 392 ;; (else 393 ;; (loop-over-frame ((cdr vars) 394 ;; (cdr vals)))))))))) 395 396 (define (env-loop env) 397 (define (scan vars vals) 398 (cond ((null? vars) 399 (env-loop 400 (enclosing-environment env))) 401 ((eq? var (car vars)) 402 (set-car! vals val)) 403 (else (scan (cdr vars) 404 (cdr vals))))) 405 (if (eq? env the-empty-environment) 406 (error "Unbound variable: SET!" var) 407 (let ((frame (first-frame env))) 408 (scan (frame-variables frame) 409 (frame-values frame))))) 410 (env-loop env)) 411 412 (define (define-variable! var val env) 413 "Add a new variable VAR with value VAL in the top frame of environment ENV. 414 415 If variable VAR already exists in the top frame, set VAR in the top 416 frame to VAL." 417 418 ;; We will CHANGE (why did I all uppercase "CHANGE" here? I 419 ;; don't remember. Maybe it is some sort of a joke?) only 420 ;; the first frame. 421 (let ((frame (first-frame env))) 422 (define (scan vars vals) 423 (cond ((null? vars) 424 (add-binding-to-frame! 425 var val frame)) 426 ((eq? var (car vars)) 427 (set-car! vals val)) 428 (else (scan (cdr vars) 429 (cdr vals))))) 430 (scan (frame-variables frame) 431 (frame-values frame)))) 432 433 ;;; 4.1.4 434 435 (define (setup-environment) 436 (let* (;; XXX: See bug below on the quoted (define 437 ;; primitive-procedures ...) 438 (primitive-procedures 439 (list (list 'car car) 440 (list 'cdr cdr) 441 (list 'cons cons) 442 (list 'null? null?) 443 (list '+ +) 444 (list '- -) 445 (list '* *) 446 (list '/ /) 447 (list '= =) 448 (list 'display display) 449 (list 'write write) 450 (list 'read read))) 451 (initial-env 452 (extend-environment 453 (primitive-procedure-names primitive-procedures) 454 (primitive-procedure-objects primitive-procedures) 455 the-empty-environment))) 456 (define-variable! 'true true initial-env) 457 (define-variable! 'false false initial-env) 458 initial-env)) 459 460 (define the-global-environment 461 (setup-environment)) 462 463 (define (primitive-procedure? proc) 464 (tagged-list? proc 'primitive)) 465 466 (define (primitive-implementation proc) 467 (cadr proc)) 468 469 ;; XXX: There is a bug here in SICP or GNU Guile or both. 470 ;; primitive-procedures is `#<unspecified>` when this library is 471 ;; being loaded. 472 '(define primitive-procedures 473 (list (list 'car car) 474 (list 'cdr cdr) 475 (list 'cons cons) 476 (list 'null? null?) 477 (list '+ +) 478 (list '- -) 479 (list '* *) 480 (list '/ /) 481 (list 'display display) 482 (list 'write write) 483 (list 'read read))) 484 485 (define (primitive-procedure-names primitive-procedures) 486 (map car 487 primitive-procedures)) 488 489 (define (primitive-procedure-objects primitive-procedures) 490 (map (lambda (proc) 491 (list 'primitive (cadr proc))) 492 primitive-procedures)) 493 494 (define (apply-primitive-procedure proc args) 495 (apply-in-underlying-scheme 496 (primitive-implementation proc) args)) 497 498 (define input-prompt ";;; M-Eval input:") 499 (define output-prompt ";;; M-Eval value:") 500 501 (define (driver-loop) 502 (prompt-for-input input-prompt) 503 (let ((input (read))) 504 (let ((output 505 (eval input 506 the-global-environment))) 507 (announce-output output-prompt) 508 (user-print output))) 509 (driver-loop)) 510 511 (define (prompt-for-input string) 512 (newline) (newline) 513 (display string) (newline)) 514 515 (define (announce-output string) 516 (newline) (display string) (newline)) 517 518 (define (user-print object) 519 (if (compound-procedure? object) 520 (display 521 (list 'compound-procedure 522 (procedure-parameters object) 523 (procedure-body object) 524 '<procedure-env>)) 525 (display object))) 526 527 ;; XXX: Already defined above: 528 ;; (define the-global-environment 529 ;; (setup-environment)) 530 531 (define (main args) 532 (match args 533 ('() 534 (error "Arguments should have something rather than nothing.")) 535 (`(,arg0) 536 (display arg0) 537 (newline)) 538 (`(,arg0 "--debug") 539 (display "DEBUG TURN ON!\n") 540 (set! *debug* #t)) 541 (args (error "Unknown arguments:" args))) 542 543 (driver-loop)) 544 545 (define (run-internal-tests) 546 (test-eq 1 547 (lookup-variable-value 'a 548 (extend-environment '(a +) 549 `(1 ,+) 550 the-empty-environment))) 551 (test-eq 1 552 (lookup-variable-value 'a 553 (extend-environment '(+ a) 554 `(,+ 1) 555 the-empty-environment))) 556 (test-eq + 557 (lookup-variable-value '+ 558 (extend-environment '(a +) 559 `(1 ,+) 560 the-empty-environment)))) 561 562 (add-form-handler! application? 563 (lambda (exp env) 564 (apply (eval (operator exp) env) 565 (list-of-values 566 (operands exp) 567 env)))) 568 (add-form-handler! (lambda (exp) 569 (tagged-list? exp 'or)) 570 (lambda (exp env) 571 (eval (let loop ((seq (cdr exp))) 572 (match seq 573 ('() 'false) 574 ((expression . rest) 575 (list (list 'lambda (list 'value) 576 (list 'if 577 'value 578 'value 579 (loop rest))) 580 expression)) 581 (otherwise 582 (error "Weird expression -- AND" 583 otherwise)))) 584 env))) 585 (add-form-handler! (lambda (exp) 586 (tagged-list? exp 'and)) 587 (lambda (exp env) 588 (eval (let loop ((seq (cdr exp))) 589 (match seq 590 ('() 'false) 591 ((expression) 592 `((lambda (value) 593 (if value 594 value 595 false)) 596 ,expression) 597 ) 598 ((expression . rest) 599 `((lambda (value) 600 (if value 601 ,(loop rest) 602 false)) 603 ,expression)) 604 (otherwise 605 (error "Weird expression -- AND" 606 otherwise)))) 607 env))) 608 (add-form-handler! cond? 609 (lambda (exp env) 610 (eval (cond->if exp) env))) 611 (add-form-handler! begin? 612 (lambda (exp env) 613 (eval-sequence 614 (begin-actions exp) 615 env))) 616 (add-form-handler! lambda? 617 (lambda (exp env) 618 (make-procedure (lambda-parameters exp) 619 (lambda-body exp) 620 env))) 621 (add-form-handler! if? eval-if) 622 (add-form-handler! definition? eval-definition) 623 (add-form-handler! assignment? eval-assignment) 624 (add-form-handler! quoted? (lambda (exp env) (text-of-quotation exp))) 625 (add-form-handler! (lambda (exp) (tagged-list? exp 'let)) 626 (lambda (exp env) 627 (eval (let->combination exp) env))) 628 (add-form-handler! (lambda (exp) (tagged-list? exp 'let*)) 629 (lambda (exp env) 630 (eval (let*->nested-lets exp) env))) 631 632 ))