exercise-6.scm (20090B)
1 (define-library (sicp solutions chapter-4 exercise-6) 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 (eval `((lambda ,vars 300 ,@body) 301 ,@exps) 302 env))))) 303 304 ;;; end 4.1.2 305 306 ;;; 4.1.3 307 308 ;; XXX: By default, false's value is #f, which is the only true 309 ;; false. All others are fake and would have the truthitousness 310 ;; property. Consult `info r7rs-small` or `C-h 311 (define (true? x) 312 (not (eq? x false))) 313 314 (define (false? x) 315 (eq? x false)) 316 317 (define (make-procedure parameters body env) 318 (list 'procedure parameters body env)) 319 (define (compound-procedure? p) 320 (tagged-list? p 'procedure)) 321 (define (procedure-parameters p) (cadr p)) 322 (define (procedure-body p) (caddr p)) 323 (define (procedure-environment p) (cadddr p)) 324 325 (define (enclosing-environment env) (cdr env)) 326 (define (first-frame env) (car env)) 327 (define the-empty-environment '()) 328 329 (define (make-frame variables values) 330 (cons variables values)) 331 (define (frame-variables frame) (car frame)) 332 (define (frame-values frame) (cdr frame)) 333 (define (add-binding-to-frame! var val frame) 334 (set-car! frame (cons var (car frame))) 335 (set-cdr! frame (cons val (cdr frame)))) 336 337 (define (extend-environment vars vals base-env) 338 (if (= (length vars) (length vals)) 339 (cons (make-frame vars vals) base-env) 340 (if (< (length vars) (length vals)) 341 (error "Too many arguments supplied" 342 vars 343 vals) 344 (error "Too few arguments supplied" 345 vars 346 vals)))) 347 348 (define (lookup-variable-value var env) 349 (define (env-loop env) 350 (define (scan vars vals) 351 (cond ((null? vars) 352 (env-loop 353 (enclosing-environment env))) 354 ((eq? var (car vars)) 355 (car vals)) 356 (else (scan (cdr vars) 357 (cdr vals))))) 358 (if (eq? env the-empty-environment) 359 (error "Unbound variable -- LOOKUP-VARIABLE-VALUE" var) 360 (let ((frame (first-frame env))) 361 (scan (frame-variables frame) 362 (frame-values frame))))) 363 (env-loop env)) 364 365 (define (set-variable-value! var val env) 366 ;; XXX: Just trying to see what it would look like if I just used named lets. 367 ;; 368 ;; (let loop-over-environments ((env env)) 369 ;; (cond 370 ;; ((eq? env the-empty-enviornment) 371 ;; (error "Unbound variable: SET!" var)) 372 ;; (else 373 ;; (let ((frame (first-frame env))) 374 ;; (let loop-over-frame ((vars (frame-variables frame)) 375 ;; (vals (frame-values frame))) 376 ;; (cond 377 ;; ((null? vars) 378 ;; (loop-over-environments (enclosing-environment env))) 379 ;; ((eq? var (car vars)) 380 ;; (set-car! vals val)) 381 ;; (else 382 ;; (loop-over-frame ((cdr vars) 383 ;; (cdr vals)))))))))) 384 385 (define (env-loop env) 386 (define (scan vars vals) 387 (cond ((null? vars) 388 (env-loop 389 (enclosing-environment env))) 390 ((eq? var (car vars)) 391 (set-car! vals val)) 392 (else (scan (cdr vars) 393 (cdr vals))))) 394 (if (eq? env the-empty-environment) 395 (error "Unbound variable: SET!" var) 396 (let ((frame (first-frame env))) 397 (scan (frame-variables frame) 398 (frame-values frame))))) 399 (env-loop env)) 400 401 (define (define-variable! var val env) 402 "Add a new variable VAR with value VAL in the top frame of environment ENV. 403 404 If variable VAR already exists in the top frame, set VAR in the top 405 frame to VAL." 406 407 ;; We will CHANGE (why did I all uppercase "CHANGE" here? I 408 ;; don't remember. Maybe it is some sort of a joke?) only 409 ;; the first frame. 410 (let ((frame (first-frame env))) 411 (define (scan vars vals) 412 (cond ((null? vars) 413 (add-binding-to-frame! 414 var val frame)) 415 ((eq? var (car vars)) 416 (set-car! vals val)) 417 (else (scan (cdr vars) 418 (cdr vals))))) 419 (scan (frame-variables frame) 420 (frame-values frame)))) 421 422 ;;; 4.1.4 423 424 (define (setup-environment) 425 (dpp 'moo) 426 (let* (;; XXX: See bug below on the quoted (define 427 ;; primitive-procedures ...) 428 (primitive-procedures 429 (list (list 'car car) 430 (list 'cdr cdr) 431 (list 'cons cons) 432 (list 'null? null?) 433 (list '+ +) 434 (list '- -) 435 (list '* *) 436 (list '/ /) 437 (list '= =) 438 (list 'display display) 439 (list 'write write) 440 (list 'read read))) 441 (initial-env 442 (extend-environment 443 (primitive-procedure-names primitive-procedures) 444 (primitive-procedure-objects primitive-procedures) 445 the-empty-environment))) 446 (define-variable! 'true true initial-env) 447 (define-variable! 'false false initial-env) 448 initial-env)) 449 450 (define the-global-environment 451 (begin 452 (dpp 'moo) 453 (setup-environment))) 454 455 (define (primitive-procedure? proc) 456 (tagged-list? proc 'primitive)) 457 458 (define (primitive-implementation proc) 459 (cadr proc)) 460 461 ;; XXX: There is a bug here in SICP or GNU Guile or both. 462 ;; primitive-procedures is `#<unspecified>` when this library is 463 ;; being loaded. 464 '(define primitive-procedures 465 (list (list 'car car) 466 (list 'cdr cdr) 467 (list 'cons cons) 468 (list 'null? null?) 469 (list '+ +) 470 (list '- -) 471 (list '* *) 472 (list '/ /) 473 (list 'display display) 474 (list 'write write) 475 (list 'read read))) 476 477 (define (primitive-procedure-names primitive-procedures) 478 (map car 479 primitive-procedures)) 480 481 (define (primitive-procedure-objects primitive-procedures) 482 (map (lambda (proc) 483 (list 'primitive (cadr proc))) 484 primitive-procedures)) 485 486 (define (apply-primitive-procedure proc args) 487 (apply-in-underlying-scheme 488 (primitive-implementation proc) args)) 489 490 (define input-prompt ";;; M-Eval input:") 491 (define output-prompt ";;; M-Eval value:") 492 493 (define (driver-loop) 494 (prompt-for-input input-prompt) 495 (let ((input (read))) 496 (let ((output 497 (eval input 498 the-global-environment))) 499 (announce-output output-prompt) 500 (user-print output))) 501 (driver-loop)) 502 503 (define (prompt-for-input string) 504 (newline) (newline) 505 (display string) (newline)) 506 507 (define (announce-output string) 508 (newline) (display string) (newline)) 509 510 (define (user-print object) 511 (if (compound-procedure? object) 512 (display 513 (list 'compound-procedure 514 (procedure-parameters object) 515 (procedure-body object) 516 '<procedure-env>)) 517 (display object))) 518 519 ;; XXX: Already defined above: 520 ;; (define the-global-environment 521 ;; (setup-environment)) 522 523 (define (main args) 524 (match args 525 ('() 526 (error "Arguments should have something rather than nothing.")) 527 (`(,arg0) 528 (display arg0) 529 (newline)) 530 (`(,arg0 "--debug") 531 (display "DEBUG TURN ON!\n") 532 (set! *debug* #t)) 533 (args (error "Unknown arguments:" args))) 534 535 (driver-loop)) 536 537 (define (run-internal-tests) 538 (test-eq 1 539 (lookup-variable-value 'a 540 (extend-environment '(a +) 541 `(1 ,+) 542 the-empty-environment))) 543 (test-eq 1 544 (lookup-variable-value 'a 545 (extend-environment '(+ a) 546 `(,+ 1) 547 the-empty-environment))) 548 (test-eq + 549 (lookup-variable-value '+ 550 (extend-environment '(a +) 551 `(1 ,+) 552 the-empty-environment)))) 553 554 (add-form-handler! application? 555 (lambda (exp env) 556 (apply (eval (operator exp) env) 557 (list-of-values 558 (operands exp) 559 env)))) 560 (add-form-handler! (lambda (exp) 561 (tagged-list? exp 'or)) 562 (lambda (exp env) 563 (eval (let loop ((seq (cdr exp))) 564 (match seq 565 ('() 'false) 566 ((expression . rest) 567 (list (list 'lambda (list 'value) 568 (list 'if 569 'value 570 'value 571 (loop rest))) 572 expression)) 573 (otherwise 574 (error "Weird expression -- AND" 575 otherwise)))) 576 env))) 577 (add-form-handler! (lambda (exp) 578 (tagged-list? exp 'and)) 579 (lambda (exp env) 580 (eval (let loop ((seq (cdr exp))) 581 (match seq 582 ('() 'false) 583 ((expression) 584 `((lambda (value) 585 (if value 586 value 587 false)) 588 ,expression) 589 ) 590 ((expression . rest) 591 `((lambda (value) 592 (if value 593 ,(loop rest) 594 false)) 595 ,expression)) 596 (otherwise 597 (error "Weird expression -- AND" 598 otherwise)))) 599 env))) 600 (add-form-handler! cond? 601 (lambda (exp env) 602 (eval (cond->if exp) env))) 603 (add-form-handler! begin? 604 (lambda (exp env) 605 (eval-sequence 606 (begin-actions exp) 607 env))) 608 (add-form-handler! lambda? 609 (lambda (exp env) 610 (make-procedure (lambda-parameters exp) 611 (lambda-body exp) 612 env))) 613 (add-form-handler! if? eval-if) 614 (add-form-handler! definition? eval-definition) 615 (add-form-handler! assignment? eval-assignment) 616 (add-form-handler! quoted? (lambda (exp env) (text-of-quotation exp))) 617 (add-form-handler! (lambda (exp) (tagged-list? exp 'let)) 618 (lambda (exp env) 619 (eval (let->combination exp) env))) 620 621 ))