From: Ramakrishnan Muthukrishnan Date: Fri, 1 Jun 2012 16:17:34 +0000 (+0530) Subject: solution to 4.31 X-Git-Url: https://git.rkrishnan.org/(%5B%5E?a=commitdiff_plain;h=554d8c6596c08583783b44d5c86fad0d1e76420b;p=sicp.git solution to 4.31 --- diff --git a/src/sicp/ex4_31.rkt b/src/sicp/ex4_31.rkt new file mode 100644 index 0000000..fc23c95 --- /dev/null +++ b/src/sicp/ex4_31.rkt @@ -0,0 +1,297 @@ +#lang racket + +(require (rename-in racket/base + (apply apply-in-underlying-scheme) + (eval eval-in-underlying-scheme))) + +(provide (all-defined-out)) + +(define (self-evaluating? expr) + (match expr + [(? number? expr) #t] + [(? char? expr) #t] + [(? string? expr) #t] + [_ #f])) + +(define (variable? expr) (symbol? expr)) + +;; sequence +(define (eval-sequence exps env) + (cond + [(last-exp? exps) (eval (first-exp exps) env)] + [else (eval (first-exp exps) env) + (eval-sequence (rest-exps exps) env)])) + +;; begin +(define (last-exp? seq) (null? (cdr seq))) +(define (first-exp seq) (car seq)) +(define (rest-exps seq) (cdr seq)) + +;; lambda +(define (make-lambda parameters body) + (cons 'lambda (cons parameters body))) + +;; environment data structures +(define (enclosing-environment env) (cdr env)) +(define (first-frame env) (car env)) +(define the-empty-environment '()) + +(define (make-frame variables values) + (let ([ht (make-hash)]) + (for-each (lambda (var val) + (hash-set! ht var val)) + variables + values) + ht)) + +(define (frame-variables frame) + (hash-keys frame)) + +(define (frame-values frame) + (hash-values frame)) + +(define (add-binding-to-frame! var val frame) + (hash-set! frame var val)) + +;; environment is a list of frames, most recent being the last +;; one consed into the list +(define (extend-environment vars vals base-env) + (let ([frame (make-frame vars vals)]) + (cons frame base-env))) + +(define (lookup-variable-value var env) + (if (eq? env the-empty-environment) + (error "unbound variable: " var) + (let ([frame (first-frame env)]) + (let ([value (hash-ref frame var (lambda () (lookup-variable-value var (enclosing-environment env))))]) + (if (eq? value '*unassigned*) + (error "evaluating a variable that is not assigned a value -- " var) + value))))) + +(define (set-variable-value! var val env) + (if (eq? env the-empty-environment) + (error "unbound variable: " var) + (let ([frame (first-frame env)]) + (if (hash-has-key? frame var) + (hash-set! frame var val) + (set-variable-value! var val (enclosing-environment env)))))) + +(define (define-variable! var val env) + (let ([frame (first-frame env)]) + (if (hash-has-key? frame var) + (hash-set! frame var val) + (add-binding-to-frame! var val frame)))) + +;; primitive procedure +(define (primitive-implementation proc) (cadr proc)) + +(define primitive-procedures + (list (list 'car car) + (list 'cdr cdr) + (list 'cons cons) + (list 'null? null?) + (list 'list list) + (list '+ +) + (list '- -) + (list '* *) + (list '/ /) + (list '= =) + (list '> >) + (list '< <))) + +(define (primitive-procedure-names) + (map car primitive-procedures)) + +(define (primitive-procedure-objects) + (map (lambda (proc) (list 'primitive (cadr proc))) + primitive-procedures)) + +(define (apply-primitive-procedure proc args) + (apply-in-underlying-scheme + (primitive-implementation proc) args)) + +;; global env +(define (make-environment) + (let ((initial-env + (extend-environment (primitive-procedure-names) + (primitive-procedure-objects) + the-empty-environment))) + (define-variable! 'true true initial-env) + (define-variable! 'false false initial-env) + initial-env)) + +(define the-global-environment (make-environment)) + +;; application +(define (list-of-values exps env) + (if (no-operands? exps) + '() + (cons (eval (first-operand exps) env) + (list-of-values (rest-operands exps) env)))) + +(define (no-operands? ops) (null? ops)) +(define (first-operand ops) (car ops)) +(define (rest-operands ops) (cdr ops)) + +;; compound procedure +(define (make-procedure params body env) + (list 'procedure params (scan-out-definitions body) env)) + +(define (procedure-parameters p) (cadr p)) +(define (procedure-body p) (scan-out-definitions (caddr p))) +(define (procedure-environment p) (cadddr p)) + +(define (apply procedure arguments env) + (match procedure + [`(primitive ,f ...) (apply-primitive-procedure procedure (list-of-arg-values arguments env))] + [`(procedure ,f ...) + (let* ([params (procedure-parameters procedure)] + [params-value-pair (create-param-value-pair params arguments env)]) + (eval-sequence + (procedure-body procedure) + (extend-environment + (map car params-value-pair) + (map cdr params-value-pair) + (procedure-environment procedure))))] + [_ (error "Unknown procedure type -- APPLY" procedure)])) + +(define (list-of-arg-values exps env) + (if (no-operands? exps) + '() + (cons (actual-value (first-operand exps) env) + (list-of-arg-values (rest-operands exps) env)))) + +(define (create-param-value-pair params arguments env) + (if (null? arguments) + '() + (let [(p (first-operand params))] + (match p + [`(,variable lazy) (cons (cons variable (delay-it (first-operand arguments) env)) + (create-param-value-pair (rest-operands params) + (rest-operands arguments) + env))] + [exp (cons (cons exp (eval (first-operand arguments) env)) + (create-param-value-pair (rest-operands params) + (rest-operands arguments) + env))])))) + +;; truth +(define (true? x) + (not (eq? x false))) + +(define (false? x) + (eq? x false)) + +;; cond +(define (cond->if clauses) + (define (seq->exp actions) + (if (empty? (cdr actions)) + (car actions) + `(begin ,@actions))) + (if (empty? clauses) + 'false + (let ([clause (car clauses)]) + (match clause + [`(else ,action ...) (seq->exp action)] + [`(,pred ,action ...) `(if ,pred + ,(seq->exp action) + ,(cond->if (cdr clauses)))])))) + +;; let +(define (let->combination lexpr) + (match-let* ([`(let ,(? (lambda (x) (or (pair? x) (empty? x))) bindings) ,body ..1) lexpr] + [`((,var ,val) ...) bindings]) + `((lambda ,var ,@body) ,@val))) + +;; named let +(define (named-let->combination lexpr) + (match-let* ([`(let ,(? symbol? name) ,(? (lambda (x) (or (pair? x) (empty? x))) bindings) ,body ..1) lexpr] + [`((,var ,val) ...) bindings]) + `(begin (define ,name (lambda ,var ,@body)) + (,name ,@val)))) + +;; let* +(define (let*->nested-lets lexpr) + (match lexpr + [`(let* (,first-binding ,rest-bindings ...) ,body ..1) + `(let (,first-binding) ,(let*->nested-lets `(let* ,rest-bindings ,@body)))] + [`(let* () ,body ..1) `(let () ,@body)])) + +;; internal definitions +(define (scan-out-definitions body) + (match body + [`((define ,var ,e) ..1 ,rest) + `((let ,(map (lambda (v) (list (car v) ''*unassigned*)) var) + ,@(map (lambda (v e) `(set! ,(car v) (lambda ,(cdr v) ,e))) var e) + ,rest))] + [_ body])) + +;; letrec +(define (letrec->combination lexpr) + (match lexpr + [`(letrec (,bindings ...) ,body ..1) + `(let ,(map (lambda (v) (list (car v) ''*unassigned*)) bindings) + ,@(map (lambda (binding) + (let ([name (car binding)] + [value (cadr binding)]) + `(set! ,name ,value))) + bindings) + ,@body)])) + + +;; thunks +(define (actual-value exp env) + ;(display (format "eval expr ~s~%" exp)) + ;(newline) + (force-it (eval exp env))) + +(define (force-it obj) + ;; (display (format "~s~%" obj)) + (match obj + [`(thunk ,exp ,env) (begin + ;(display (format "eval's output ~s~%" obj)) + ;(newline) + (actual-value exp env))] + [_ obj])) + +(define (delay-it exp env) + (list 'thunk exp env)) + + +;; eval +(define (eval exp env) + ;; use this display statement to visualize the recursive evaluation process + ;;(display (format "~s~%" exp)) + (match exp + [(? self-evaluating? exp) exp] + [(? variable? exp) (lookup-variable-value exp env)] + [`(quote ,x) x] + [`(set! ,var ,val) (set-variable-value! var (eval val env) env)] + [`(define ,(? (lambda (x) (not (pair? x))) var) ,b) (define-variable! var (eval b env) env)] + [`(define ,(? pair? var) ,b ..1) (define-variable! (car var) (eval (make-lambda (cdr var) b) env) env)] + [`(if ,pred ,consequent ,alternative) (if (true? (eval pred env)) (eval consequent env) (eval alternative env))] + [`(unless ,condition ,consequent ,alternative) (if (true? (eval condition env)) (eval alternative env) (eval consequent env))] + [`(lambda ,parameters ,body ..1) (make-procedure parameters body env)] + [`(begin ,exp ...) (eval-sequence exp env)] + [`(cond ,clauses ...) (eval (cond->if clauses) env)] + [`(let ,(? (lambda (x) (or (pair? x) (empty? x))) bindings) ,body ..1) (eval (let->combination exp) env)] + [`(let ,(? symbol? name) ,bindings ,body ..1) (eval (named-let->combination exp) env)] + [`(let* ,bindings ,body ..1) (eval (let*->nested-lets exp) env)] + [`(letrec ,bindings ,body ..1) (eval (letrec->combination exp) env)] + [`(,f ,x ...) (apply (actual-value f env) + x + env)] + [_ (error "unable to evaluate expression -- EVAL " exp)])) + + +(define (interpret) + (let loop ([input (read)] + [env the-global-environment]) + (let ([output (eval input env)]) + (display output) + (loop (read) env)))) + +(define (t expr) + (let ([t1 (current-inexact-milliseconds)]) + (eval expr the-global-environment) + (displayln (- (current-inexact-milliseconds) t1)))) \ No newline at end of file