From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Fri, 1 Jun 2012 16:17:34 +0000 (+0530)
Subject: solution to 4.31
X-Git-Url: https://git.rkrishnan.org/pf/content/en/seg/sub?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