From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com>
Date: Sat, 31 Dec 2011 06:48:44 +0000 (+0530)
Subject: Meta circular evaluator using Racket's `match' form. It also uses
X-Git-Url: https://git.rkrishnan.org/?a=commitdiff_plain;h=a0529379a6d3e2f31488c8c6b518ea35bfa278a5;p=sicp.git

Meta circular evaluator using Racket's `match' form. It also uses
immutable hash table to represent frames.
---

diff --git a/src/sicp/metacircular2.rkt b/src/sicp/metacircular2.rkt
new file mode 100644
index 0000000..7196c94
--- /dev/null
+++ b/src/sicp/metacircular2.rkt
@@ -0,0 +1,203 @@
+#lang racket
+
+(require (rename-in racket/base
+                    (apply apply-in-underlying-scheme)
+                    (eval eval-in-underlying-scheme)))
+
+(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 (list 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)])
+        (hash-ref frame var (lambda () (lookup-variable-value var (enclosing-environment env)))))))
+
+(define (set-variable-value! var val env)
+  (if (eq? env the-empty-environment)
+      (error "unbound variable")
+      (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 '/ /)))
+
+(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 (setup-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 (setup-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 body env))
+
+(define (procedure-parameters p) (cadr p))
+(define (procedure-body p) (caddr p))
+(define (procedure-environment p) (cadddr p))
+
+(define (apply procedure arguments)
+  (match procedure
+    [`(primitive ,f ...)  (apply-primitive-procedure procedure arguments)]
+    [`(procedure ,f ...)  (eval-sequence
+                       (procedure-body procedure)
+                       (extend-environment
+                        (procedure-parameters procedure)
+                        arguments
+                        (procedure-environment procedure)))]
+    [_                (error "Unknown procedure type -- APPLY" procedure)]))
+
+;; 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 ,bindings ,body) lexpr]
+               [`((,var ,val) ...) bindings])
+    `((lambda ,var ,body) ,@val)))
+
+;; eval
+(define (eval exp env)
+  (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) (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))]
+    [`(lambda ,parameters ,body ...) (make-procedure parameters body env)]
+    [`(begin ,exp ...) (eval-sequence exp env)]
+    [`(cond ,clauses ...) (eval (cond->if clauses) env)]
+    [`(let ,bindings ,body) (eval (let->combination exp) env)]
+    [(list f x ...) (apply (eval f env) (list-of-values 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))))
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