--- /dev/null
+#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))))
\ No newline at end of file
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