自己評価型ラケットインタプリタ

Question

私はそれ自体評価できるラケットインタプリタを書こうとしてきましたが、何らかの理由でそれを動作させることができません。 interpreter.rktのコードはかなり標準です。 interpreter-test.rktのコードが問題になることはありますか？よく分かりません。

interpreter.rkt

#lang racket 

(provide eeval) 

(define (eeval lines) 
    ; returns (key . val) if key in frame, #f otherwise 
    (define (lookup-in-frame key frame) 
    (cond 
     [(null? frame) #f] 
     [(eq? key (mcar (mcar frame))) (mcar frame)] 
     [else (lookup-in-frame key (mcdr frame))])) 

    ; returns (key . val) if key in env, #f otherwise 
    (define (lookup-in-env key env) 
    (cond 
     [(null? env) #f] 
     [else (let ([key-val-pair (lookup-in-frame key (mcar env))]) 
       (if key-val-pair 
        key-val-pair 
        (lookup-in-env key (mcdr env))))])) 

    (define (add-to-env! key value env) 
    (set-mcar! env 
       (mcons (mcons key value) 
         (mcar env)))) 

    (define (update-env! key value env) 
    (cond 
     [(null? env) 
     (error "set!: assignment disallowed; cannot set variable before its definition \nvariable:" key)] 
     [else (let ([key-val-pair (lookup-in-frame key (mcar env))]) 
       (if key-val-pair 
        (set-mcdr! key-val-pair value) 
        (update-env! key value (mcdr env))))])) 

    (define (extend-env keys values env) 
    (define (new-frame keys values) 
     (cond 
     ((null? keys) '()) 
     (else (mcons (mcons (car keys) (car values)) 
        (new-frame (cdr keys) (cdr values)))))) 
    (mcons (new-frame keys values) env)) 

    (define global-env (mcons '() '())) 

    (define (myeval expr env) 
    (cond 
     [(and (not (null? expr)) (not (pair? expr))) 
     (cond 
     [(boolean? expr) expr] 
     [(number? expr) expr] 
     [(string? expr) expr] 
     [(symbol? expr) 
      (let ([key-value (lookup-in-env expr env)]) 
      (if key-value 
       [mcdr key-value] 
       [if [member expr 
          '(void void? null? member 
            pair? list cons car cdr cddr 
            mpair? mcons mcar mcdr 
            set-mcar! set-mcdr! 
            first second third fourth 
            boolean? false? not 
            number? = + - */expt 
            string? 
            symbol? eq? equal? 
            foldl error)] 
        [lambda() (list 'primitive expr)] 
        [error expr "undefined"]]))])] 
     [(null? expr) (error "()" "missing procedure expression.")] 
     [(eq? (car expr) 'quote) 
     (second expr)] 
     [(eq? (car expr) 'lambda) 
     (lambda() (list 'non-primitive 
         (second expr) 
         (cddr expr) 
         env))] 
     [(eq? (car expr) 'define) 
     (if [not (pair? (second expr))] 
      [if [false? (lookup-in-frame (second expr) (mcar env))] 
       [add-to-env! (second expr) (myeval (third expr) env) env] 
       [error "duplicate definition for identifier in" 
         (second expr)]] 
      [myeval (list 'define 
         (car (second expr)) 
         (cons 'lambda 
           (cons (cdr (second expr)) 
            (cddr expr)))) 
        env])] 
     [(eq? (car expr) 'set!) 
     (update-env! (second expr) 
        (myeval (third expr) env) 
        env)] 
     [(eq? (car expr) 'begin) 
     (eval-sequence (cdr expr) env)] 
     [(eq? (car expr) 'cond) 
     (evcond (cdr expr) env)] 
     [(eq? (car expr) 'if) 
     (myeval (list 'cond 
        (list (second expr) (third expr)) 
        (list 'else (fourth expr))) 
       env)] 
     [(eq? (car expr) 'and) (evand (cdr expr) env)] 
     [(eq? (car expr) 'or) (evor (cdr expr) env)] 
     [(eq? (car expr) 'let) 
     (eval-sequence (cddr expr) 
         (extend-env 
         (map first (second expr)) 
         (map second (second expr)) 
         env))] 
     [else (myapply (myeval (car expr) env) 
        (eval-args (cdr expr) env))] 
    )) 

    (define (eval-sequence lines env) 
    (if [null? lines] 
     [void] 
     (if [null? (cdr lines)] 
      [myeval (car lines) env] 
      [begin (myeval (car lines) env) 
        (eval-sequence (cdr lines) env)]))) 

    (define (evcond lines env) 
    (cond 
     [(null? lines) (void)] 
     [(eq? 'else (first (car lines))) 
     (myeval (second (car lines)) env)] 
     [(myeval (first (car lines)) env) 
     (myeval (second (car lines)) env)] 
     [else (evcond (cdr lines) env)])) 

    (define (evand args env) 
    (cond 
     [(null? args) #t] 
     [(null? (cdr args)) (myeval (car args) env)] 
     [else [let ([val (myeval (car args) env)]) 
       (if [false? val] 
        #f 
        [evand (cdr args) env])]])) 

    (define (evor args env) 
    (if [null? args] 
     #f 
     [let ([val (myeval (car args) env)]) 
      (if val 
       val 
       (evor (cdr args) env))])) 

    (define (eval-args args env) 
    (cond 
     [(null? args) '()] 
     [else (cons (myeval (car args) env) 
        (eval-args (cdr args) env))])) 

    (define (myapply func vals) 
    (cond 
     [(eq? (first (func)) 'primitive) 
     (apply-primitive (second (func)) vals)] 
     [(eq? (first (func)) 'non-primitive) 
     (eval-sequence (third (func)) 
         (extend-env 
         (second (func)) 
         vals 
         (fourth (func))))] 
     [else (error func "unexpected case in myapply")])) 

    (define (apply-primitive name vals) 
    (cond 
     [(eq? name 'void) (void)] 
     [(eq? name 'void?) (void? (first vals))] 
     [(eq? name 'null?) (null? (first vals))] 
     [(eq? name 'member) (member (first vals) (second vals))] 
     [(eq? name 'pair?) (pair? (first vals))] 
     [(eq? name 'list) 
     (begin 
     (define (helper vals) 
      (if [null? vals] 
       '() 
       [cons (car vals) (helper (cdr vals))])) 
     (helper vals))] 
     [(eq? name 'cons) (cons (first vals) (second vals))] 
     [(eq? name 'car) (car (first vals))] 
     [(eq? name 'cdr) (cdr (first vals))] 
     [(eq? name 'cddr) (cddr (first vals))] 
     [(eq? name 'mpair?) (mpair? (first vals))] 
     [(eq? name 'mcons) (mcons (first vals) (second vals))] 
     [(eq? name 'mcar) (mcar (first vals))] 
     [(eq? name 'mcdr) (mcdr (first vals))] 
     [(eq? name 'set-mcar!) (set-mcar! (first vals) (second vals))] 
     [(eq? name 'set-mcdr!) (set-mcdr! (first vals) (second vals))] 
     [(eq? name 'first) (first (first vals))] 
     [(eq? name 'second) (second (first vals))] 
     [(eq? name 'third) (third (first vals))] 
     [(eq? name 'fourth) (fourth (first vals))] 
     [(eq? name 'boolean?) (boolean? (first vals))] 
     [(eq? name 'false?) (false? (first vals))] 
     [(eq? name 'not) (not (first vals))] 
     [(eq? name 'number?) (number? (first vals))] 
     [(eq? name '=) 
     (begin 
     (define (helper x l) 
      (cond 
      [(null? l) #t] 
      [(= (car l) x) (helper x (cdr l))] 
      [else #f])) 
     (if [or (null? vals) 
       (null? (cdr vals))] 
      [error "=" 
        "arity mismatch; expects at least 2 arguments."] 
      [helper (car vals) (cdr vals)]))] 
     [(eq? name '+) (foldl + 0 vals)] 
     [(eq? name '-) (- (car vals) (foldl + 0 (cdr vals)))] 
     [(eq? name '*) (foldl * 1 vals)] 
     [(eq? name '/) (/ (car vals) (foldl * 1 (cdr vals)))] 
     [(eq? name 'expt) (expt (first vals) (second vals))] 
     [(eq? name 'string?) (string? (first vals))] 
     [(eq? name 'symbol?) (symbol? (first vals))] 
     [(eq? name 'eq?) (eq? (first vals) (second vals))] 
     [(eq? name 'equal?) (equal? (first vals) (second vals))] 
     [(eq? name 'foldl) (foldl (first vals) 
           (second vals) 
           (third vals))] 
     [(eq? name 'error) (error (first vals) (second vals))])) 

    (eval-sequence lines global-env) 
) 

(eeval 
'(
    (define (even? n) 
    (if [= n 0] 
     #t 
     [odd? (- n 1)])) 

    (define (odd? n) 
    (if [= n 0] 
     #f 
     [even? (- n 1)])) 

    (define x #f) 
    (set! x (even? 6)) 
    x 
    )) 

正しいREPLプリント#t。別のファイルに続いて 、： - 貼り付けコードから(require "interpreter.rkt")から1つずつ

interpreter-test.rkt

#lang racket 

(require "interpreter.rkt") 

(eeval 
'(
    (define (eeval lines) ...) ;; copy paste code from interpreter.rkt 
    )) 

だから、私は、REPLは二回#tを印刷することを期待しています。代わりに私が貼り付けコードから(require "interpreter.rkt")から#tと役に立たないエラーメッセージが表示されます。

; mcdr: contract violation 
; expected: mpair? 
; given: '(lookup-in-env expr env) 

私は問題が何であるか見当がつかない。引用符の動作とは何か関係がありますか？任意のポインタが評価されるだろう。

更新： オスカー・ロペスは、プログラム全体にmconsを使用する必要があるかもしれないと示唆しました。しかし、コピーペーストコードを大きく変更する必要があるため、この種の自己評価インタプリタの目的は敗北します。だから、私はset-carを許可するので、代わりにR5RSに変更しようとしました！そしてset-cdr！

interpreter-r5rs.rkt

#lang R5RS 

(#%provide eeval) 

(define (eeval lines) 

    (define first car) 
    (define second cadr) 
    (define third caddr) 
    (define fourth cadddr) 

    (define (foldl proc init lst) 
    (cond 
     ((null? lst) init) 
     (else (foldl proc (proc (car lst) init) (cdr lst))))) 

    ; returns (key . val) if key in frame, #f otherwise 
    (define (lookup-in-frame key frame) 
    (cond 
     ((null? frame) #f) 
     ((eq? key (car (car frame))) (car frame)) 
     (else (lookup-in-frame key (cdr frame))))) 

    ; returns (key . val) if key in env, #f otherwise 
    (define (lookup-in-env key env) 
    (cond 
     ((null? env) #f) 
     (else (let ((key-val-pair (lookup-in-frame key (car env)))) 
       (if key-val-pair 
        key-val-pair 
        (lookup-in-env key (cdr env))))))) 

    (define (add-to-env! key value env) 
    (set-car! env 
       (cons (cons key value) 
        (car env)))) 

    (define (update-env! key value env) 
    (cond 
     ((null? env) 
     (myerror "set!: assignment disallowed; cannot set variable before its definition \nvariable:" key)) 
     (else (let ((key-val-pair (lookup-in-frame key (car env)))) 
       (if key-val-pair 
        (set-cdr! key-val-pair value) 
        (update-env! key value (cdr env))))))) 

    (define (extend-env keys values env) 
    (define (new-frame keys values) 
     (cond 
     ((null? keys) '()) 
     (else (cons (cons (car keys) (car values)) 
        (new-frame (cdr keys) (cdr values)))))) 
    (cons (new-frame keys values) env)) 

    (define global-env (cons '() '())) 

    (define (myeval expr env) 
    (cond 
     ((and (not (null? expr)) (not (pair? expr))) 
     (cond 
     ((boolean? expr) expr) 
     ((number? expr) expr) 
     ((string? expr) expr) 
     ((symbol? expr) 
      (let ((key-value (lookup-in-env expr env))) 
      (if key-value 
       (cdr key-value) 
       (if (member expr 
          '(member null? pair? 
            list cons car cdr cddr 
            set-car! set-cdr! 
            cadr caddr cadddr 
            boolean? not 
            number? = + - */expt 
            string? 
            symbol? eq? equal? 
            display)) 
        (lambda() (list 'primitive expr)) 
        (myerror expr "undefined"))))))) 
     ((null? expr) (myerror "()" "missing procedure expression.")) 
     ((eq? (car expr) 'quote) 
     (second expr)) 
     ((eq? (car expr) 'lambda) 
     (lambda() (list 'non-primitive 
         (second expr) 
         (cddr expr) 
         env))) 
     ((eq? (car expr) 'define) 
     (if (not (pair? (second expr))) 
      (if (lookup-in-frame (second expr) (car env)) 
       (myerror "duplicate definition for identifier in" 
         (second expr)) 
       (add-to-env! (second expr) (myeval (third expr) env) env)) 
      (myeval (list 'define 
         (car (second expr)) 
         (cons 'lambda 
           (cons (cdr (second expr)) 
            (cddr expr)))) 
        env))) 
     ((eq? (car expr) 'set!) 
     (update-env! (second expr) 
        (myeval (third expr) env) 
        env)) 
     ((eq? (car expr) 'begin) 
     (eval-sequence (cdr expr) env)) 
     ((eq? (car expr) 'cond) 
     (evcond (cdr expr) env)) 
     ((eq? (car expr) 'if) 
     (myeval (list 'cond 
        (list (second expr) (third expr)) 
        (list 'else (fourth expr))) 
       env)) 
     ((eq? (car expr) 'and) (evand (cdr expr) env)) 
     ((eq? (car expr) 'or) (evor (cdr expr) env)) 
     ((eq? (car expr) 'let) 
     (eval-sequence (cddr expr) 
         (extend-env 
         (map first (second expr)) 
         (map second (second expr)) 
         env))) 
     (else (myapply (myeval (car expr) env) 
        (eval-args (cdr expr) env))) 
    )) 

    (define (eval-sequence lines env) 
    (cond 
     ((not (null? lines)) 
     (if (null? (cdr lines)) 
      (myeval (car lines) env) 
      (begin (myeval (car lines) env) 
        (eval-sequence (cdr lines) env)))))) 

    (define (evcond lines env) 
    (cond 
     ((not (null? lines)) 
     (cond 
     ((eq? 'else (first (car lines))) 
      (myeval (second (car lines)) env)) 
     ((myeval (first (car lines)) env) 
      (myeval (second (car lines)) env)) 
     (else (evcond (cdr lines) env)))))) 

    (define (evand args env) 
    (cond 
     ((null? args) #t) 
     ((null? (cdr args)) (myeval (car args) env)) 
     (else (let ((val (myeval (car args) env))) 
       (if val 
        (evand (cdr args) env) 
        #f))))) 

    (define (evor args env) 
    (if (null? args) 
     #f 
     (let ((val (myeval (car args) env))) 
      (if val 
       val 
       (evor (cdr args) env))))) 

    (define (eval-args args env) 
    (cond 
     ((null? args) '()) 
     (else (cons (myeval (car args) env) 
        (eval-args (cdr args) env))))) 

    (define (myapply func vals) 
    (cond 
     ((eq? (first (func)) 'primitive) 
     (apply-primitive (second (func)) vals)) 
     ((eq? (first (func)) 'non-primitive) 
     (eval-sequence (third (func)) 
         (extend-env 
         (second (func)) 
         vals 
         (fourth (func))))) 
     (else (myerror func "unexpected case in myapply")))) 

    (define (apply-primitive name vals) 
    (define (list-helper vals) 
     (if (null? vals) 
      '() 
      (cons (car vals) (list-helper (cdr vals))))) 
    (define (=helper x l) 
     (cond 
     ((null? l) #t) 
     ((= (car l) x) (=helper x (cdr l))) 
     (else #f))) 
    (cond 
     ((eq? name 'member) (member (first vals) (second vals))) 
     ((eq? name 'null?) (null? (first vals))) 
     ((eq? name 'pair?) (pair? (first vals))) 
     ((eq? name 'list) (list-helper vals)) 
     ((eq? name 'cons) (cons (first vals) (second vals))) 
     ((eq? name 'car) (car (first vals))) 
     ((eq? name 'cdr) (cdr (first vals))) 
     ((eq? name 'cddr) (cddr (first vals))) 
     ((eq? name 'set-car!) (set-car! (first vals) (second vals))) 
     ((eq? name 'set-cdr!) (set-cdr! (first vals) (second vals))) 
     ((eq? name 'cadr) (cadr (first vals))) 
     ((eq? name 'caddr) (caddr (first vals))) 
     ((eq? name 'cadddr) (cadddr (first vals))) 
     ((eq? name 'boolean?) (boolean? (first vals))) 
     ((eq? name 'not) (not (first vals))) 
     ((eq? name 'number?) (number? (first vals))) 
     ((eq? name '=) 
     (if (or (null? vals) 
       (null? (cdr vals))) 
      (myerror "=" 
        "arity mismatch; expects at least 2 arguments.") 
      (=helper (car vals) (cdr vals)))) 
     ((eq? name '+) (foldl + 0 vals)) 
     ((eq? name '-) (- (car vals) (foldl + 0 (cdr vals)))) 
     ((eq? name '*) (foldl * 1 vals)) 
     ((eq? name '/) (/ (car vals) (foldl * 1 (cdr vals)))) 
     ((eq? name 'expt) (expt (first vals) (second vals))) 
     ((eq? name 'string?) (string? (first vals))) 
     ((eq? name 'symbol?) (symbol? (first vals))) 
     ((eq? name 'eq?) (eq? (first vals) (second vals))) 
     ((eq? name 'equal?) (equal? (first vals) (second vals))) 
     ((eq? name 'display) (display (first vals))) 
    )) 


    (define (myerror expr1 expr2) 
    (begin 
     (display expr1) 
     (display " ") 
     (display expr2) 
     (newline))) 

    (eval-sequence lines global-env) 
) 

(eeval 
'(
    (define (even? n) 
    (if (= n 0) 
     #t 
     (odd? (- n 1)))) 

    (define (odd? n) 
    (if (= n 0) 
     #f 
     (even? (- n 1)))) 

    (define x #f) 
    (set! x (even? 6)) 
    (display x) 
    )) 

interpreter-r5rs-test.rkt

#lang R5RS 

(#%require "interpreter-r5rs.rkt") 

(eeval 
'(
    (define (eeval lines) ...) ;; copy paste code from interpreter.rkt 
    )) 

いますが、彼らは「ていることを確認し、可変ペアを使用するつもりなら、私はまだエラー

; application: not a procedure; 
; expected a procedure that can be applied to arguments 
; given: (mcons 'expr (mcons 'env)) 
; arguments...: [none] 

Answer 1

を得ました再使用e非常に。例えば、このような表現を変換：

(cons 'x 'y) 

この中に、：

(mcons 'x 'y) 

そして、この：この中へ

'(a b c) 

：

(require compatibility/mlist) 
(mlist 'a 'b 'c) 

Answer 2

私はあなたが表現することをお勧めします環境、フレームa構造としての束縛。メーリングリストラケットユーザーのマタイアス・フェライセンへ

#lang racket 
; From SICP: 
; An environment is a sequence of frames. 
(struct environment (frames) #:mutable #:transparent) 
; Each frame is a table (possibly empty) of bindings, 
; which associate variable names with their corresponding values. 
; (A single frame may contain at most one binding for any variable.) 
; Each frame also has a pointer to its enclosing environment, unless, 
; for the purposes of discussion, the frame is considered to be global. 
(struct frame (bindings parent) #:mutable #:transparent) 
; The value of a variable with respect to an environment is the value 
; given by the binding of the variable in the first frame in the environment 
; that contains a binding for that variable. 
(struct binding (key value) #:mutable #:transparent) 
; If no frame in the sequence specifies a binding for the variable, 
; then the variable is said to be unbound in the environment. 

(define (lookup-in-env key env) 
    (match env 
    [(environment frames) 
    (lookup-in-frames key frames)])) 

(define (lookup-in-frames key frames) 
    (match frames 
    ['()   #f] ; unbound 
    [(cons f fs) (or (lookup-in-frame key f) 
        (lookup-in-frames key fs))])) 

(define (lookup-in-frame key f) 
    (match f 
    [(frame bindings parent) 
    (lookup-in-bindings key bindings)])) 

(define (lookup-in-bindings key bindings) 
    (match bindings 
    ['()   #f] ; unbound 
    [(cons b bs) (if (eq? key (binding-key b)) 
        b ; binding with key-value paring 
        (lookup-in-bindings key bs))])) 

(define (add-frame-to-env! f env) 
    (match env 
    [(environment frames) 
    (set-environment-frames! env (cons f frames))])) 

(define (update-env! key value env) 
    (let ([b (lookup-in-env key env)]) 
    (if b 
     (set-binding-value! b value) 
     (error 'update-env! (~a "no binding for " key))))) 

(define (extend-env keys values env) 
    (match env 
    [(environment (cons top-frame frames)) 
    (define bs (map binding keys values)) 
    (define new-f (frame bs top-frame)) 
    (set-environment-frames! env (cons new-f (cons top-frame frames)))])) 

(define global-env (environment (list (frame '() #f)))) 

(lookup-in-env '+ global-env) ; #f since plus is unbound 
(extend-env '(+ - * /) (list + - * /) global-env) 
(lookup-in-env '+ global-env) 

Answer 3

ありがとう：https://groups.google.com/forum/#!topic/racket-users/aFfGgh7Rfgc、私は問題を発見しました。それは、cons、mcons、quotesとは関係ありません。

問題はinterpreter.rktの間違いでした。通訳で。RKT、myevalの定義の下、let式の場合には、それがされている必要があります：

また

[(eq? (car expr) 'let) 
(eval-sequence (cddr expr) 
       (extend-env 
       (map first (second expr)) 
       (eval-args (map second (second expr)) env) 
       env))] 

、何らかの理由で、組み込みfoldlのを使用することはできません。それを自分で定義し、組み込み関数のリストからそれを削除すると動作します：

(define (foldl proc init lst) 
    (cond 
    ((null? lst) init) 
    (else (foldl proc (proc (car lst) init) (cdr lst))))) 

interpreter.rkt

#lang racket 

(provide eeval) 

(define (eeval lines) 

    ;; The global environment is a mutable list of frames, 
    ;; where each frame is a mutable list of 
    ;; mutable variable-value pairs. 
    ;; When a function is called, it creates a new frame 
    ;; which is a mutable list of parameter-argument pairs. 
    ;; Then it mcons the new frame to the enviroment the 
    ;; function was defined in. 
    (define global-env (mcons '() '())) 

    ; returns (mcons key val) if key in frame, #f otherwise 
    (define (lookup-in-frame key frame) 
    (cond 
     [(null? frame) #f] 
     [(eq? key (mcar (mcar frame))) (mcar frame)] 
     [else (lookup-in-frame key (mcdr frame))])) 

    ; returns (mcons key val) if key in env, #f otherwise 
    (define (lookup-in-env key env) 
    (cond 
     [(null? env) #f] 
     [else (let ([key-val-pair (lookup-in-frame key (mcar env))]) 
       (if key-val-pair 
        key-val-pair 
        (lookup-in-env key (mcdr env))))])) 

    (define (add-to-env! key value env) 
    (set-mcar! env 
       (mcons (mcons key value) 
         (mcar env)))) 

    (define (update-env! key value env) 
    (cond 
     [(null? env) 
     (error "set!: assignment disallowed; cannot set variable before its definition \nvariable:" key)] 
     [else (let ([key-val-pair (lookup-in-frame key (mcar env))]) 
       (if key-val-pair 
        (set-mcdr! key-val-pair value) 
        (update-env! key value (mcdr env))))])) 

    (define (extend-env keys values env) 
    (define (new-frame keys values) 
     (cond 
     ((null? keys) '()) 
     (else (mcons (mcons (car keys) (car values)) 
        (new-frame (cdr keys) (cdr values)))))) 
    (mcons (new-frame keys values) env)) 

    (define (myeval expr env) 
    (cond 
     [(and (not (null? expr)) (not (pair? expr))) 
     (cond 
     [(boolean? expr) expr] 
     [(number? expr) expr] 
     [(string? expr) expr] 
     [(symbol? expr) 
      (let ([key-value (lookup-in-env expr env)]) 
      (if key-value 
       [mcdr key-value] 
       [if [member expr 
          '(void void? null? member 
            pair? list cons car cdr cddr 
            mpair? mcons mcar mcdr 
            set-mcar! set-mcdr! 
            first second third fourth 
            boolean? false? not 
            number? = + - */expt 
            string? 
            symbol? eq? equal? 
            display error)] 
        [lambda() (list 'primitive expr)] 
        [error expr "undefined"]]))])] 
     [(null? expr) (error "()" "missing procedure expression.")] 
     [(eq? (car expr) 'quote) 
     (second expr)] 
     [(eq? (car expr) 'lambda) 
     (lambda() (list 'non-primitive 
         (second expr) 
         (cddr expr) 
         env))] 
     [(eq? (car expr) 'define) 
     (if [not (pair? (second expr))] 
      [if [false? (lookup-in-frame (second expr) (mcar env))] 
       [add-to-env! (second expr) (myeval (third expr) env) env] 
       [error "duplicate definition for identifier in" 
         (second expr)]] 
      [myeval (list 'define 
         (car (second expr)) 
         (cons 'lambda 
           (cons (cdr (second expr)) 
            (cddr expr)))) 
        env])] 
     [(eq? (car expr) 'set!) 
     (update-env! (second expr) 
        (myeval (third expr) env) 
        env)] 
     [(eq? (car expr) 'begin) 
     (eval-sequence (cdr expr) env)] 
     [(eq? (car expr) 'cond) 
     (evcond (cdr expr) env)] 
     [(eq? (car expr) 'if) 
     (myeval (list 'cond 
        (list (second expr) (third expr)) 
        (list 'else (fourth expr))) 
       env)] 
     [(eq? (car expr) 'and) (evand (cdr expr) env)] 
     [(eq? (car expr) 'or) (evor (cdr expr) env)] 
     [(eq? (car expr) 'let) 
     (eval-sequence (cddr expr) 
         (extend-env 
         (map first (second expr)) 
         (eval-args (map second (second expr)) env) 
         env))] 
     [else (myapply (myeval (car expr) env) 
        (eval-args (cdr expr) env))] 
    )) 

    (define (eval-sequence lines env) 
    (if [null? lines] 
     [void] 
     (if [null? (cdr lines)] 
      [myeval (car lines) env] 
      [begin (myeval (car lines) env) 
        (eval-sequence (cdr lines) env)]))) 

    (define (evcond lines env) 
    (cond 
     [(null? lines) (void)] 
     [(eq? 'else (first (car lines))) 
     (myeval (second (car lines)) env)] 
     [(myeval (first (car lines)) env) 
     (myeval (second (car lines)) env)] 
     [else (evcond (cdr lines) env)])) 

    (define (evand args env) 
    (cond 
     [(null? args) #t] 
     [(null? (cdr args)) (myeval (car args) env)] 
     [else [let ([val (myeval (car args) env)]) 
       (if [false? val] 
        #f 
        [evand (cdr args) env])]])) 

    (define (evor args env) 
    (if [null? args] 
     #f 
     [let ([val (myeval (car args) env)]) 
      (if val 
       val 
       (evor (cdr args) env))])) 

    (define (eval-args args env) 
    (cond 
     [(null? args) '()] 
     [else (cons (myeval (car args) env) 
        (eval-args (cdr args) env))])) 

    (define (myapply func vals) 
    (cond 
     [(eq? (first (func)) 'primitive) 
     (apply-primitive (second (func)) vals)] 
     [(eq? (first (func)) 'non-primitive) 
     (eval-sequence (third (func)) 
         (extend-env 
         (second (func)) 
         vals 
         (fourth (func))))] 
     [else (error func "unexpected case in myapply")])) 

    (define (apply-primitive name vals) 
    (cond 
     [(eq? name 'void) (void)] 
     [(eq? name 'void?) (void? (first vals))] 
     [(eq? name 'null?) (null? (first vals))] 
     [(eq? name 'member) (member (first vals) (second vals))] 
     [(eq? name 'pair?) (pair? (first vals))] 
     [(eq? name 'list) 
     (begin 
     (define (helper vals) 
      (if [null? vals] 
       '() 
       [cons (car vals) (helper (cdr vals))])) 
     (helper vals))] 
     [(eq? name 'cons) (cons (first vals) (second vals))] 
     [(eq? name 'car) (car (first vals))] 
     [(eq? name 'cdr) (cdr (first vals))] 
     [(eq? name 'cddr) (cddr (first vals))] 
     [(eq? name 'mpair?) (mpair? (first vals))] 
     [(eq? name 'mcons) (mcons (first vals) (second vals))] 
     [(eq? name 'mcar) (mcar (first vals))] 
     [(eq? name 'mcdr) (mcdr (first vals))] 
     [(eq? name 'set-mcar!) (set-mcar! (first vals) (second vals))] 
     [(eq? name 'set-mcdr!) (set-mcdr! (first vals) (second vals))] 
     [(eq? name 'first) (first (first vals))] 
     [(eq? name 'second) (second (first vals))] 
     [(eq? name 'third) (third (first vals))] 
     [(eq? name 'fourth) (fourth (first vals))] 
     [(eq? name 'boolean?) (boolean? (first vals))] 
     [(eq? name 'false?) (false? (first vals))] 
     [(eq? name 'not) (not (first vals))] 
     [(eq? name 'number?) (number? (first vals))] 
     [(eq? name '=) 
     (begin 
     (define (helper x l) 
      (cond 
      [(null? l) #t] 
      [(= (car l) x) (helper x (cdr l))] 
      [else #f])) 
     (if [or (null? vals) 
       (null? (cdr vals))] 
      [error "=" 
        "arity mismatch; expects at least 2 arguments."] 
      [helper (car vals) (cdr vals)]))] 
     [(eq? name '+) (foldl + 0 vals)] 
     [(eq? name '-) (- (car vals) (foldl + 0 (cdr vals)))] 
     [(eq? name '*) (foldl * 1 vals)] 
     [(eq? name '/) (/ (car vals) (foldl * 1 (cdr vals)))] 
     [(eq? name 'expt) (expt (first vals) (second vals))] 
     [(eq? name 'string?) (string? (first vals))] 
     [(eq? name 'symbol?) (symbol? (first vals))] 
     [(eq? name 'eq?) (eq? (first vals) (second vals))] 
     [(eq? name 'equal?) (equal? (first vals) (second vals))] 
;  [(eq? name 'foldl) (foldl (first vals) 
;        (second vals) 
     ;        (third vals))] 
     ((eq? name 'display) (display (first vals))) 
     [(eq? name 'error) (error (first vals) (second vals))])) 

    (define (foldl proc init lst) 
    (cond 
     ((null? lst) init) 
     (else (foldl proc (proc (car lst) init) (cdr lst))))) 

    (define (eval-print-sequence lines) 
    (if [null? lines] 
     [void] 
     [let ([result (myeval (car lines) global-env)]) 
      (if [void? result] 
       [eval-print-sequence (cdr lines)] 
       [begin (display result) 
        (display "\n") 
        (eval-print-sequence (cdr lines))])])) 

    (eval-print-sequence lines) 
) 

(eeval 
'(
    (define (even? n) 
    (if [= n 0] 
     #t 
     [odd? (- n 1)])) 

    (define (odd? n) 
    (if [= n 0] 
     #f 
     [even? (- n 1)])) 

    (define x #f) 
    (set! x (even? 6)) 
    x 
    ))