racket - 自我评估球拍解释器

Question

我一直在尝试编写一个可以自我评估的 Racket 解释器，但由于某种原因我无法让它工作。的代码interpreter.rkt非常标准。来自的代码interpreter-test.rkt可能是问题所在？我不确定。

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
   ))

正确的 repl 打印#t。然后，在另一个文件中：

interpreter-test.rkt

#lang racket

(require "interpreter.rkt")

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

所以，我希望 repl 打印#t两次——一个来自(require "interpreter.rkt")粘贴的代码，一个来自粘贴的代码。相反，我从粘贴的代码中得到了一条#t来自(require "interpreter.rkt")和无用的错误消息：

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

我不知道问题是什么。它与引号的行为方式有关吗？任何指针将不胜感激。

更新：Oscar Lopez 建议我可能需要在整个程序中使用 mcons。但是，这违背了自我评估解释器的目的，因为我需要大量修改复制粘贴的代码。所以，我尝试改用 R5RS，因为它允许坐车！和设置-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
   ))

但我仍然收到错误

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

Answer 1

如果您要使用可变对，请确保它们在任何地方都被使用。例如，像这样转换表达式：

(cons 'x 'y)

进入这个：

(mcons 'x 'y)

和这个：

'(a b c)

进入这个：

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

Answer 2

我建议您将环境、框架和绑定表示为结构。

#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

感谢 Racket 用户邮件列表中的 Matthias Felleisen：https ://groups.google.com/forum/#!topic/racket-users/aFfGgh7Rfgc ，我发现了问题。它与 cons、mcons 或引号无关。

问题是interpreter.rkt 中的一个错误。在interpreter.rkt 中，在myeval 的定义下，在let 表达式的情况下，它应该是：

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

此外，由于某种原因，使用内置 foldl 不起作用。自己定义它并将其从内置函数列表中删除是可行的：

(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
   ))