Self evaluating Racket interpreter

Question

I've been trying to write a Racket interpreter that can evaluate itself but I can't get it to work for some reason. The code for interpreter.rkt is pretty standard. The code from interpreter-test.rkt might be the problem? I'm not sure.

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
   ))

The repl print #t which is correct. Then, in another file:

interpreter-test.rkt

#lang racket

(require "interpreter.rkt")

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

So, I expect the repl to print #t twice - one from (require "interpreter.rkt") and one from the pasted code. Instead I get a #t from (require "interpreter.rkt") and an unhelpful error message from the pasted code:

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

I have no idea what the problem is. Does it have something to do with how quotes behave? Any pointers would be appreciated.

UPDATE: Oscar Lopez suggested that it might be that I need to use mcons for the whole program. However, that kind of defeats the purpose of a self-evaluating interpreter since I would need to modify the copy pasted code heavily. So, I tried changing to R5RS instead since it allows set-car! and 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
   ))

but I still got the error

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

Answer 1

If you're going to use mutable pairs, make sure that they're used everywhere. For instance, transform expressions like this:

(cons 'x 'y)

Into this:

(mcons 'x 'y)

And this:

'(a b c)

Into this:

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

Answer 2

I recommend that you represent the environment, frames and bindings as structures.

#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

Thanks to Matthias Felleisen on the Racket users mailing list: https://groups.google.com/forum/#!topic/racket-users/aFfGgh7Rfgc, I found the problem. It has nothing to do with cons, mcons, or quotes.

The problem was a mistake in interpreter.rkt. In interpreter.rkt, under the definition of myeval, in the case of a let expression, it should have been:

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

Also, for some reason, using the builtin foldl doesn't work. Defining it yourself and removing it from the list of builtin functions works:

(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
   ))