方案宏展开显示变量未绑定

Question

当我阅读 Lisp in Small Pieces 时，我正在编写一个简单的 lisp 解释器，但我在这个错误上停留了 2 个多小时。我 am 定义了一个本地 genv 变量，但我仍然收到此错误。一定有一些宏扩展的东西我看不懂，我用引号检查了扩展，似乎还可以。请如果有人可以阐明它会很棒。 （代码适用于 r5rs 语言和 guile 方案）

;; Macro to print it's given arguments line by line and end with a ------
(define-syntax println
  (syntax-rules ()
    ((_ expr expr* ...) (begin (display expr)
                               (newline)
                               (println expr* ...)))
    ((_ expr) (begin (display expr) (newline)))
    ((_) (display "--------\n"))))

(define (evaluate expr env)
  (begin (println "Evaluating" expr)
  (if (not (pair? expr))
      (cond ((symbol? expr) (lookup env expr))
            ((or (number? expr) (string? expr) (char? expr) (boolean? expr) (vector? expr)) expr)
            (else (error "Cannot evaluate" expr)))

      ;; not atom
      (case (car expr)
        ((quote) (cadr expr))

        ;; (define name expr)
        ((define) (update (cadr expr) (caddr expr) env))

        ((if) (if (evaluate (cadr expr) env)
                  (evaluate (caddr expr) env)
                  (evaluate (cadddr expr) env)))

        ((begin) (eprogn (cdr expr) env))

        ((set!) (update (cadr expr) (evaluate (caddr expr) env) env))

        ((lambda) (make-function (cadr expr) (cddr expr) env))

        (else (invoke (evaluate (car expr) env)
                      (evlis (cdr expr) env)))))))

;; Evaluates all the expressions (exprs) in the given environment (env)
(define (eprogn exprs env)
  (if (pair? exprs)

      ;; False when exprs contains just one item
      (if (pair? (cdr exprs))

          (begin (evaluate (car exprs) env)
                 (eprogn (cdr exprs) env))

          (evaluate (car exprs) env))

      '()))


(define (evlis exprs env)
  (if (pair? exprs)
      (cons (evaluate (car exprs) env)
            (evlis (cdr exprs) env))
      '()))

;; Makes a new applicable function, that closes the environment (env)
(define (make-function vars body env)
  (lambda (vals)
    (eprogn body (extend-environment env vars vals))))


(define (invoke fn args)
  (if (procedure? fn)
      (fn args)
      (error "Not a function" fn)))

;; Environment suite

;; Helper macros for working with an environment vector

;; Returns the parent environment of (env)
(define-syntax parent-env-of
  (syntax-rules ()
    ((parent-env-of env) (vector-ref env 0))))

;; Returns the bind-map of (env)
(define-syntax bind-map-of
  (syntax-rules ()
    ((bind-map-of env) (vector-ref env 1))))

;; Sets the parent environment of (env)
(define-syntax set-parent-env!
  (syntax-rules ()
    ((set-parent-env! env parent-env) (vector-set! env 0 parent-env))))

;; Sets the bind-map of (env)
(define-syntax set-bind-map!
  (syntax-rules ()
    ((set-bind-map! env bind-map) (vector-set! env 1 bind-map))))

;; Makes a new environment with the parent env set to (parent-env)
(define (make-new-environment parent-env)
  (let ((new-env (vector #f #f)))
    (begin
      (set-parent-env! new-env parent-env)
      (set-bind-map! new-env '())
      new-env)))

;; Searches for the value of (sym) in (env), raises
;; error if it can't find
(define (lookup env sym)
  (if (null? env)
      (error "Unbound name" sym)
      (let ((val (assoc sym (bind-map-of env))))
        (if (equal? val #f) (lookup (parent-env-of env) sym) (cdr val)))))

;; Create the binding update the (sym)'s value to (value) in the given (env)
(define (update sym value env)
  (begin (println "Called update with env: " env "sym: " sym "value: " value)
         (define new-bind-map (assoc-set! (bind-map-of env) sym value))
         (set-bind-map! env new-bind-map)))

;; Extends an (env) by creating a new environment and setting the
;; bindings specified by the list of symbols (vars) and the
;; list of values (vals)
(define (extend-environment vars vals env)
  (define new-env (make-new-environment env))
  (update-all vars vals env))

;; Helper function
(define (update-all vars vals env)
  (cond ((pair? vars) (if (not (pair? vals))
                          (error "More symbols than values to bind with")
                          (begin (update (car vars) (car vals) env)
                                 (extend (cdr vars) (cdr vals) env))))
        ((null? vars) (if (not (null? vals))
                          (error "More values than symbols to bind with")
                          env))))

;; Helper macros for initializing the global env bind map

有问题的代码：

;; ------------PROBLEM IN THESE MACROS------------------

(define-syntax _def-initial
  (syntax-rules ()
    ((_def-initial name)
     (update 'name 'void genv))
    ((_def-initial name value)
     (update 'name value genv))))

(define-syntax _def-primitive
  (syntax-rules ()
    ((_def-primitive name value arity)
     (_def-initial name (lambda (args)
                          (if (equal? arity (length args))
                              (apply value args)
                              (error "Incorrect arity" (list 'name value))))))))

(define-syntax _fill-global-env
  (syntax-rules ()
    ((_fill-global-env)
     (begin
       (println "Filling the environment")
       (_def-primitive + (lambda (x y) (+ x y)) 2)
       (_def-primitive - (lambda (x y) (- x y)) 2)
       (_def-primitive * (lambda (x y) (* x y)) 2)
       (_def-primitive / (lambda (x y) (/ x y)) 2))
    )))


;; Racket and Guile SAY genv IS UNBOUND

(define get-global-environment
  ;; name must be `genv' coz of the above macros
  (let ( (genv #f) )
    (lambda ()
      (if (equal? genv #f) ;; If uninitialized
          (begin (set! genv (make-new-environment '()))
                 (println "Before filling: "genv)
                 (_fill-global-env)
                 (println "After filling: " genv)
                 genv)
          genv))))

;; ------------------- END OF PROBLEMATIC CODE(IT SEEMS) ---------------

继续：

;; - Start the interpreter
(define (main args)
  ;; Define the global environment
  (define genv (get-global-environment))
  (println "Global environment: " genv)
  (let loop ((expr (read (current-input-port))))
    (if (eof-object? expr)
        (println "Done")
        (begin (println (evaluate expr genv))
               (loop (read (current-input-port)))))))

(main "")

这是我从 Racket 收到的错误（在有问题的代码 get-global-environment 的正文中，而不是在 main 的正文中）：

. . genv: undefined;
 cannot reference undefined identifier

Answer 1

方案宏是hygienic。您在get-global-environment 中定义的genv 与您在_def-initial 中的genv 不同（它使用在定义_def-initial 时存在的任何genv，在这种情况下将是顶级一个，正如你指出的那样不存在）。

为了使你的宏工作，你必须修改_fill-global-env、_def-primitive和_def-initial都带一个genv参数，这样_def-initial使用那个@ 987654333@ 而不是顶级的。