How do I test whether a variable is defined before referencing it?

Question

I would like to be able to test whether a variable is defined, prior to accessing it.

I like to have a global that specifies a \"debug level\". If debug level is 0, no

Answer 1

Clunky but workable solution for R5RS. Use the often neglect/forgotten ability of let-syntax to redefine keywords. this is clunky because your whole file is wrapped in a let-syntax and because it adds some overhead to each define. I use a associative list to remember definitions, a hash table would be a beter choice.

(define define-list '())
(define define-list-add 
  (lambda (key value)
    (set! define-list (cons `(,key ,value) define-list))))

(let-syntax (
             (define (syntax-rules ()
                       ((_ (pro-name args ...) body ...) 
                        (begin
                          (define (pro-name args ...) body ...)
                          (define-list-add pro-name  '((pro-name args ...) body ...))))
                       ((_ pro-name pro) (begin
                                           (define pro-name pro)
                                           (define-list-add 'pro-name 'pro)))

                       ))
             (defined?
               (syntax-rules ()
                 ((_ sym) (begin (if (assoc (quote sym) define-list) #t #f)))))
             )
  (define y (lambda () x))

  (display (defined? y))
  (newline)
  (display (defined? x))
  )

prints

#t
#f

Below in racket: a module is used to redefine define to store each symbol and definition in a list called define-list. The macro defined? looks in this list to see if weather or not the symbol has been defined.

(module qdefine mzscheme
  (provide ;(all-from-except mzscheme let)
   (rename define olddefine)
   (rename quote-define define)
   defined?)

  (define define-list '())
  (define define-list-add 
    (lambda (key value)
      (set! define-list (cons `(,key ,value) define-list))))

  (define-syntax quote-define
    (syntax-rules ()
      ((_ (pro-name args ...) body ...) 
       (begin
         (define (pro-name args ...) body ...)
         (define-list-add pro-name  '((pro-name args ...) body ...))))
      ((_ pro-name pro) (begin
                          (define pro-name pro)
                          (define-list-add 'pro-name 'pro)))

      ))

  (define-syntax defined?
    (syntax-rules ()
      ((_ sym) (begin (if (assoc (quote sym) define-list) #t #f)))))
  )
(require 'qdefine)

(define y (lambda () x))

(defined? y)
(defined? x)

In guile it is just defined? apparently: http://www.delorie.com/gnu/docs/guile/guile_289.html