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Pascal J. Bourguignon  
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 More options Apr 21 2011, 2:33 pm
Newsgroups: comp.lang.lisp
From: "Pascal J. Bourguignon" <p...@informatimago.com>
Date: Thu, 21 Apr 2011 20:33:13 +0200
Local: Thurs, Apr 21 2011 2:33 pm
Subject: Re: flet vars
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t...@sevak.isi.edu (Thomas A. Russ) writes:

> TheFlyingDutchman <zzbba...@aol.com> writes:

>> > (defvar *x* 'global)

>> How is this is represented internally. Are there now two entries in a
>> symbol table (or similar structure) for "*x*" and "global"? Does the
>> entry for "*x*" point to a 5 cell structure known as a symbol and
>> likewise for the "global" entry. Does the "symbol value cell" of the
>> *x* symbol structure point to the symbol structure for the "global"
>> symbol?

> How this is represented internally is not specified.  

There isn't even an API to know if a symbol has been declared special.
But we can detect it:

(defun specialp (symbol)
  "Detects whether the symbol has been declared special."
  (eval `(flet ((f () ,symbol)) (let ((,symbol t)) (not (nth-value 1 (ignore-errors (f))))))))

(defvar a 1)

(values
 (funcall (compile nil (lambda ()
                         (let ((b 1))
                           (declare (special b))
                           (list (specialp 'a) (specialp 'b) (specialp 'c))))))
 (list (specialp 'a) (specialp 'b) (specialp 'c)))

--> (T T NIL)
    (T NIL NIL)

SPECIAL-ness is a dynamic property, in the global environment.

--
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.


 
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