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From: Stefan Monnier <monnier@iro.umontreal.ca>
To: help-gnu-emacs@gnu.org
Subject: Re: Replacement for Common Lisp's GENSYM in Emacs Lisp
Date: Sun, 07 Mar 2010 14:06:14 -0500	[thread overview]
Message-ID: <jwv4oksm005.fsf-monnier+gnu.emacs.help@gnu.org> (raw)
In-Reply-To: 87tysssj78.fsf@rapttech.com.au

> This is actually a non-trivial problem.  Best approach would probably be
> to do something similar to how they generate universally unique IDs.

The best approach often is to not use "unique symbols" at all.  A common
technique is to rely on lexical scoping to precisely control which
variables are visible to each part of a macro's argument during its
evaluation.  E.g.:

  (defmacro dotimes (xl e)
    (let ((x (car xl))
          (l (cadr xl)))
      `(let ((body (lambda (,x) ,e))
             (list ,l))
         (while (consp list)
           (funcall body (car list))
           (setq list (cdr list))))))

Notice how the expressions `l' and `e' are evaluated in a context where
neither of `body', nor `list' exist.  No need for gensym.

Of course Elisp's lack of good support for lexical scoping makes such
a technique impractical.  But `make-symbol' can be used instead of
`gensym' and works very well for that purpose.


        Stefan


  reply	other threads:[~2010-03-07 19:06 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <mailman.2359.1267888813.14305.help-gnu-emacs@gnu.org>
2010-03-06 16:10 ` Replacement for Common Lisp's GENSYM in Emacs Lisp David Kastrup
2010-03-06 16:35   ` Teemu Likonen
2010-03-07  7:07 ` Tim X
2010-03-07 19:06   ` Stefan Monnier [this message]
2010-03-07 22:07     ` Helmut Eller
2010-03-08  4:55       ` Stefan Monnier
2010-03-08  7:01         ` Helmut Eller
2010-03-09  2:33           ` Stefan Monnier
2010-03-06 15:19 Teemu Likonen
2010-03-07  5:48 ` tomas
2010-03-07  6:13   ` Teemu Likonen
     [not found]   ` <mailman.2390.1267942438.14305.help-gnu-emacs@gnu.org>
2010-03-11 22:48     ` Pascal J. Bourguignon

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