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From: Stefan Monnier <monnier@iro.umontreal.ca>
Newsgroups: gmane.emacs.help
Subject: Re: Declaring a local dynamic variable?
Date: Tue, 24 Sep 2013 17:40:41 -0400
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> CL has the (declare (special var…)) declaration for that.

And cl.el does provide some support for it, indeed.

> (defun f ()
>   (declare (special var))
>   (print var))

Since Elisp's `defun' macro also supports `declare', we could provide
support for the above, tho you can just as well write it as

   (defun f ()
     (defvar var)
     (print var))

>   (let ((var 42))
>      (declare (special var))
>      (f)))

Supporting for byte-compiled files would be fairly easy, but when
interpreting code it it would be very costly, so Elisp does not
support this.  With Emacs-24.3's eager macro-expansion the cost would be
a lot more reasonable, but I still don't find it worth the trouble.

Instead you have to write:

   (defvar var)
   (let ((var 42))
      (f)))


-- Stefan