Path: utzoo!attcan!uunet!cs.utexas.edu!hellgate.utah.edu!cdr.utah.edu!moore
From: moore%cdr.utah.edu@cs.utah.edu (Tim Moore)
Newsgroups: comp.lang.lisp
Subject: Re: scope/extent interaction with flet and load
Message-ID: <1990Oct26.122108.27095@hellgate.utah.edu>
Date: 26 Oct 90 18:21:08 GMT
References: <60375@bbn.BBN.COM> <1990Oct26.133156@cs.yale.edu>
Distribution: comp
Organization: University of Utah CS Dept
Lines: 39

In article <1990Oct26.133156@cs.yale.edu> engelson-sean@cs.yale.edu (Wisp Lizard) writes:
>In article <60375@bbn.BBN.COM>, sboisen@bbn.com (Sean Boisen) writes:
>|> 
>|>[rebinding macros and functions doesn't work as expected when loading]
>
>This behavior is correct because of Common Lisp's lexical scoping; the
>contents of the file are outside the lexical scope of the flet.  To do
>what you want with functions, you need to change the global function
>definition, as follows:
>
>(let ((old+ #'+)) 
>  (setf (symbol-function '+) #'cons)
>  (unwind-protect
>      (load "fu")  ; note the correct spelling :-)
>    (setf (symbol-function '+) old+))
>
>Note the use of unwind-protect in case an error occurs while loading.  I
>don't know how to do this with macros, however.

I'm sure the function names are for illustrative purposes, but
remember that redefining CL functions like #'+ is a really bad idea.

You can do the same trick with macros:

(let ((old-macro-def (macro-function 'foo)))
  (setf (macro-function 'foo) (macro-function 'bar))
  (unwind-protect
       (load "foo")
    (setf (macro-function 'foo) old-macro-def)))

With the new macro functions in CLtL2 it's not hard to write a macro
that dynamically binds macro definitions. Of course, this only will
only work for forms that are being explicitly evaluated (either by
eval or load).

>|> Sean
Tim Moore                    moore@cs.utah.edu {bellcore,hplabs}!utah-cs!moore
"Ah, youth. Ah, statute of limitations."
		-John Waters