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From: engelson-sean@cs.yale.edu (Wisp Lizard)
Newsgroups: comp.lang.lisp
Subject: Re: scope/extent interaction with flet and load
Message-ID: <1990Oct26.133156@cs.yale.edu>
Date: 26 Oct 90 17:31:56 GMT
References: <60375@bbn.BBN.COM>
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In article <60375@bbn.BBN.COM>, sboisen@bbn.com (Sean Boisen) writes:
|> 
|> Scenario: assume a file foo.lisp which contains the following single form.
|> 
|> 	(print (+ 1 2))
|> 
|> What i expected to see when executing the following
|> 
|> > (flet ((+ (a b) (cons a b))) (load "foo"))
|> 
|> was something like
|> (1 . 2)
|> 
|> Instead (at least under Lucid and Allegro on a Sun) i see 
|> 
|> 3
|> 
|> In other words, my flet "redefinition" of + wasn't in effect when the
|> file got loaded, even though it seems to me the loading is inside the
|> scope of the flet, according to the rules in CLtL. Can somebody
|> explain to me why this behavior is correct (assuming it is)? 

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.

|> Sean

Coincidence?

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