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From: charest@ai-cyclops.jpl.nasa.gov
Newsgroups: comp.lang.lisp
Subject: Self-modifying function
Message-ID: <11609@jpl-devvax.JPL.NASA.GOV>
Date: 27 Feb 91 23:01:27 GMT
Sender: news@jpl-devvax.JPL.NASA.GOV
Organization: NASA/Jet Propulsion Lab
Lines: 27

I want to define a function that performs a certain computation on its first
invocation and performs a different computation on all subsequent invocations.
Consider the trivial (but representative) example below:

(defun mutate (list)
  (let ((len (length list)))
    (prog1 
      list
      (compile 'mutate
               `(lambda ()
                  ,len)))))

1> (mutate '(a b c))
=> (a b c)
2> (mutate '(a b c))
=> Error: too many arguments to function mutate.
3> (mutate)
=> 3

My question is, Will this function behave as expected (i.e., as described
above) in any Common LISP? More specifically, is it safe to modify the contents
of a symbols' function cell even as the contents are being executed? I can find
nothing in CLtL2 that addresses this. I welcome all opinions, learned or
otherwise.

-Len Charest
charest@ai-cyclops.jpl.nasa.gov