Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!samsung!munnari.oz.au!yoyo.aarnet.edu.au!sirius.ucs.adelaide.edu.au!levels!majgr
From: Jenny.Rowland@levels.sait.edu.au
Newsgroups: comp.lang.lisp
Subject: Re: Self-modifying function
Message-ID: <15983.27dfd64c@levels.sait.edu.au>
Date: 14 Mar 91 09:30:12 GMT
References: <11609@jpl-devvax.JPL.NASA.GOV>
Organization: University of South Australia
Lines: 43

In article <11609@jpl-devvax.JPL.NASA.GOV>, charest@ai-cyclops.jpl.nasa.gov writes:
> 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


You could try something like:

(defun mutate (list)
  (let ((len (length list)))
    (setf (symbol-function 'mutate) #'(lambda () len))
    list))

I am not one for altering function definitions on the fly - but if one must,
this version seems 'nicer' than the other. I would welcome any comments on it
however.

-Jenny Rowland
jenny.rowland@sait.edu.au