"Wayne O. Cochran" <[EMAIL PROTECTED]> writes:
> I've cut and pasted together what seems
> to be the concensus answer:
>
> > MAX is predefined in Common Lisp.
>
> > MAX is a builtin function.
>
> > your problem comes from redefining a standard Common Lisp function.
>
> There were also a couple of suggestions to place max in a different
> namespace or give it another name, and of course I could do
> that. The students also suggested the aforementioned namespace
> collision with a builtin. My response way "bah" -- Lisp allows you
> to redefine functions -- even the builtin ones (although this is not
> generally a good idea). To be a language lawyer, here is a quote
> from Steel's "Common Lisp," sec 5.3.1, p.85.
>
> "It is permissible to use defun to redefine a function,
> to install a corrected version of an incorrect definition,
> for example. It is permissible to redfine a macro as
> a function."
To be a /real/ language lawyer, you have to quote the HyperSpec:
# 11.1.2.1.2 Constraints on the COMMON-LISP Package for Conforming
# Programs
#
# Except where explicitly allowed, the consequences are undefined if
# any of the following actions are performed on an external symbol of
# the COMMON-LISP package:
# ...
# 2. Defining, undefining, or binding it as a function.(Some
# exceptions are noted below.)
(none of the exceptions apply).
> If one could not redefine the builtin functions then one would have
> to keep mental track of the very large set of functions, macros,
> etc... that are predefined in Lisp.
That's right.
> Also, even if one could not redefine the builtin functions, the
> problem should manifest itself at definition time:
>
> * (defun max (a b) (if (> a b) a b))
> MAX
> * #'max
> #<Interpreted Function MAX {4801E451}>
>
> Everythings seems fine at this point. Shouldn't I have at least been
> issued a warning of some sort?!?
You have been told how to protect yourself in CMUCL. Other
implementations have their own ways. In LispWorks, check the variable
*PACKAGES-FOR-WARN-ON-REDEFINITION*, for instance.
> > YOU created an infinite regress.
>
> Assuming what I did was wrong,
It was.
> I am a little bit curious as to how this manifested itself. I have a
> hard time seeing how this could have triggered an "infinite
> regress." Either my max or the system's max should have been
> invoked. Can anyone explain how the system could have been confused
> into issuing multiple recursive calls?
>
> In any case, I wouldn't call this a "feature."
Well, it may do anything it likes... such are the joys of undefined
behavior.
> Any more thoughts? Thanks for your time.
Do you know /why/ it is undefined behavior? Think a bit about it
(hint: macros...)!
Regards,
--
Nils G�sche
"Don't ask for whom the <CTRL-G> tolls."
PGP key ID 0x0655CFA0
