🚀 go-pugleaf

In article <LGM.91Jun18195405@cbnewsc.ATT.COM> lgm@cbnewsc.ATT.COM (lawrence.g.mayka) writes:
>In article <1991Jun17.210108.29347@Think.COM> barmar@think.com (Barry Margolin) writes:
>In article <ALANR.91Jun17130803@chopin.media-lab.media.mit.edu> alanr@chopin.media-lab.media.mit.edu (Alan Ruttenberg) writes:
>>(deftype C () '(or A B))
>>
>>(defmethod common ((instance C))
>> ...)
>>
>>This doesn't work. (can't find class C)
>>
>>My only two choices seem to be to either replicate the code, or to factor the
>>body of the definition of common into a defun which is called by both.
>
>If A and B have any ancestors in common you could define the method on a
>common ancestor, and then use (check-type instance 'C) to catch misuse.
>But that's also pretty inelegant.  

I'm having a similar problem.  I wrote an AVL tree in Lisp a few
months back using a 4-element list as my basic data structure (left,
data, height, right).  Now I'm trying to convert it to CLOS.  I
defined the methods with parameter specializers that make them take
my avl-tree class, but I need to be able to have "no avl-tree", which
I was implementing as "nil".  But of course none of the avl-tree
methods recognize the null stuff.  Is factoring the only way to
get my methods to recognize both avl-tree and null as valid instances
of avl-tree?  I tried (defclass avl-tree (null) blah blah blah) but
it didn't help.

If there is no really good solution to this, I'll be amazed, since
out of the OOP paradigms I've seen so far, CLOS has been the best.

Steve Boswell         | This opinion is distributed in the hopes that it
whatis@ucsd.edu       | will be useful, but WITHOUT ANY WARRANTY...
whatis@gnu.ai.mit.edu |
-- 
Steve Boswell         | This opinion is distributed in the hopes that it
whatis@ucsd.edu       | will be useful, but WITHOUT ANY WARRANTY...
whatis@gnu.ai.mit.edu |