🚀 go-pugleaf

   Date: 27 Jun 91 04:03:27 GMT
   Organization: UC San Diego -- 1991 "Wannabe Real College" finalist
   From: sboswell@sdcc13.ucsd.edu (....What Is?....)
   Sender: commonloops-request@cis.ohio-state.edu


   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 |

You need to have methods for both avl-tree and null.  I really doubt you
want avl-tree to inherit the characteristics of NIL.  The other possiblity
is to have a null-avl-tree class to use instead of nil and then avl-tree
can be a subclass of that.  You still need different methods since the
null-avl-tree has no slots.