CPL anxieties

[edsel!bhopal!jonl@navajo.stanford.edu (Jon L White)]

Re: If the currently proposed algorithm is used, you will not be able to see
    the difference between merging turtle and animate-thing, and/or merging
    gren and colored things.  . . . 
The "anxiety" followed from merging Colored-things and Animate-things,
rather than either of those with their direct inferiors.  When they are
merged (and they have no similar methods so it would be a disjoint
merger), then the "up to join" nature of the pre-order treewalk forces
a semi-breadth-first ordering.  Which is what the original source of
this example was trying to achieve.


Re                          . . . The order proposed puts both turtle and
    animate before both green and colored, so the latter two will have no
    effect wrt the two methods of concern.  I claim this is correct.
This is simply the depth-first order.  While it enforces *some* ordering
on the problem, there is no reason in the topology of the inheritance
graph to believe that one ordering is preferable to another.  In such
circumstances, either there are some methods which would be inherited from
essentially *incomparable* classes, or there are no such methods.  In the
latter case, it is a "moot" point, but in the former, shouldn't an error
of some sort be signalled?  or shouldn't there be some way for the user
to specify more exactly what he wants?

The anxieties, then, can be summarized as
1) defining a "mix in" subclass forces the programmer to specify an order
   among the direct suprs; but this ordering may have no relation whatsoever
   to the application problem, and it will probably determine all the
   method inheritance choices.
2) merging/spliting a class "way up" in the inheritance graph can override
   what methods the new class has;  but the programmer wanted to be able to 
   say something (anything?) which would insure the selection of the correct 
   blush and pen-up methods.

Maybe there should be an "unordered" defclass, for which the CPL is built
exactly like in the "ordered" case, but for which the error signalling
conditions might be more stringent.

-- JonL --