restarts and MI as possible omissions

[Alan Bawden <Alan@lcs.mit.edu>]

   Date: Fri, 19 Apr 96 19:27:31 BST
   From: Jeff Dalton <jeff@aiai.ed.ac.uk>

   Alan Bawden wrote:
   > See?  Multiple inheritance doesn't have to be scary!

   That is the sort of thing I had in mind by saying that MI was not 
   the only way to express that "recovery strategies for several types
   of conditions might be appropriate for a particular exceptional
   situation" (which is what Richard Kelsey said made MI necessary).

   What you describe can be seen as MI, true, but it's not what people
   will expect ...

Hmmm...  Well then before you guys start having heated arguments about
condition inheritance you need to get your terminology straight.  I had
imagined that most people would agree that every condition system was
either NI (No Inheritance), SI (Single Inheritance) or MI (Multiple
Inheritance).  If I have to fit the Emacs Lisp condition system into one of
those categrories, it's pretty clearly in MI.

   You probably didn't intend this as a proposal -- that a condition be
   respresented by a list of symbols -- but I think it's instructive to
   treat it as one.

I thought it was clear that I wasn't making a proposal.

     * Even it we adopt the basic idea, conditions should be records
       that contain, in some field, a list of error types (or whatever
       we call them).  Having conditions just _be_ such lists is too
       limiting and insufficiently abstract.  [We we're back again to
       the questions of records, inheritance, and object systems.]

Well, they really don't need to be records either.  I think you will find
that association-lists already have all the right properties.