Hi Lispers,
When I learned about object oriented programming one of the views that
was taught was that the set of methods of a class should preserve a
set of invariants related to the class. I.e. the object state before
and after a (official or exported) method is run should satisfy a set
of constraints, which of course could be violated while running the
method. I like this view, since it defines clearly the responsibility
of the method writer with respect to the method user.
With CLOS the methods don't belong to a class but I think the view
could be adopted, especially in the non-multi method case. E.g. if a
slot has an accessor before/after and around methods could be used for
ensuring that constraints are satisfied. However, there are often
inter-class constraints that needs to be preserved. In my graph editor
application, each node has an out-edges slot with a list of edges and
each edge has a from-node slot which point back. A constraint for the
node-edge relation could be formulated as:
(member edge (out-edges node)) <=> (eq (from-node edge) node),
which means that whenever an edge is in the out-edges list of a node,
the from-node slot of edge should point to this node. I have written a
macro, define-slot-relation, that expands such a constraint into two
setf-around-methods, one for each slot.
My question is whether it's best to generate setf-around-methods to
take care of such a constraint, or should I make another layer of
methods, e.g. set-node-out-edges and set-edge-from-node. Personally, I
prefer using setf-methods, if they shouldn't be used for such things
what should they be used for? In addition, we already have a layer
below, (setf slot-value). A problem I see, is that the user of the
setf-method perhaps expects the implementation to have low
computational complexity e.g. O(1) instead of my O(n). Anyway, I have
discussed this with my colleagues and we don't agree, so I thought I'd
ask other lispers for their opinion.
Regards, Hallvard
--
Hallvard Traetteberg
Dept. of Knowledge Based Systems
Center for Industrial Research
Box 124 Blindern, 0314 Oslo 3
NORWAY
Tlf: +47 2 45 29 83 or +47 2 45 20 10
Fax: +47 2 45 20 40
Email: ···················@si.no