Re: Canonical DB
Date: Sat, 24 Jun 2006 21:31:38 +0200
On Sat, 24 Jun 2006 14:18:26 +0200, mAsterdam wrote:
>> mAsterdam wrote:
>>> ...A correct model would have the constraints >>> in place to preserve the invariant of the >>> structure while allowing all valid modifications. >> >> Yes. But the problem is (apart from expressing the constraints, which >> itself might be far from trivial: like an acyclic graph constraint etc), >> that you might lose the structure.
> Is your 'lose the structure' just another way of
> stating 'the invariant is not preserved',
> or is there more to it?
Sort of. Though one could argue that the invariant is in some sense the structure abstracted from the elements.
>> To me this all is the infamous LSP problem.
> LSP? Label-switch path as in
> http://www.ietf.org/rfc/rfc3209.txt ?
Liskov Substitutability Principle
Alas, but LSP does not work, for the reason that any non-trivial modification of a structure changes it, and so produces a different structure.
>>>I tried modeling the C3 MRO relationally >>>but haven't found a solution (or the conviction >>>that it can't be done) yet. >>> >>>http://www.python.org/2.3/mro.html >>> >>>and (background) >>> >>>http://www.webcom.com/haahr/dylan/linearization-oopsla96.html#C3-line >>> >>>It is a nice (and IMHO very useful) challenge. >>>Any takers? >> >> Ah, multiple dispatch, it is a hard problem. I'm unsatisfied with presently >> existing solutions. >> >> BTW, it might be interesting to consider relational structures of types.
> Yes, please. Could you provide some background
> (and maybe some references to/elaboration on to presently
> existing solutions)?
In short, what I want is that dispatch would never fail. I.e. either an operation is properly inherited or it is overridden. Then I want to be able to handle exponential growth of variants. In particular I want to be able to express symmetries, like in commutative operations (+,*) and ones with an inverse (-. /). I also want to be sure that overriding be consistent, i.e. overriding real+int would also force me to override real+real (while int+real be deduced).
For relational structures I have only a vague idea. But there is a bad problem of parallel type hierarchies. For example: you have one hierarchy of types and another of pointers to them. Or containers of, or whatsoever. It could be a candidate for some relational algebra on the types. Maybe all type relations (derived-base, container-element, pointer-target, function-argument etc) could be unified in some way. Maybe it is just rubbish, I don't know.
-- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.deReceived on Sat Jun 24 2006 - 21:31:38 CEST