Re: RM VERY STRONG SUGGESTION 4: TRANSITION CONSTRAINTS

On 10 sep, 14:29, Brian <br..._at_selzer-software.com> wrote:

> Incidentally, the transition constraints defined on pages 220-221 of
> TTM are not enforceable

Of course they are enforceable. Every declarable constraint that effectively evaluates to a boolean is enforceable by evaluating it and inspecting the boolean result and acting upon that result.

So declarable constraints are enforceable by definition.

> since they implicitly assume that each value
> for S# permanently identifies a supplier.  What if the supplier number
> for a given supplier changes?

Then the machinery underlying the transition constraints will fail to raise some exception that you would like the system to raise.

However, no matter what construct one wishes to use, it is materially impossible for any system to guaranteeably raise that exception in every case where you would want to see it raised.

> I really don't understand why you're fighting this so vehemently,
> seeing as how you are on record that you don't think := should be
> supported anyway.- Tekst uit oorspronkelijk bericht niet weergeven -

That record states pretty unmistakably that I find it a bad idea to expose direct relational assignment as an update vehicle that is directly available to the user. That record also pretty unmistakably states my reason for having that belief : I dislike the implicitness of the delete that would arise. I dislike the idea of giving the user the possibility to _implicitly_ assert the falseness of a bunch of facts, where said user isn't even required to know which facts those are ... It perhaps doesn't, and perhaps it should, clarify that I am _not_ considering the context of relvars that are local to a running instance of some program, but that I only consider database relvars, i.e. relvars that survive the lifetime of a program, and that are shared among _all_ programs.

I can also confirm that D&D agree that what I _do_ support in SIRA_PRISE, that is, insert+delete+MA, amounts to exactly the same thing as supporting assignment. So at least your way of expressing yourself when you say that I believe that assignment shouldn't be supported, gets it wrong in more than one way.

>
> - Tekst uit oorspronkelijk bericht weergeven -
Received on Fri Sep 10 2010 - 20:08:43 CEST