Re: choice of character for relational division

From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 01 Apr 2007 14:37:53 GMT
Message-ID: <5tPPh.13566$6m4.8082@pd7urf1no>

Marshall wrote:
> On Mar 31, 2:47 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>

>>One needs to have set equality and set containment operators. If one has
>>those, the query seems simple. What does a special operator for it buy one?

>
>
> Do we need all of those? We need set equality for RVAs if we
> want to join on RVAs (which we do) but do we need a top level
> equality? I mean, I would expect it, but do we need it? Do we
> ever ask in logic whether these propositions are the same as
> those propositions? Doesn't seem so.
>
> If we have equality (=) and join (&), we don't need containment:
>
> A & B = A <=> A ⊆ B.
> ...

Yes, and to see if we have a proper subset I suppose something like:

(A & B = A) & (A & B != B).

Personally, I would rather not write that out very often and given the keyboard I have, I'm content to overload. In the above, which resembles a number of current languages, I think '&' is actually overloaded. In ordinary language or computer languages, it is types that tell what an op means, eg., people can be married and so can ideas but those are different kinds of marriages.

Rather than write out the above to find a proper subset, I'd prefer to ask whether A < B? or A <= B? and use the above as the definition of A < B. But if I were writing a parser I think I'd let the user substitute their own symbols - they might not have a lowest-common-denominator keyboard like mine.

I think Bob B is driving at a more important question - what ops should there be? There is a difference between having a small number of fundamental operators versus a small number of symbols that we use to ease expression.

Being a little interested in amateur genealogy, I can't help but like JOG's use of RVA's to name sets of friends, which is not at all a neat problem using the gedcom/hierarchical methods most street genealogists use. It is very clumsy to record who knew whom with those systems so I think most people avoid trying.

p Received on Sun Apr 01 2007 - 09:37:53 CDT