Re: The IDS, the EDS and the DBMS

"mAsterdam" <mAsterdam_at_vrijdag.org> wrote in message news:414214df$0$568$e4fe514c_at_news.xs4all.nl...
> Marshall Spight wrote:
>
> > erk wrote:
> >>Type implementation; I think Date had it right in suggesting that type
> >>implementations could be done in a variety of languages, and even that
> >>those languages might be different than those used in the rest of the
> >>app...
> >
> > I don't see any value to Date's position here. His canonical polar/rectangular
> > point example is uninspiring: you can't even represent (1,1) precisely
> > in polar; you can't represent (45 degrees, 1) in rectangular.
> > type definition is an area where you can't get away from the
> > limitations of the underlying data structure.
>
> Is this your reasoning? :
> To represent polar(45,1) as a cartesian(x,x) we would need an exact
> way to represent the square root of 2. We don't have that so
> "you can't represent (45 degrees, 1) in rectangular."

Yes.

> That begs the question: why, exactly? or: is it really impossible?

There is only one way to do it, which is not to evaluate the expression, but keep it as an expression: so sqrt(2) remains exactly "sqrt(2)" internally, so that if you were to square that expression, you'd get exactly two. No programming language I know of does this, but I believe Mathematica does.

Other than that, yes, it really is impossible.

> And, related: is this an implementation issue or a type-model issue?
> And: is the (current) impossibilty of an implementation enough
> reason to abandon a model?

If it provides no particular benefits, and adds complexity, then it shouldn't be done. I'm not clear whether it provides any benefits; it certainly adds complexity, and the canonical example doesn't work!

> ((10 divided by 3) times 3) does not give you 10 in any
> programming language (barring optimization effects :-)
> It does not realy stop us from thinking of these operators
> as the ones we are familiar with from math.

Au contraire! Any language with a true rational type will handle this just fine. I believe Lisp and various other languages provide this.

The difference here is between rationals and irrationals; rationals present no huge difficulties but irrationals cannot be represented extensionally in finite space.

> [...]
> > And Date's idea that there *not be* a way to define new types in
> > the application language is disastrous!
>
> Are you sure?
> D&D Tutorial D supports UDT's (User Defined Types) -
> or am I misunderstanding your statement?

IIRC TTM explicitly states that the implementation of a UDT should be in a different language than the application language.

Marshall Received on Tue Sep 14 2004 - 04:15:29 CEST