Re: Mixing OO and DB

From: Dmitry A. Kazakov <mailbox_at_dmitry-kazakov.de>
Date: Sat, 16 Feb 2008 12:39:54 +0100
Message-ID: <fc3z6xqtrely.qqz8d2iw8spx$.dlg_at_40tude.net>


On Fri, 15 Feb 2008 17:49:38 -0800 (PST), Marshall wrote:

> On Feb 15, 12:22 pm, "Dmitry A. Kazakov" <mail..._at_dmitry-kazakov.de>
> wrote:

>> On Fri, 15 Feb 2008 08:43:35 -0800 (PST), Marshall wrote:
>>> On Feb 15, 3:13 am, "Dmitry A. Kazakov" <mail..._at_dmitry-kazakov.de>
>>> wrote:
>>
>>>> Ah, this is a crucial point. Circle value is not an ellipse value. These
>>>> have different types.
>>
>>> You shouldn't let your history with type systems influence your
>>> thinking to the degree that you say obviously false things like
>>> "Circle value is not an ellipse value." Such unqualified statements
>>> obscure rather than illuminate. It would be better to say that
>>> within a given type system we might give up the mathematical
>>> fact that a circle is an ellipse in favor of other properties we
>>> might find more useful.
>>
>> I don't give up anything. Mathematical "is" merely means that there exists
>> an injective mapping from the set of circles to the set of ellipses. As
>> this stays true for the values, circle is an ellipse. Yet circle value is
>> not an ellipse value.

>
> Mathematically, a circle value is a set of points, and an ellipse
> value is a set of points.

Geometry does not operate "values". It deals with plain circles and ellipses. In a computational system that models geometrical objects a circle value serves as a model of circle.

> Mathematically, we can describe a set of points
> with an equation, and the equation that describes ellipses reduces
> to the equation that describes circles in certain cases. Do you want
> to say that two different appearances of the same equation may be
> different depending on what they were reduced from? That dog
> won't hunt.

It can also define a circle using its center and radius, or as a conic section, or via complex exponent, or by an uncountable number of other ways. All these definitions are said equivalent. It has nothing to do with being "same value".

> I happen to think type systems are just about the most interesting
> thing in computer science, but I don't make the mistake of thinking
> that they are somehow fundamentally necessary, or worse, that
> the state of the art of today's industrial languages (which is decades
> behind the state of the art of today's academic languages) is
> somehow the end of the road.

Just a comment, mathematicians understood need in typed systems hundred years before us.

> Here's a polemic on the topic written by someone smarter
> than anyone who has ever posted in comp.databases.theory
> or comp.object:
>
> http://citeseer.ist.psu.edu/416093.html

Thanks for the reference.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
Received on Sat Feb 16 2008 - 12:39:54 CET

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