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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: null record extension mechanism
Date: 1999/11/10
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In article <38295873.990923A1@interact.net.au> , G 
<Dizzy@interact.net.au>  wrote:

>   The whole point of having an [abstract][tagged][null][record] is
> so that it can be extended.  Is that correct ?

That's the point of tagged types.  A tagged null record just happens not
to have any components; it's like counting by starting at zero.


> It can not itself be
> instantiated, but derivations of it can.

That's what an abstract tagged type is.  You'd typically declare a
tagged type as abstract because you have an abstract operation.


> So, it appears that one of the major activities programmers (in this language)
> undergo is the careful sculpturing/engineering of types. (?)

Yes.  That's the essence of object-oriented programming: programming
with abstractions, implemented as types.


> I guess that at some undetermined time in the future, the "penny will drop"
> and I will no longer be stumbling in the darkness with this.

That's exactly right.  Just keep doing it, and you'll learn it.


> Is there any useful online information about strategies for doing this design
> work ?

Do an online search for "object-oriented programming," and see what
drops out.  There's a web-site, called Cetis-Links or something like
that, that has all the OO technology stuff you could ever want.

Matt

--
Why stop at evolution and cosmology, though? Let's make sure that the
schoolkids of Kansas get a really first-rate education by loosening up
the teaching standards for other so-called scientific ideas that are,
after all, just theories. The atomic theory, for example. The theory of
relativity. Heck, the Copernican theory--do we really know that the
universe doesn't revolve around the earth?

John Rennie, Scientific American, Oct 1999