On Tue, 5 Oct 1999, George Russell wrote:
> Perhaps I'm being stupid. (It certainly wouldn't be the first time!)
> But what does OO give me that I can't get with existential types (in
> datatype definitions) and multiparameter type classes? The latter seem
> to me much more powerful, since I can add dancing and singing methods
> to objects without having to go back to the original class definition.
1) Dynamic types. You can't cast up. That is you can't recover the
original type from an object in a existential collection. You need to
use a dynamic type library for that. And the library proved with hugs
and ghc leaves a lot to be desired. In an OO langauge all classes
automatically cary dynamic typing information.
2) More specific types, you can't _easilly_ call the more general type.
For example in OO this is very commen:
class Base
virtual foo()
do stuff....
class Derived, extends Base
foo()
call Base::foo()
doo stuff
3) Encapsulation. You can't have private and protected members. Some
of this can be done using modules. However it is more work.
4) Cleaner more natural syntax.
---
Kevin Atkinson
[EMAIL PROTECTED]
http://metalab.unc.edu/kevina/
