[Haskell-cafe] [Newbie] Quest for inheritance

Cédric Paternotte cpaternotte at gmail.com
Mon Jun 6 12:00:14 EDT 2005

Hi Chris,

On 6 Jun 2005 14:53:11 -0000, ChrisRathman.6155983 at bloglines.com
<ChrisRathman.6155983 at bloglines.com> wrote:
> The differences in the two shape examples doesn't have to do with code inheritance
> - it has to do with subtype polymorphism.  

Does this mean that I was wrong in saying in my initial post that
existential types can be used to get "code inheritance" ? Or is it
just that the Shapes example has never been meant to illustrate that
side of things ?

> Existential types allow you to
> throw different subtypes into a collection and have the function dispatch
> on the common superclass type.  The example without existential types demonstrates
> parametric polymorphism, but not subclass polymorphism.

Okay, I think I've finally spotted the difference : Using existential
types you actually managed to put both CircleInstance and
RectangleInstance in the same list. I didn't notice that detail at
first. All the difference lies in that line ;) So I understand this is
something you obviously cannot do when using classic subtyping.
Incredible. This is weird, I mean that's the kind of feature I would
take for granted.

That makes existential types much more useful indeed. Even essential I
would say.

> As far as inheritance, there's actually two kinds
> that occur in most OO languages.  The first being type inheritance (which
> really just gets you subtype polymorphism).  And then there's code inheritance
> - which is what you are trying to achieve.  

Thanks for the clarification.

> At any rate, I consider
> inheritance to be a convenience mechanism, and not vitally necessary to the
> concepts of encapsulation, type inheritance, and polymorphism, though I realize
> that many do consider it important.  

Well, on the -way too big for my head- projects I'm working on,
inheritance saves my life many times a day.

> Chris Rathman

Thanks for a lot for your time Chris, think I'm slowly starting to get
the picture of all this.


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