[Haskell-cafe] [Newbie] Quest for inheritance

[ChrisRathman.6155983 at bloglines.com]

The differences in the two shape examples doesn't have to do with code inheritance
- it has to do with subtype polymorphism.  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.

Generally speaking,
OO programming is based on three properties (a) encapsulation; (b) inheritance;
and (c) polymorphism.  Well, Haskell has pretty good means of encapsulation
(though I'm spoiled by ML at the moment).  And existential types can get you
subtype polymorphism.  

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.  A language like Sather actually
treats type inheritance and code inheritance as seperate things.  Languages
that mix the two concepts have their own set of difficulties, as the two forms
of inheritance are sometimes at odds with each other.

Anyhow, the classic
case for code inheritance is from a re-use perspective - i.e. not having to
repeat the code in multiple places.  The classic problem with code inheritance
is that it breaks encapsulation, as classes that reuse code from a superclass
become tightly coupled with the implementation.

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.  From a Java perspective, you usually
have to decide whether you want single inheritance via class extension, or
multiple inheritance via interface implementation.

>From the perspective
of Java, the shape example that does not have code inheritance corresponds
to the Java world of Interfaces - with Interfaces not allowing code inheritance.


Chris Rathman

--- Cédric Paternotte <cpaternotte at gmail.com wrote:
> 1. Through existential types
> 
> As shown in the Shapes example at
>
http://www.angelfire.com/tx4/cus/shapes/haskell.html.
> However, I couldn't
spot the advantage over the standard version using
> normal classes at
>
http://www.angelfire.com/tx4/cus/shapes/haskell98.html
> 
> The getX function
still needs to be defined in both RectangleInstance
> and CircleInstance.
This is not what I call inheritance. Inheritance
> would make it possible
to define getX only once in ShapeInstance. Or
> maybe the point was only
to demonstrate polymorphism. But then what is
> the advantage of using existential
types ? It just looks like more
> work compared to the standard version that
also makes use of
> polymorphism. Or am I missing something ?