Re: [Haskell-beginners] Re: Encapsulation and Polymorphism

From what I understand (not much), It seems that type-classes are more-or-less equivalent to OO interfaces (I think OP mentioned this), and that containers with existential types are less more-or-less equivalent to OO containers whose elements implement an interface, the biggest exception being that you can't use anything that isn't a part of the class on the contained values.

As I understand, something like this could be relevant:

class AquaticLifeform a where nutrition :: Int reproduce :: Maybe (a, a)

On Thu, Aug 26, 2010 at 10:08 AM, Stephen Tetley wrote:

Hi Drew

Bear in mind though that existentials are not equivalent to subtyping in OO.

For instance, with example 2.1 from [1] all you can do with an Obj is show it, so for the list xs all you can do is show the elements:

data Obj = forall a. (Show a) => Obj a

xs :: [Obj] xs = [Obj 1, Obj "foo", Obj 'c']

Because Obj is an existential you can't do an case analysis on it - so you can't write a function like this:

add_one_if_int (Obj (n::Int)) = Obj (n+1) add_one_if_int (Obj other) = Obj other

There really is nothing you can do with Obj other than show it.

If you are trying to transliterate OO designs, you might quickly find existentials are too inert to be useful.

Best wishes

Stephen

[1] http://www.haskell.org/haskellwiki/Existential_type

On 26 August 2010 07:45, Drew Haven wrote:

...

I think I found the answers to all my questions at http://www.haskell.org/haskellwiki/Existential_type

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