[Haskell-beginners] Re: Encapsulation and Polymorphism

[Stephen Tetley]

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 <drew.haven at gmail.com> wrote:
> I think I found the answers to all my questions at
> http://www.haskell.org/haskellwiki/Existential_type