[Haskell-beginners] Re: Encapsulation and Polymorphism

[Ozgur Akgun]

On 26 August 2010 08:08, Stephen Tetley <stephen.tetley at gmail.com> 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.
>

But that's because you use Show while defining the Obj data type. You can
implement other functionalities, by introducing a custom type class, and
implementing functionalities in instance declarations.

class Show a => CustomTC a where
    add_one_if_int :: a -> a

instance CustomTC Int where
    add_one_if_int x = x + 1

instance CustomTC Char where
    add_one_if_int = id

instance CustomTC String where
    add_one_if_int = id

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

xs' :: [Obj]
xs' = map (\ (Obj i) -> Obj (add_one_if_int i) ) xs
-- xs' = [Obj 2,Obj "foo",Obj 'c']



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
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-- 
Ozgur Akgun
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