[Haskell-cafe] Data structure containing elements which are instances of the same type class

[David Feuer]

Has anyone used existential types to represent items on a schedule in a
scheduled lazy data structure?
On Aug 11, 2012 4:15 AM, <oleg at okmij.org> wrote:

>
> > data A = A deriving Show
> > data B = B deriving Show
> > data C = C deriving Show
> >
> > data Foo = forall a. Show a => MkFoo a (Int -> Bool)
> >
> > instance Show Foo where
> >    show (MkFoo a f) = show a
>
> I'd like to point out that the only operation we can do on the first
> argument of MkFoo is to show to it. This is all we can ever do:
> we have no idea of its type but we know we can show it and get a
> String. Why not to apply show to start with (it won't be evaluated
> until required anyway)? Therefore, the data type Foo above is in all
> respects equivalent to
>
> > data Foo = MkFoo String (Int -> Bool)
>
> and no existentials are ever needed. The following article explains
> elimination of existentials in more detail, touching upon the original
> problem, of bringing different types into union.
>
>         http://okmij.org/ftp/Computation/Existentials.html
>
>
>
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