[Haskell-cafe] Existential quantification problem

[Ronald Guida]

On Thu, 10 July 2008, Marco Túlio Gontijo e Silva wrote:
> how do I unbox a existential quantificated data type?

Dan Doel wrote:
>    elim :: L a -> (forall l. l a -> r) -> r
>    elim (L e) f = f e

Just one catch: You can't actually write a function 'f' of type
(forall l. l a -> r) without knowing something about the forgotten
type of l.

One way to deal with this is by restricting the type of l in the data
declaration.  For example, you could restrict it to the typeclass
Foldable, and then you have access to the methods of that typeclass.

\begin{code}
{-# LANGUAGE ExistentialQuantification #-}

module Main
    where

import qualified Data.Foldable as F

data L a = forall l. (F.Foldable l) => L (l a)

toList :: L a -> [a]
toList (L x) = F.foldr (:) [] x

main :: IO ()
main = do
  let x = L [1..10]
  print $ toList x
\end{code}

See also http://www.haskell.org/haskellwiki/Existential_type