Re: [Haskell-cafe] What does the `forall` mean ?

Dan Piponi wrote:

To use these types with ghc you need to use the compilation flag -XExistentialQuantification.

Or, more portably, add {-# LANGUAGE ExistentialQuantification #-} at the top of the source file. It should now compile in any computer that supports this feature without any special command-line flags.

There's more to be found here: http://www.haskell.org/haskellwiki/Existential_typ

Amusingly, half of this article is still the text that I wrote. ;-)