[Haskell-cafe] Problem with existential quantification

[Henning Thielemann]

On Thu, 25 Sep 2008, David Menendez wrote:

> That looks like it should work. I'm guessing you enabled
> ExistentialQuantification, but not Rank2Types or RankNTypes. The
> former allows you to use the forall keyword in data declarations, but
> you need one of the others to allow universal quantification in
> signatures, which is what build is using.
>
> This could be better documented. The GHC manual lists all the
> extensions it supports, but it doesn't use the same terminology as the
> Extension codes (e.g. "arbitrary-rank polymorphism" instead of
> "RankNTypes"), and there is a hierarchy of extensions that appears to
> be entirely implicit. For example,
>
> RankNTypes implies Rank2Types
> Rank2Types implies ExistentialQuantification
> Rank2Types implies PolymorphicComponents

Something to add to
    http://www.haskell.org/haskellwiki/Rank-N_types ?