[Haskell-cafe] Problem with existential quantification

[Eric]

Thanks!

E.

David Menendez wrote:
> On Thu, Sep 25, 2008 at 1:15 PM, Eric <eeoam at ukfsn.org> wrote:
>   
>> Dear all,
>>
>> I've written a function with the following type:
>>
>> build :: Bifunctor s => (forall b. (s a b  ->  b) -> b)  ->  Fix s a
>>
>> When I try to compile I get the following error:
>>
>> Illegal polymorphic or qualified type: forall b. (s a b -> b) -> b
>> In the type signature for `build':
>>  build :: (Bifunctor s) => (forall b. (s a b -> b) -> b) -> Fix s a
>>
>> What's happening?
>>     
>
> 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
>
>