Re: [Haskell-cafe] Moving "forall" over type constructors

On Tue, 2008-06-10 at 11:28 -0700, Ryan Ingram wrote:

On 6/10/08, Derek Elkins wrote:

...

This is the lack of impredicativity.

W :: a -> W a To get the result type W (forall a. t a), W must instantiate the a in W's type to (forall a. t a). Further we then pass it to (.) which has type (b -> c) -> (a -> b) -> a -> c and thus require instantiating both a and b to higher-rank types. A predicative type system does not allow instantiating type variables to quantified types.

Although if I am reading the literature correctly, that definition will likely typecheck in the next GHC...

http://research.microsoft.com/~simonpj/papers/boxy/

Actually some support was hacked into the current version of GHC. http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extension...