If GHC handles the explicit "forall" in constructor "T1" in the same way as it does for function "f", we have:

data T a where T1 :: (forall b. b -> b) -> Int -> T a

Which is totally fine! The main question is then why the "forall"s are handled differently?

On Tue, Jul 31, 2012 at 9:07 PM, MigMit <miguelimo38@yandex.ru> wrote:
It really seems to me that the error message you've got explains everything quite clear.

Отправлено с iPad

31.07.2012, в 22:59, Shayan Najd Javadipour <sh.najd@gmail.com> написал(а):

Hi, 

I wonder why the following code doesn't typecheck in GHC 7.4.1:

{-# LANGUAGE GADTs,RankNTypes #-}
data T a where T1 :: (forall b. b -> b) -> (forall a. Int -> T a)

{- Error:
Data constructor `T1' returns type `forall a. Int -> T a'
      instead of an instance of its parent type `T a'
    In the definition of data constructor `T1'
    In the data type declaration for `T'
Failed, modules loaded: none. -}

While:


{-# LANGUAGE GADTs,RankNTypes #-}

f :: (forall b. b -> b) -> (forall a. Int -> Maybe a)
f = undefined

{- 
ghci> :t f
f :: (forall b. b -> b) -> Int -> Maybe a
-} 

Thanks, 
 Shayan
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe