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 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

On Tue, Jul 31, 2012 at 2:59 PM, Shayan Najd Javadipour wrote:

{-# 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'

This looks to me like other cases where GHC requires an exact type match even though you used something equivalent. Similarly, for example, it rejects (contrived example) foo :: Num a => a -> a -> a foo 0 0 = -1 foo = (+) because the explicit arity of the cases must match exactly, even though (+)'s type matches the required arity. I am under the impression that it's difficult to make those kinds of things work nicely in the typechecker. -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms