Re: [Haskell-cafe] Explicit forall - Strange Error

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 wrote:

It really seems to me that the error message you've got explains everything quite clear.

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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

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