[Haskell-cafe] Explicit forall - Strange Error

[MigMit]

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 at 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
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