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

31 Jul 2012


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

MigMit