problems with impredicativity

[Wolfgang Jeltsch]

Hello,

this code is accepted by GHC 7.0.4:

> {-# LANGUAGE ImpredicativeTypes #-}
> 
> polyId :: (forall a. a) -> a
> polyId x = x
> 
> polyIdMap :: [forall a. a] -> [forall a. a]
> polyIdMap xs = fmap polyId xs

However, this one isn’t:

> {-# LANGUAGE ImpredicativeTypes #-}
> 
> polyId :: (forall a. Maybe a) -> Maybe a
> polyId x = x
> 
> polyIdMap :: [forall a. Maybe a] -> [forall a. Maybe a]
> polyIdMap xs = fmap polyId xs

Is there a way to make it accepted?

Best wishes,
Wolfgang