problems with impredicativity

[wagnerdm at seas.upenn.edu]

Quoting Wolfgang Jeltsch <g9ks157k at acme.softbase.org>:

> this code is accepted by GHC 7.0.4:
> <snip>
> 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?

Yep, fix the type signature. There is no type you can substitute for  
"a" in "Maybe a" that results in "forall a. Maybe a". But GHC accepts  
the same code with the following type signature, which should make  
clear what I mean:

polyIdMap :: [forall a. Maybe a] -> [Maybe (forall a. a)]

~d