See our previous discussion on this topic here:
http://www.nabble.com/Fwd:-Unification-for-rank-N-types-td23942179.html
Thanks,
Vladimir
On Wed, Oct 14, 2009 at 10:35 PM, Martijn van Steenbergen <
mart...@van.steenbergen.nl> wrote:
> Dear café,
>
> {-# LANGUAGE Rank2Types #-}
>> {-# LANGUAGE ImpredicativeTypes #-}
>>
>> type Void = forall a. a
>>
>> newtype Mono a = Mono { runMono :: [Void] }
>>
>> beep :: Mono a -> Mono a
>> beep (Mono vs) = Mono (map undefined vs)
>>
>
> Compiling this with GHC results in:
>
> Monotype.hs:9:28:
>> Cannot match a monotype with `Void'
>> Expected type: Void
>> Inferred type: a
>>
>
> What does this error mean and why does the code not compile?
>
> Thanks!
>
> Martijn.
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