[Haskell-cafe] Monotype error

[Vladimir Reshetnikov]

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 <
martijn at 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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