* Martijn van Steenbergen <mart...@van.steenbergen.nl> [2009-10-14
20:35:06+0200]
> 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?
It works if you annotate the type of undefined:
beep (Mono vs) = Mono (map (undefined :: Void -> Void) vs)
--
Roman I. Cheplyaka :: http://ro-che.info/
"Don't let school get in the way of your education." - Mark Twain
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