[Haskell-cafe] Monotype error

[Roman Cheplyaka]

* Martijn van Steenbergen <martijn at 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