[Haskell-cafe] Monotype error

[Martijn van Steenbergen]

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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