Re: [Haskell-cafe] Monotype error

14 Oct 2009


      * Martijn van Steenbergen  [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

Re: [Haskell-cafe] Monotype error

Roman Cheplyaka