tcfail165.hs:19:23: error:
    • Couldn't match expected type ‘forall a. Show a => a -> String’
                  with actual type ‘b0 -> String’
    • In the second argument of ‘putMVar’, namely
        ‘(show :: forall b. Show b => b -> String)’
      In a stmt of a 'do' block:
        putMVar var (show :: forall b. Show b => b -> String)
      In the expression:
        do var <- newEmptyMVar ::
                    IO (MVar (forall a. Show a => a -> String))
           putMVar var (show :: forall b. Show b => b -> String)