Problem using type alias

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-- section 1
type Foo m n = [(m, Maybe n)]
t :: (Show m, Show n) => Foo m n -> String
t f =
  intercalate "." $ map (\(a, b) ->
                          show a ++ show b
                        ) f

-- section 2
type JobArgs k a = M.Map k a
start :: (Ord k, Exception e) => JobArgs k a -> (a -> IO b) -> IO (M.Map k (Either e b))