foo :: Int -> (forall a. a -> (a -> String) -> r) -> r
foo n k
  | even n = k (show n) id
  | otherwise = k n show

bar :: Int -> String
bar n =
  foo n $ \x1 f1 ->
  foo (n+1) $ \x2 f2 ->
  f1 x1 ++ f2 x2

main :: IO ()
main = putStrLn (bar 42)