# Pastebin uckM5m9r
-- | allocates a string of size 'n' of repeating @'a'@ characters
createStuff :: CInt -- ^ n
            -> IO (Maybe (ForeignPtr CChar))
createStuff n = do
  traverse (newForeignPtr p_free) . mf =<< c_makeStuff n
  where mf p
          | p == nullPtr = Nothing
          | otherwise = Just p


main = do
  -- Let's read a number from input...
  putStrLn "Enter a nonnegative integer: "
  n <- read <$> getLine
  -- use this number with the C function a couple times
  traverse f =<< createStuff n
  traverse f =<< createStuff (n * 2)
  -- note that f does not deallocate its argument
  -- that is done automatically by the GC later
  -- withForeignPtr is used to "downgrade" ForeignPtr to a Ptr
  where
    f fPtr = withForeignPtr fPtr (\p -> peekCString p >>= putStrLn)