roundtrip :: (URL -> URL) -> String -> String
roundtrip f x = case importURL x of
                  Just u  -> exportURL . f $ u
                  Nothing -> x
roundtrip :: (URL -> URL) -> String -> String
roundtrip f x = case importURL x of
                  Just u  -> exportURL . f $ u
                  Nothing -> x
roundtrip f x = case importURL x of
                  Just u  -> exportURL . f $ u
                  Nothing -> x