let n = c_strcmp s' t'
  -- free s'
  -- free t'
  return $ case () of
    _ | n == 0    -> EQ
      | n <  0    -> LT
      | otherwise -> GT

Two questions:

1. May I safely use unsafePerformIO in such cases?

2. What’s the proper way of using free here?  If I uncomment the above,
   the function returns incorrect results.