I don't think it's a function of complexity per se, but determinism. This is why Haskellers love the IO monad. Used well, it lets you quarantine IO to a thin to

I don't think it's a function of complexity per se, but determinism. This is why Haskellers love the IO monad. Used well, it lets you quarantine IO to a thin top-level layer, below which all functions are pure and easy to unit test.