At the end of the day most of us have differing opinions on what constitutes simplicity and elegance. It's certainly true that a "pure" annotation like you're proposing is a much smaller change to introduce in an imperative setting. I recollect D or Rust or something is doing this. But in the functional programming context the monadic solution is more general, and a two-function type class with 3 (IIRC) algebraic laws is not considered an overbearing amount of complexity, though there are of course interesting alternatives with their own merits.