This is, arguably, the biggest reason to use Monads in the first place, the typeclass thing. You don't even need Monads to have completely functional IO code.

This is, arguably, the biggest reason to use Monads in the first place, the typeclass thing. You don't even need Monads to have completely functional IO code. Clean's uniqueness types are another method, but you can actually do it with just plain types if you're dedicated. Oddly, monads just make things easier. Really.