If you look at the other reply to my comment, someone has linked the Unison docs on their effect handlers implementation - which is what is used for IO, and with any luck, error handling as well. Haskell has several packages for effect handlers, and they're taking off in programming and category theory circles as alternatives to monads for IO.

The short summary is that you can write code with side-effects that you don't define the implementation for, and then let call sites handle those effects or defer them.