io is not a monad. theres nothing stopping you from stashing a global io "object" and just passing the global wherever you interface with the stdlib.

It's dependency injection. and yes, you can model dependecies like a monad but most people, even in less pure fp langs, don't.

i don't really say this to just be a pedant, but if you're an fp enjoyer, you will be disappointed if you get the picture that zig is fp-like, outside of a few squint-and-it-looks-like things