Agreed, Zig's IO is closer to the effect handler / capability passing model. And by closer, I mean exactly the same [1]. However, it's related to monads by duality. A comonadic program is a program that depends on context, which captures the notion of passing capabilities around.

[1] Languages designed around capability passing often have other features, like capture checking to ensure capabilities aren't used outside the scope where they are active. There are only two such languages I know of. Effekt (see https://effekt-lang.org/tour/captures) and Scala 3 (see https://docs.scala-lang.org/scala3/reference/experimental/cc...) However, this is not core to the idea of capability passing.

> I don't understand why Zig's `Io` is a "monad".

I don't see how it's true in any meaningful sense. It seems about the same as stating that any function is an example of the reader monad.

The whole point of monads in programming languages is as an _abstraction_ that allows one to ignore internals like how the IO token is passed around.

Maybe Zig is a language for people who are scared of abstraction. Otherwise they'd presumably be using something more powerful like Rust.

I guess that if a burrito can illustrate what is a monad, anything can be casted as a projection of a monad in some perspective.

https://i.imgflip.com/65gu3j.jpg