Comment by noelwelsh

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.