I think this design is very reasonable. However, I find Zig's explanation of it pretty confusing: they've taken pains to emphasize that it solves the function coloring problem, which it doesn't: it pushes I/O into an effect type, which essentially behaves as a token that callers need to retain. This is a form of coloring, albeit one that's much more ergonomic.

(To my understanding this is pretty similar to how Go solves asynchronicity, expect that in Go's case the "token" is managed by the runtime.)