One and a half colors

Similar to Go, which also has "one and a half colors". Functions taking a context.Context as their first argument are most likely performing I/O. It's not required though: a number-crunching function performing no I/O might still check the context for cancellation every so often. Likewise, I/O without taking a context.Context is possible.

Similarly, in async languages (Rust, ...), a function marked async might end up performing no I/O at all (no await points). A function marked sync might spawn its own runtime (or grab the global one) and start spawning futures (= called async functions) on it.

Lots of grey areas.

One thing I wonder about is mixing threading for CPU-bound work and eventing for I/O-bound work. In Rust, one solution is having the application be fundamentally async (tokio, ...) and hand around a dedicated threadpool (rayon, ...). If there's enough available parallelism, both can coexist without interference and communicate via channels. Rust makes this explicit and relatively ergonomic at compile time (Send + Sync, ...). I wonder how equivalent Zig code would look like (I suppose Io would be the evented variant, and for the CPU-bound work just spawn OS threads normally).