Personally I find this really cool.

One thing I like about the design is it locks in some of the "platforms" concepts seen in other languages (e.g. Roc), but in a way that goes with Zig's "no hidden control flow" mantra.

The downstream effect is that it will be normal to create your own non-posix analogue of `io` for wherever you want code to hook into. Writing a game engine? Let users interact with a set of effectful functions you inject into their scripts.

As a "platform" writer (like the game engine), essentially you get to create a sandbox. The missing piece may be controlling access to calling arbitrary extern C functions - possibly that capability would need to be provided by `io` to create a fool-proof guarantees about what some code you call does. (The debug printing is another uncontrolled effect).