As far as I understand it, the idea is that the IO interface is "magic" in the sense that it affects which version of a function gets called: either the synchronous, linear version, or the asynchronous, stackless, suspensible state machine version.

This has a lot of implications, especially for function pointers and other forms of dynamic dispatch.

My personal take is that async code is fundamentally different from sync code, because the weights of various tradeoffs change dramatically. In my view, "colorless" functions are not really a worthwhile goal in the first place. But I've been wrong before, so it may turn out to be great, we'll see.