Without actual code solving an actual problem discussing this is all a bit wishy washy, but I think you're mixing 'levels'.
The sans-io design here is about building reusable parts that any runtime or thread can drive and incorporate into the colored interface they expose.
Function coloring is about (the annoyance when) composing interfaces designed around those different runtimes/thread that might use the same sans-io library internally.
Yes async compiles down to a statemachine, but we're talking about the elegance of the language syntax/composition. Most languages require _a lot_ of ritual (or overhead) if you want to use the async-statemachine as a pure statemachines and sidestep the build in runtime-side-effect channels.
The sans-io design here is about building reusable parts that any runtime or thread can drive and incorporate into the colored interface they expose.
Function coloring is about (the annoyance when) composing interfaces designed around those different runtimes/thread that might use the same sans-io library internally.
Yes async compiles down to a statemachine, but we're talking about the elegance of the language syntax/composition. Most languages require _a lot_ of ritual (or overhead) if you want to use the async-statemachine as a pure statemachines and sidestep the build in runtime-side-effect channels.