Comment by hxtk

The problem with function coloring is that it makes libraries difficult to implement in a way that's compatible with both sync and async code.

In Python, I needed to write both sync and async API clients for some HTTP thing where the logical operations were composed of several sequential HTTP requests, and doing so meant that I needed to implement the core business logic as a Generator that yields requests and accepts responses before ultimately returning the final result, and then wrote sync and async drivers that each ran the generator in a loop, pulling requests off, transacting them with their HTTP implementation, and feeding the responses back to the generator.

This sans-IO approach, where the library separates business logic from IO and then either provides or asks the caller to implement their own simple event loop for performing IO in their chosen method and feeding it to the business logic state machine, has started to appear as a solution to function coloring in Rust, but it's somewhat of an obtuse way to support multiple IO concurrency strategies.

On the other hand, I do find it an extremely useful pattern for testability, because it results in very fuzz-friendly business logic implementation, isolated side-effect code, and a very simple core IO loop without much room in it for bugs, so despite being somewhat of a pain to write I still find it desirable at times even when I only need to support one of the two function colors.