Function coloring is specifically about requiring syntax for a function, eg. the async keyword. So if you want an async and non-async function you need to write both in code. If you pass the "coloring" as an argument you avoid the need for extra syntax and multiple function definitions and therefor the function has no color. You can solve this in various ways with various tradeoffs but as long as there is a single function (syntactically) is all that matters for coloring.

> Function coloring is specifically about requiring syntax for a function, eg. the async keyword.

Someone should tell the inventor of the phrase, because they don't mention the async keyword at all[1]. As-written, function coloring is about callbacks (since that's semantic mechanism that JavaScript happens to pick for their asynchronous model).

Function coloring is just an informal way to describe encoding a function's effect. You can encode that in syntax if you want (an `async` keyword), or in the type system (returning `() -> T` instead of `T`), or in the runtime itself (by controlling all I/O and treating it the same). But you can't avoid it.

[1]: https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...

> Function coloring is specifically about requiring syntax for a function, eg. the async keyword.

It isn't really. It's about having two classes of functions (async and sync), and not being able to await async functions from sync ones.

It was originally about Javascript, where it is the case due to how the runtime works. In a sync function you can technically call an async one, but it returns a promise. There's no way to get the actual result before you return from your sync function.

That isn't the case for all languages though. E.g. in Rust: https://docs.rs/futures/latest/futures/executor/fn.block_on....

I think maybe Python can do something similar but don't quote me on that.

There's a closely related problem about making functions generic over synchronicity, which people try and solve with effects, monads, etc. Maybe people call that "function colouring" now, but that wasn't exactly the original meaning.