> 1) zig's io is not an effect type, you can in principle declare a global io variable and use it everywhere that any library calls for it.

That's an effect, akin to globally intermediated I/O in a managed runtime.

To make it intuitive: if you have a global token for I/O, does your concurrent program need to synchronize on it in order to operate soundly? Do programs that fail to obtain the token behave correctly?

> If calling the same function with a different argument would be considered 'function coloring', than every function in a program is 'colored' and the word loses its meaning ;)

Well, yes, but in this case the colors (= effects) are actually important. The implications of passing an effect through a system are nontrivial, which is why some languages choose to promote that effect to syntax (Rust) and others choose to make it a latent invariant (Java, with runtime exceptions). Zig chooses another path not unlike Haskell's IO.

> Zig actually also had solved the coloring problem in the old and abandondend async-await solution because the compiler simply stamped out a sync- or async-version of the same function based on the calling context (this works because everything is a single compilation unit).

AFAIK this still leaked through function pointers, which were still sync or async (and this was not visible in their type)

The subject of the function coloring article was callback APIs in Node, so an argument you need to pass to your IO functions is very much in the spirit of colored functions and has the same limitations.

> If calling the same function with a different argument would be considered 'function coloring', than every function in a program is 'colored' and the word loses its meaning ;)

I mean, the concept of "function coloring" in the first place is itself an artificial distinction invented to complain about the incongruent methods of dealing with "do I/O immediately" versus "tell me when the I/O is done"--two methods of I/O that are so very different that it really requires very different designs of your application on top of those I/O methods: in a sync I/O case, I'm going to design my parser to output a DOM because there's little benefit to not doing so; in an async I/O case, I'm instead going to have a streaming API.

I'm still somewhat surprised that "function coloring" has become the default lens to understand the semantics of async, because it's a rather big misdirection from the fundamental tradeoffs of different implementation designs.

If your functions suddenly requires (currently)unconstructable instance "Magic" which you now have to pass in from somewhere top level, that indeed suffers from the same issue as async/await. Aka function coloring.

But most functions don't. They require some POD or float, string or whatever that can be easily and cheaply constructed in place.

> Actually it seems like they just colored everything async and you pick whether you have worker threads or not.

There is no 'async' anywhere yet in the new Zig IO system (in the sense of the compiler doing the 'state machine code transform' on async functions).

AFAIK the current IO runtimes simply use traditional threads or coroutines with stack switching. Bringing code-transform-async-await back is still on the todo-list.

The basic idea is that the code which calls into IO interface doesn't need to know how the IO runtime implements concurrency. I guess though that the function that's called through the `.async()` wrapper is expected to work properly both in multi- and single-threaded contexts.

> 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.

It's not a monad because it doesn't return a description of how to carry out I/O that is performed by a separate system; it does the I/O inside the function before returning. That's a regular old interface, not a monad.

How are the tradeoffs meaningfully different? Imagine that, instead of passing an `Io` object around, you just had to add an `async` keyword to the function, and that was simply syntactic sugar for an implied `Io` argument, and you could use an `await` keyword as syntactic sugar to pass whatever `Io` object the caller has to the callee.

I don't see how that's not the exact same situation.

It's sans-io at the language level, I like the concept.

So I did a bit of research into how this works in Zig under the hood, in terms of compilation.

First things first, Zig does compile async fns to a state machine: https://github.com/ziglang/zig/issues/23446

The compiler decides at compile time which color to compile the function as (potentially both). That's a neat idea, but... https://github.com/ziglang/zig/issues/23367

> It would be checked illegal behavior to make an indirect call through a pointer to a restricted function type when the value of that pointer is not in the set of possible callees that were analyzed during compilation.

That's... a pretty nasty trade-off. Object safety in Rust is really annoying for async, and this smells a lot like it. The main difference is that it's vaguely late-bound in a magical way; you might get an unexpected runtime error and - even worse - potentially not have the tools to force the compiler to add a fn to the set of callees.

I still think sans-io at the language level might be the future, but this isn't a complete solution. Maybe we should be simply compiling all fns to state machines (with the Rust polling implementation detail, a sans-io interface could be used to make such functions trivially sync - just do the syscall and return a completed future).

> The one thing you could say is that using some kind of token makes it dead simple to have different tokens. But that's really not something I run into often at all when using async.

It's valuable to library authors who can now write code that's agnostic of the users' choice of runtime, while still being able to express that asynchronicity is possible for certain code paths.