Zig's New Async I/O
(kristoff.it)345 points by afirium a day ago
345 points by afirium a day ago
The key difference to typical async function coloring is that `Io` isn't something you need specifically for asynchronicity; it's something which (unless you make a point to reach into very low-level primitives) you will need in order to perform any IO, including reading a file, sleeping, getting the time, etc. It's also just a value which you can keep wherever you want, rather than a special attribute/property of a function. In practice, these properties solve the coloring problem:
* It's quite rare for a function to unexpectedly gain a dependency on "doing IO" in general. In practice, most of your codebase will have access to an `Io`, and only leaf functions doing pure computation will not need them.
* If a function does start needing to do IO, it almost certainly doesn't need to actually take it as a parameter. As in many languages, it's typical in Zig code to have one type which manages a bunch of core state, and which the whole codebase has easy access to (e.g. in the Zig compiler itself, this is the `Compilation` type). Because of this, despite the perception, Zig code doesn't usually pass (for instance) allocators explicitly all the way down the function call graph! Instead, your "general purpose allocator" is available on that "application state" type, so you can fetch it from essentially wherever. IO will work just the same in practice. So, if you discover that a code path you previously thought was pure actually does need to perform IO, then you don't need to apply some nasty viral change; you just grab `my_thing.io`.
I do agree that in principle, there's still a form of function coloring going on. Arguably, our solution to the problem is just to color every function async-colored (by giving most or all of them access to an `Io`). But it's much like the "coloring" of having to pass `Allocator`s around: it's not a problem in practice, because you'll basically always have easy access to one even if you didn't previously think you'd need it. I think seasoned Zig developers will pretty invariably agree with the statement that explicitly passing `Allocator`s around really does not introduce function coloring annoyances in practice, and I see no reason that `Io` would be particularly different.
Colouring every function async-coloured is something that's been attempted in the past; it was called "threads".
The innovation of async over threads is simply to allocate call stack frames on the heap, in linked lists or linked DAGs instead of fixed-size chunks. This sounds inefficient, and it is: indexing a fixed block of memory is much cheaper. It comes with many advantages as well: each "thread" only occupies the amount of memory it actually uses, so you can have a lot more of them; you can have non-linear graphs, like one function that calls two functions at the same time; and by reinventing threading from scratch you avoid a lot of thread-local overhead in libraries because they don't know about your new kind of threads yet. Because it's inefficient (and because for some reason we run the new threading system on top of the old threading system), it also became useful to run CPU-bound functions in the old kind of stack.
If you keep the linked heap activation records but don't colour functions, you might end up with Go, which already does this. Go can handle a large number of goroutines because they use linked activation records (but in chunks, so that not every function call allocates) and every Go function uses them so there is no colour.
You do lose advantages that are specific to coloured async - knowing that a context switch will not occur inside certain function calls.
As usual, we're beating rock with paper in the moment, declaring that paper is clearly the superior strategy, and missing the bigger picture.
> I do agree that in principle, there's still a form of function coloring going on. Arguably, our solution to the problem is just to color every function async-colored
I feel like there are two issues with this approach:
- you basically rely on the compiler/stdlib to silently switch the async implementation, effectively implementing a sort of hidden control flow which IMO doesn't really fit Zig
- this only solves the "visible" coloring issue of async vs non-async functions, but does not try to handle the issue of blocking vs non-blocking functions, rather it hides it by making all functions have the same color
- you're limiting the set of async operations to the ones supported in the `Io`'s vtable. This forces it to e.g. include mutexes, even though they are not really I/O, because they might block and hence need async support. But if I wrote my own channel how would this design support it?
> It's quite rare for a function to unexpectedly gain a dependency on ...
If this was true in general, the function coloring problem wouldn't be talked about.
However, the second point is more interesting. I think there's a bit of a Stockholm syndrome thing here with Zig programmers and Allocator. It's likely that Zig programmers won't mind passing around an extra param.
If anything, it would make sense to me to have IO contain an allocator too. Allocation is a kind of IO too. But I guess it's going to be 2 params from now on.
> If anything, it would make sense to me to have IO contain an allocator too. Allocation is a kind of IO too.
Io in zig is for “things that can block execution”. Things that could semantically cause a yield of any kind. Allocation is not one of those things.
Also, it’s perfectly reasonable and sometimes desireable to have 13 different allocators in your program at once. Short lived ones, long lived ones, temporary allocations, super specific allocators to optimize some areas of your game…
There are fewer reasons to want 2 different strategies to handle concurrency at the same time in your program as they could end up deadlocking on each other. Sure, you may want one in debug builds, another in release, another when running tests, but there are much fewer usecases of them running side by side.
> But I guess it's going to be 2 params from now on.
>> So, if you discover that a code path you previously thought was pure actually does need to perform IO, then you don't need to apply some nasty viral change; you just grab `my_thing.io
> Arguably, our solution to the problem is just to color every function async-colored.
This is essentially how Golang achived color-blindness.
It's basically how Java does it (circa 17) as well.
It's something you really can't do without a pretty significant language runtime. You also really need people working within your runtime to prefer being in your runtime. Environments that do a lot of FFI don't work well with a colorblind runtime. That's because if the little C library you call does IO then you've got an incongruous interaction that you need to worry about.
> It's quite rare for a function to unexpectedly gain a dependency on "doing IO" in general.
From the code sample it looks like printing to stdio will now require an Io param. So won’t you now have to pass that down to wherever you want to do a quick debug printf?
std.debug.print(..) prints to stderr whuch does not need an io param.
I think the key is, if you don't have an "io" in your call stack you can always create one. At least I hope that is how it would work. Otherwise it is equally viral to async await.
> * It's quite rare for a function to unexpectedly gain a dependency on "doing IO" in general.
I don't know where you got this, but it's definitely not the case, otherwise async would never cause problems either. (Now the problem in both cases is pretty minor, you just need to change the type signature of the call stack, which isn't generally that big, but it's exactly the same situation)
> In practice, most of your codebase will have access to an `Io`, and only leaf functions doing pure computation will not need them.
So it's exactly similar to making all of your functions async by default…
You are skipping the massive point here.
If you are using a library in rust, it has to be async await, tokio, send+sync and all the other crap. Or if it is sync api then it is useless for async application.
This approach of passing IO removes this problem and this is THE main problem.
This way you don’t have to use procedural macros or other bs to implement multi versioning for the functions in your library, which doesn’t work well anyway in the end.
https://nullderef.com/blog/rust-async-sync/
You can find 50 other ones like this by searching.
To be honest I don’t hope they will solve cooperative scheduling, high performance, optionally thread-per-core async soon and the API won’t be that good anyway. But hope it solves all that in the future.
> Or if it is sync api then it is useless for async application.
The rest is true, but this part isn't really an issue. If you're in an async function you can call sync functions still. And if you're worried it'll block and you can afford that, I know tokio offers spawn_blocking for this purpose.
> If you are using a library in rust, it has to be async await, tokio, send+sync and all the other crap
Send and sync is only required if you want to access something from multiple threads, which isn't required by async await (parallelism vs concurrency)
1) You can use async await without parallelism and 2) send and sync aren't a product of async/await in Rust, but generally memory safety, i.e. you need Send generally when something can/is allowed to move between threads.
Yes, but async Rust is basically built on tokio's runtime, which is what most the big async libraries depend on, like hyper/axum/tokio etc. And tokio is a thread-per-core work-stealing architecture, which requires Send + Sync bounds everywhere. You can avoid them if you depend on tokio-proper, but it's more icky when building on something like axum, where your application handlers also require these bounds.
A good article on this: https://emschwartz.me/async-rust-can-be-a-pleasure-to-work-w...
Iirc I had a situation a while back, in which I used async await with tokio with a non Send or Sync type and it compiled when I didn't use spawn[1] (implying multithreading) but a simple loop with sequential processing.
Only when I wanted to enable parallelism using spawn, I got a compilation error.
> Send and sync is only required if you want to access something from multiple threads, which isn't required by async await (parallelism vs concurrency)
In theory this is correct. In practice, a lot of APIs (including many in tokio) require both traits even for single-thread use cases.
I'm not skipping anything. And in fact acknowledge this exact point:
> That being said, I don't think Zig's implementation here is bad. If anything, it does a great job at abstracting the usage from the implementation. This is something Rust fails at spectacularly.
This comment is justly flatly incorrect. You don't need Tokio at all to write an async library. Nor do you need send sync. Not sure what other crap you are speaking of, either.
Sync APIs can be spawned in worker threads as futures, too. Generally executors have helper methods for that.
I'll let a real category theorist get into the details that I'll likely flub, but the IO monad is where you end up if you start on this path. That context can be implicit, but it's there, and if you want any help from the compiler (to, for example, guide Claude Code towards useful outcomes) you've got to reify it as a real thing in the formality of the system.
Async and coroutines are the graveyard of dreams for systems programming languages, and Andrew by independently rediscovering the IO monad and getting it right? Hope of a generation.
Functions in the real world have colors: you can have predictable rules for moving between colors, or you can wing it and get C++ co_await and tokio and please kill me.
This is The Way.
it's not a monad, since you can do unholy (for fp) things with it, like stash it in a struct and pass the struct around (even to functions which have no clue theres an io call) or just grab a globalized io object and use that at will arbitrarily at many entrypoints in your function.
most importantly, besides the obvious situations (creating the io object, binding it to another object), it's not generally going to be returned from a function as part of its value.
When I write in Haskell, I find myself mentally glossing the returned monadic state, along the lines of, "Oh, an M x is just an x that does monady stuff to get the x". This becomes natural once you get the hang of do-notation and sometimes monad combinators. So I'm not really thinking about the monadic state in the return value a lot.
It's not really any less natural than thinking stateful programming, except now the state is a reified thing, which I think is strictly advantageous once you get used to it.
i'm relatively confident that andrew will happily break the language again (god love him for that) when it becomes clear that you really want that algebra.
though i will say, for a systems language, it's probably better to invert the lift/unlift relationship, default to do-notation and explicitly unlift into pure functions. that's almost what const meant in C++ to begin with but it lost it's way.
Here's a trick to make every function red (or blue? I'm colorblind, you decide):
var io: std.Io = undefined;
pub fn main() !void {
var impl = ...;
io = impl.io();
}
Jokes aside, I agree that there's obviously a non-zero amount of friction to using the `Io` intreface, but it's something qualitatively very different from what causes actual real-world friction around the use of async await.
> but the general problem behind function coloring is that of context
I would disagree, to me the problem seems, from a practical perspective that:
1. Code can't be reused because the async keyword statically colors a function as red (e.g. python's blocking redis client and asyncio-redis). In Zig any function that wants to do Io, be it blue (non-async) or red (async) still has to take in that parameter so from that perspective the Io argument is irrelevant.
2. Using async and await opts you automatically into stackless coroutines with no way of preventing that. With this new I/O system even if you decide to use a library that interally uses async, you can still do blocking I/O, if you want.
To me these seems the real problems of function coloring.
Well, it's not really a joke. That's a valid strategy that languages use. In Go, every function is "async". And it basically blocks you from doing FFI (or at least it used to?). I wonder if Zig will run into similar issues here.
> 1. Code can't be reused because the async keyword statically colors a function
This is fair. And it's also a real pain point with Rust. However, it's funny that the "What color is your function?" article doesn't even really mention this.
> 2. Using async and await opts you automatically into stackless coroutines with no way of preventing that
This however I don't think is true. Async/await is mostly syntax sugar.
In Rust and C# it uses stackless coroutines.
In JS it uses callbacks.
There's nothing preventing you from making await suspend a green thread.
I should have specified that better, of course async and await can be lowered to different things (that's what Zig does afterall), what I wanted to say is that that's how it works in general. JS is a good counter example, but for all other mainstream languages, async means stackless coroutines (python, ruby, c#, rust, ...).
Which means that if I want to use a dependency that uses async await, it's stackless coroutines for me too whether I like it or not.
There are plenty of libraries out there which require users to do an init() call of some sorts at startup. It is perfectly possible to design a library that only works with 1 io instance and gets it at init(). Whether people like or want that… I have no clue.
you could still have a library-global io, let the user set it as desired.
> use of two different implementations of io
functionally rare situation.
Go also suffers from this form of “subtle coloring”.
If you’re working with goroutines, you would always pass in a context parameter to handle cancellation. Many library functions also require context, which poisons the rest of your functions.
Technically, you don’t have to use context for a goroutine and could stub every dependency with context.Background, but that’s very discouraged.
Having all async happen completely transparently is not really logically possible. asynchronous logic is frequently fundamentally different from synchronous logic, and you need to do something different one way or the other. I don't think that's really the same as "function colouring".
And context is used for more than just goroutines. Even a completely synchronous function can (and often does) take a context, and the cancellation is often useful there too.
I think the main point is in something like Go, the approach is non-viral. If you are 99 levels deep in synchronous code and need to call something with context, well, you can just create one. With C#, if you need to refactor all 99 levels above (or use bad practices which is of course what everyone does).
Also, in general cancellation is something that you want to optionally have with any asynchronous function so I don't think there really exists an ideal approach that doesn't include it. In my opinion the approach taken by Zig looks pretty good.
Like you said you dont NEED context. Its just something thats available if you need it. I still think Go/Erlang has one of the best concurrency stories out there.
Context is not required in Go and I personally encourage you to avoid it. There is no shame in blazing a different path.
It's not required, but eschewing it ends up going against the grain, since so much of the ecosystem is written to use contexts, including the standard library.
For example, say you instead of contexts, you use channels for cancellation. You can have a goroutine like this:
go func() {
for {
select {
case <-stop:
return
case <-time.After(1*time.Second):
resp := fetchURL(url)
processResult(resp.Body) // Simplified, of course
}
}
}()
Likewise, processResult() may be doing stuff that cannot be aborted just by closing the stop channel. You could pass the stop channel to it, but now you're just reinventing contexts.
Of course, you can choose to only use contexts where you're forced to, and invent wrappers around standard library stuff (e.g. the HTTP client), but at that point you're going pretty far to avoid them.
I do think the context is problematic. For the purposes of cancellation, it's invasive and litters the call graph with parameters and variables. Goroutines really ought to have an implicit context inherited from its parent, since everything is using it anyway.
Contexts are wildly abused for passing data around, leading to bloated contexts and situations where you can't follow the chain of data-passing without carefully reviewing the entire call graph. I always recommend not being extremely discriminating about where to pass values in context. A core principle is that it has to be something that is so pervasive that it would be egregious to pass around explicitly, such as loggers and application-wide feature flags.
Why do you encourage avoiding it? Afaik it's the only way to early-abort an operation since Goroutines operate in a cooperative, not preemptive, paradigm. To be very clear, I'm asking this completely in good faith looking to learn something new!
What would you use in its place? I've never had an issue with it. I use it for 1) early termination 2) carrying custom request metadata.
I don't really think it is fully the coloring problem because you can easily call non-context functions from context functions (but not other way around, so one way coloring issue), but you need to be aware the cancellation chain of course stops then.
The thing about context is it can be a lot more than a cancellation mechanism. You can attach anything to it—metadata, database client, logger, whatever. Even Io and Allocator if you want to. Signatures are future-proof as long as you take a context for everything.
At the end of the day you have to pass something for cooperative multitasking.
Of course it’s also trivial to work around if you don’t like the pattern, “very discouraged” or not.
> If you’re working with goroutines, you would always pass in a context parameter to handle cancellation.
The utility of context could be called a subtle coloring. But you do NOT need context at all. If your dealing with data+state (around queue and bus processing) its easy to throw things into a goroutine and let the chips fall where they will.
> which poisons the rest of your functions. You are free to use context dependent functions without a real context: https://pkg.go.dev/context#TODO
Agree that with something like go, there is truly no function coloring at all. However, since most real world async things require cancellation, a context parameter is always present so there is some "coloring" do to that. Still, it is much less viral than C# style async await as if you don't have a context in your call stack you can still create one when needed and call the function. I don't think it is reasonable to abstract cancellation in a way that nothing has to be passed in so perhaps the approach presented here is realistically as good as it gets.
Aside from the ridiculous argument that function parameters color them, the assertion that you can’t call a function that takes IO from inside a function that does not is false, since you can initialize one to pass it in
To me, there's no difference between the IO param and async/await. Adding either one causes it to not be callable from certain places.
As for the second thing:
You can do that, but... You can also do this in Rust. Yet nobody would say Rust has solved function coloring.
Also, check this part of the article:
> In the less common case when a program instantiates more than one Io implementation, virtual calls done through the Io interface will not be de-virtualized, ...
Doing that is an instant performance hit. Not to mention annoying to do.
>To me, there's no difference between the IO param and async/await.
You can't pass around "async/await" as a value attached to another object. You can do that with the IO param. That is very different.
> Adding either one causes it to not be callable from certain places.
you can call a function that requires an io parameter from a function that doesn't have one by passing in a global io instance?
as a trivial example the fn main entrypoint in zig will never take an io parameter... how do you suppose you'd bootstrap the io parameter that you'd eventually need. this is unlike other languages where main might or might not be async.
You’re allowed to not like it, but that doesn’t change that your argument that this is a form of coloring is objectively false. I’m not sure what Rust has to do with it.
> you can’t call a function that takes IO from inside a function that does not is false, since you can initialize one to pass it in
that's not true. suppose a function foo(anytype) takes a struct, and expects method bar() on the struct.
you could send foo() the struct type Sync whose bar() does not use io. or you could send foo() the struct type Async whose bar uses an io stashed in the parameter, and there would be no code changes.
if you don't prefer compile time multireification, you can also use type erasure and accomplish the same thing with a vtable.
It's hard to parse your comment, but I think we are agreeing? I was refuting the point of the parent. you have given another example of calling an IO-taking function inside a non-IO taking function. the example I gave was initializing an IO inside the non-IO taking function. you could also, as pointed out elsewhere, use global state.
> Well, you don't have async/sync/red/blue anymore, but you now have IO and non-IO functions.
> However, the coloring problem hasn't really been defeated.
Well, yes, but if the only way to do I/O were to have an Io instance to do it with then Io would infect all but pure(ish, non-Io) functions, so calling Io functions would be possible in all but those contexts where calling Io functions is explicitly something you don't want to be possible.
So in a way the color problem is lessened.
And on top of that you get something like Haskell's IO monad (ok, no monad, but an IO interface). Not too shabby, though you're right of course.
Next Zig will want monadic interfaces so that functions only have to have one special argument that can then be hidden.
Zig's not really about hiding things but you could put it in an options struct that has defaults unless overridden at compile time.
> In order to call such a function you also need to provide the context. Zig hasn't really solved this.
It is much more flexible though since you don't need to pass the IO implementation into each function that needs to do IO. You could pass it once into an init function and then use that IO impl throughout the object or module. Whether that's good style is debatable - the Zig stdlib currently has containers that take an allocator in the init function, but those are on the way out in favour of explicitly taking the allocator in each function that needs to allocate - but the user is still free to write a minimal wrapper to restore the 'pass allocator into init' behaviour.
Odin has an interesting solution in that it passes an implicit context pointer into each function, but I don't know if the compiler is clever enough to remove the overhead for called functions that don't access the context (since it also needs to peek into all called functions - theoretically Zig with it's single-compilation-unit approach could probably solve that problem better).
So this is a tangent from the main article, but this comment made me curious and I read the original "What color is Your Function" post.
It was an interesting read, but I guess I came away confused about why "coloring" functions is a problem. Isn't "coloring" just another form of static typing? By giving the compiler (or interpreter) more meta data about your code, it can help you avoid mistakes. But instead of the usual "first argument is an integer" type meta data, "coloring" provides useful information like: "this function behaves in this special way" or "this function can be called in these kinds of contexts." Seems reasonable?
Like the author seems very perturbed that there can be different "colors" of functions, but a function that merely calculates (without any IO or side-effects) is different than one that does perform IO. A function with only synchronous code behaves very differently than one that runs code inside another thread or in a different tick of the event loop. Why is it bad to have functions annotated with this meta data? The functions behave in a fundamentally different way whether you give them special annotations/syntax or not. Shouldn't different things look different?
He mentions 2015 era Java as being ok, but as someone that’s written a lot of multithreaded Java code, it’s easy to mess up and people spam the “synchronized” keyword/“color” everywhere as a result. I don’t feel the lack of colors in Java makes it particularly intuitive or conceptually simpler.
> He mentions 2015 era Java as being ok, but as someone that’s written a lot of multithreaded Java code, it’s easy to mess up and people spam the “synchronized” keyword/“color” everywhere as a result. I don’t feel the lack of colors in Java makes it particularly intuitive or conceptually simpler.
Async as a keyword doesn’t solve this or make writing parallel code any easier. You can still mess this up even if every function is annotated as async.
> A function with only synchronous code behaves very differently than one that runs code inside another thread or in a different tick of the event loop.
I think this is conflating properties of multiple runtimes. This is true in JavaScript because the runtime works on an event loop. In Java an “async” function that reads from a file or makes an http call doesn’t run in a different threads and doesn’t run in a different tick of an event loop. So what value does it have in that type of runtime?
Personally for me I think “async” is putting pain on a lot of developers where 99% of all code is not parallel and doesn’t share memory.
Yes, the main character of that article really is mostly JavaScript. The main issue there is that some things must be async, and that doesn't mesh well with things that can't be.
If you're writing a game, and you need to render a new enemy, you might want to reduce performance by blocking rather than being shot by an invisible enemy because you can only load the model async.
But even the article acknowledges that various languages tackle this problem better. Zig does a good job, but claiming it's been defeated completely doesn't really fly for me.
> It was an interesting read, but I guess I came away confused about why "coloring" functions is a problem. Isn't "coloring" just another form of static typing?
It is. Function coloring is static typing.
But people never ever agree on what to put in typing system. For example, Java's checked exceptions are a form of typing... and everyone hates them.
Anyway it's always like that. Some people find async painful and say fuck it I'm going to manage threads manually. In the meanwhile another bunch of people work hard to introduce async to their language. Grass is always greener on the other side.
> But people never ever agree on what to put in typing system. For example, Java's checked exceptions are a form of typing... and everyone hates them.
I love checked exceptions. Checked errors are fantastic and I think most developers would agree they want errors to be in the type system, but Java as a language just hasn’t provided the language syntax to make them usable. They haven’t made it easy to “uncheck” when you can’t possibly handle an error. You have to write boilerplate:
Something s;
try {
s = something();
} catch (SomethingException e) {
throw new RuntimeException(e);
}
var s = try! something();
try {
someCall(s -> {
try {
another(s);
} catch (CheckedException ex) {
throw new UncheckedException(ex);
}
});
} catch (UncheckedException ex) {
// handle somehow
}
[0] https://docs.scala-lang.org/scala3/reference/experimental/ca...
> Isn't "coloring" just another form of static typing?
In a very direct way. Another example in languages that don't like you ignoring errors, changing a function from infallible to fallible is a breaking change, a la "it's another colour".
I'm glad it is: if a function I call can suddenly fail, at the very least I want to know that it can, even if the only thing I do is ignore it (visibly).
> Isn't "coloring" just another form of static typing?
Yes, and so is declaring what exceptions a function can throw (checked exceptions in Java).
> Why is it bad to have functions annotated with this meta data? The functions behave in a fundamentally different way whether you give them special annotations/syntax or not. Shouldn't different things look different?
It really isn't a problem. The article makes people think they've discovered some clever gotcha when they first read it, but IMHO people who sit down for a bit and think through the issue come to the same conclusion you have - Function coloring isn't a problem in practice.
> but IMHO people who sit down for a bit and think through the issue come to the same conclusion you have - Function coloring isn't a problem in practice.
I dunno man, have you seen people complain about async virality in Rust being annoying? Have you ever tried to read a backtrace from a program that does stackless coroutines (it's not fun)? Have you seen people do basically duplicate work to maintain a blocking and an async version of the same networking library?
I am not very experienced in async Rust, but it seems there are some pieces of async Rust that rely too much on tokio internals, so using an alternative runtime (like pollster) results in broken code.
Searching for comments mentioning "pollster" and "tokio" on HN brings a few results, but not one I recall seeing a while ago where someone demonstrated an example of a library (using async Rust) that crashes when not using tokio as the executor.
Related documentation: https://rust-lang.github.io/async-book/08_ecosystem/00_chapt...
There's two details that are important to highlight. tokio is actually 2 components, it's the async scheduler, and it's the IO runtime. Pollster is only a scheduler, and does not offer any IO functionality. You can actually use tokio libraries with pollster, but you need to register the IO runtime (and spawn a thread to manage it) - this is done with Runtime::enter() and it configures the thread local interface so any uses of tokio IO know what runtime to use.
There are ideas to abstract the IO runtime interface into the async machinery (in Rust that's the Context object that schedulers pass into the Future) but so far that hasn't gotten anywhere.
Yep. The old async wars in the Rust ecosystem. It's the AsyncRead and AsyncWrite traits. Tokio has its own, there was a standard brewing at the same time in the futures crate. Tokio did their own thing, people burnt out, these traits were never standardized to std.
So you cannot use most of the async crates easily outside Tokio.
Sure. Let's do an imaginary scenario. Let's say that you are the author of a http request library.
Async hasn't been added yet, so you're using `std::net::TcpStream`.
All is well until async comes along. Now, you have a problem. If you use async, your previous sync users won't be able to (easily) call your functions. You're looking at an API redesign.
So, you swallow your pride and add an async variant of your functionality. Since Tokio is most popular, you use `tokio::net::TcpStream`.
All is well, until a user comes in and says "Hey, I would like to use your library with smol (a different async runtime)". Now what do you do? Add a third variant of your code using `smol::net::TcpStream`? It's getting a bit ridiculous, and smol isn't the only alternative runtime.
One solution is to do what Zig does, but there isn't really an agreed upon solution. The stdlib does not even provide AsyncRead/AsyncWrite so you could invert your code and just work with streams provided from above and keep your libary executor agnostic.
> Technically you could pass in a new executor, but is that really what you want?
why does it have to be new? just use one executor, set it as const in some file, and use that one at every entrypoint that needs io! now your io doesn't propagate downwards.
I think of it this way.
Given an `io` you can, technically, build another one from it with the same interface.
For example given an async IO runime, you could create an `io` object that is blocking (awaits every command eagerly). That's not too special - you can call sync functions from async functions. (But in JavaScript you'd have trouble calling a sync function that relies on `await`s inside, so that's still something).
Another thing that is interesting is given a blocking posix I/O that also allows for creating processes or threads, you could build in userspace a truly asynchronous `io` object from that blocking one. It wouldn't be as efficient as one based directly on iouring, and it would be old school, but it would basically work.
Going either way (changing `io` to sync or async) the caller doesn't actually care. Yes the caller needs a context, but most modern apps rely on some form of dependency injection. Most well-factored apps would probably benefit from a more refined and domain-specific "environment" (or set of platform effects, perhaps to use the Roc terminology), not Zig's posix-flavoured standard library `io` thing.
Yes rust achieves this to some extent; you can swap an async runtime for another and your app might still compile and run fine.
Overall I like this alot - I am wondering if Richard Feldmann managed to convince Andrew Kelley that "platforms" are cool and some ideas were borrowed from Roc?
This is like saying arithmetic is a form of calculus, the simplest form. Ie it reduces the concept (DI) to a meaningless tautology.
The original “function colouring” blogpost has done irreparable damage to PL discussions because it’s such a stupid concept to begin with. Of course I want async functions to be “coloured” differently, they do different things! How else is a “normal function” supposed to call a function that gives you a result later——obviously you want to be forced to say what to do with the result; await it, ignore it, .then() in JS terms, etc. these are important decisions that you can’t just ignore because it’s “painful”
There is nothing obvious around that - it is driven by what abstractions the language provides related to concurrency, and with different choices you will end needing different ways to interact with it.
So yes, given how the language designers of C# and JavaScript choose to implement concurrency and the APIs around that, then coloring is necessary. But it is very much implementation driven and implementation of other concurrency models then other ways to do it that don't involve keywords can make sense. So when people complain about function coloring, they are complaining about the choice of concurrency model that a language uses.
Note that this same concept is "sans io" and was previously discussed for it's use in Rust:
https://www.firezone.dev/blog/sans-io
If the functions are still calling I/O methods directly rather than the I/O being externally driven, I don't think that qualifies as sans-io, based on my previous exposure / based on your second link:
> For byte-stream based protocols, the protocol implementation can use a single input buffer and a single output buffer. For input (that is, receiving data from the network), the calling code is responsible for delivering code to the implementation via a single input (often via a method called receive_bytes, or something similar). The implementation will then append these bytes to its internal byte buffer. At this point, it can choose to either eagerly process those bytes, or do so lazily at the behest of the calling code.
> When it comes to generating output, a byte-stream based protocol has two options. It can either write its bytes to an internal buffer and provide an API for extracting bytes from that buffer, as done by hyper-h2, or it can return bytes directly when the calling code triggers events (more on this later), as done by h11. The distinction between these two choices is not enormously important, as one can easily be transformed into the other, but using an internal byte buffer is recommended if it is possible that the act of receiving input bytes can cause output bytes to be produced: that is, if the protocol implementation sometimes automatically responds to the peer without user input.
I'm confused. The trouble with "colored" functions is that they either do processing on the stack, or unwind the stack. They claim defeat of function coloring, and describe that IO implementation can use blocking/thread pool/green threads. But... these are all blocking methods, which weren't the problem in the first place! If you keep convention to never do IO using global state, you could do that practically in any language. Stackless coroutines being left for later feels like "draw the rest of the owl" situation.
To actually have truly universal functions, I think there are two solutions:
- Make every function async, and provide extra parameter indicating to not actually unwind the stack and execute synchronously instead. Comes with performance penalty.
- Compile each function twice, picking appropiate variant at call site. Increases code size and requires some hackery with handling function pointers.
I am not on the core team but i believe the plan is to do exactly what you are talking about, but after the API is nailed down and kinks have been ironed out by users of the existing semiblocking implementation (to possibly include the compiler), as the default LLVM coro state machine compiler has problems (for example: I think I remember Andrew complaining that it has an obligatory libc/malloc dependency).
since the new io interface has userland async/await methods, then dropping in a proper frame jumping solution will be less painful, and easier to debug, and if using coroutines proves to be challenging with the api hopefully changes to io api would be minor, versus going after stackless coroutines NOW and making large API changes often as the warts with the system uncover themselves.
> Make every function async, and provide extra parameter indicating to not actually unwind the stack and execute synchronously instead. Comes with performance penalty.
I think ValueTask<T> in C#/.NET can approach this use case - It avoids overhead if the method actually completes synchronously. Otherwise, you can get at the Task<T> if needed. From a code perspective, you await it like you normally would and the compiler/runtime figures out what to do.
I'm generally a fan of Zig, but it's a little sad seeing them go all in on green threads (aka fibers, aka stackful coroutines). Rust got rid of their Runtime trait (the rough equivalent of Zig's Io) before 1.0 because it performed badly. Languages and OS's have had to learn this lesson the hard way over and over again:
https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2018/p13...
> While fibers may have looked like an attractive approach to write scalable concurrent code in the 90s, the experience of using fibers, the advances in operating systems, hardware and compiler technology (stackless coroutines), made them no longer a recommended facility.
If they go through with this, Zig will probably top out at "only as fast as Go", instead of being a true performance competitor. I at least hope the old std.fs sticks around for cases where performance matters.
I'm not sure how you got the perception that we're going "all in" on green threads, given that the article in OP explicitly mentions that we're hoping to have an implementation based on stackless coroutines, based on this Zig language proposal: https://github.com/ziglang/zig/issues/23446
Performance matters; we're not planning to forget that. If fibers turn out to have unacceptable performance characteristics, then they won't become a widely used implementation. Nothing discussed in this article precludes stackless coroutines from backing the "general purpose" `Io` implementation if that turns out to be the best approach.
That is lovely to hear. I think the general conscious is that not a single programming language has done Async right. So people are a little sceptical. But Andrew and the team so far seems to have the do it right mentality. So I guess people should be a little more optimistic.
Cant wait for 0.15 coming out soon.
Does the BDFL want this though, or is it just one person's opinion that it might be nice? Given how he has been aggressively pruning proposals, I don't put any hope in them anymore unless I see some kind of positive signal from him directly.
e.g. I'd feel a lot more confident if he had made the coroutine language feature a hard dependency of the writergate refactor.
I’m confused about the assertion that green threads perform badly. 3 of the top platforms for high concurrency servers use or plan to use green threads (Go, Erlang, Java). My understanding was that green threads have limitations with C FFI which is why lower level languages don’t use them (Rust). Rust may also have performance concerns since it has other constraints to deal with.
Green threads have issues with C FFI mostly due to not being able to preempt execution, when the C thing is doing something that blocks. This is a problem when you have one global pool of threads that execute everything. To get around it you essentially need to set up a dedicated thread pool to handle those c calls.
Which may be fine - go doesn't let the user directly create thread pools directly but do create one under the hood for ffi interaction.
It's hardly "all-in" if it is merely one choice of many, and the choice is made in the executable not in the library code.
I have definitely gotten the impression that green threads will be the favored implementation, from listening to core team members and hanging around the discord. Stackless coroutines don't even exist in the language currently.
What does "favored" mean if event loop and direct blocking are relatively trivial and provided also/ If I can trivially use them, what do I care what Andrew or someone in core thinks? The control is all mine, and near zero cost (potential vtable indirection).
And would Rust be "all-in" if tokio was in std, so you could use its tasks everywhere? That would be a very similar level of "all-in" to Zig's current plan, but with a seemingly better API.
I understand the benefit of not being in std, but really not a fundamental issue, IMO.
In the 2026 roadmap talk Andrew Kelley spoke of the fact that stackless coroutines with iouring is the end goal here (but the requires an orthogonal improvement in the compiler for inlining that data to the stack where possible).
Do you have the timestamp? I watched that video when it came out and don't remember hearing it.
That paper (P1364R0) was contentious, and I regard it as being severely motivated reasoning, published only to kill off competing approaches to C++ coroutines.
Some discussions of that paper:
* https://old.reddit.com/r/cpp/comments/1jwlur9/stackful_corou...
* https://old.reddit.com/r/programming/comments/dgfxde/fibers_...
It actually has much the same benefits of Rust removing green threads and replacing them with a generic async runtime.
The point here is that "async stuff is IO stuff is async stuff". So rather than thinking of having pluggable async runtimes (tokio, etc) Zig is going with pluggable IO runtimes (which is kinda the equivalent of "which subset of libc do you want to use?").
But in both moves the idea is to remove the runtime out of the language and into userspace, while still providing a common pluggable interface so everyone shares some common ground.
Oh man. I think the biggest mistake Rust has ever made is their async model. It’s been nothing short of a disaster. Zig supporting green threads and other options is spectacularly exciting. Thus far no one has “solved” async. So very exciting to see what Zig can come up with.
Seeing a systems language like Zig require runtime polymorphism for something as common as standard IO operations seems off to me -- why force that runtime overhead on everyone when the concrete IO implementation could be known statically in almost all practical cases?
I/O strikes me as one place where dynamic dispatch overhead would likely be negligible in practice. Obviously it depends on the I/O target and would need to be measured, but they don't call them "I/O bound" (as opposed to "CPU bound") programs for no reason.
> why force that runtime overhead on everyone
pretty sure the intent is for systems that only use one io to have a compiler optimization that elides the cost of double indirection... but also, you're doing IO! so usually something else is the bottleneck, one extra indirection is likely to be peanuts.
I think it's just the Zig philosophy to care more about binary size than speed. Allocators have the same tradeoff, ArrayListUnmanaged is not generic over the allocator, so every allocation uses dynamic dispatch. In practice the overhead of allocating or writing a file will dwarf the overhead of an indirect call. Can't argue with those binary sizes.
(And before anyone mentions it, devirtualization is a myth, sorry)
> (And before anyone mentions it, devirtualization is a myth, sorry)
In Zig it's going to be a language feature, thanks to its single unit compilation model.
Wouldn't this only work if there's only one implementation throughout the entire compliation unit? If you use 2 allocators in your app, your restricted function type has 2 possible callees for each entry, and you're back to the same problem.
It can also mean faster compilation (and sometimes better performance? https://nical.github.io/posts/rust-custom-allocators.html)
Just templating everything doesn’t mean it will be faster every time
Runtime polymorphism isn’t something inherently bad.
It is bad if you are introducing branching in a tight loop or preventing compiler from inlining things it would inline otherwise and other similar things maybe?
Although I'm not wild about the new `io` parameter popping up everywhere, I love the fact that it allows multiple implementations (thread based, fiber based, etc.) and avoids forcing the user to know and/or care about the implementation, much like the Allocator interface.
Overall, I think it's a win. Especially if there is a stdlib implementation that is a no overhead, bogstock, synchronous, blocking io implementation. It follows the "don't pay for things you don't use" attitude of the rest of zig.
Isn’t “don’t pay for what you don’t use” a myth? Some other person will using unless you are a very small team with discipline, and you will pay for it.
Or just passing around an “io” is more work than just calling io functions where you want them.
> io.async expresses asynchronicity (the possibility for operations to happen out of order and still be correct) and it does not request concurrency, which in this case is necessary for the code to work correctly.
This is the key point for me. Regardless of whether you’re under an async event loop, you can specify that the order of your io calls does not imply sequencing. Brilliant. Separate what async means from what the io calls do.
Seeing the example code made me wonder if this would allow introducing capability based security. E.g. passing an `io` instance to a library which can only read a subtree of the filesystem.
Edit: not quite https://news.ycombinator.com/item?id=44549430
I wrote a simple ssh server in zig to learn the language in my spare time.
The new design makes the event loop / io much easier to reason about. Thanks Andy
I miss the mention of boost::asio in this thread. At first glance this new Io interface feels not that dissimilar to it:
Both are generic interfaces over an event loop/executor supporting async or blocking operations. Both ship a thread-pool and a stackful coroutine backend and both can be used through their respective language's stackless coroutine implementation (co_yield in cpp and yet-unimplemented in zig).
I don't know if it's still true in the recent versions of Scala (stopped caring in 2018) but it used to have implicit parameters designed specifically for passing context like this.
A notable example was passing around an implicit ExecutionContext for thread pools, e.g. in Akka :)
I think this design is a regression from the previous design, in which you could use compile time introspection to check whether things are actually async (calling convention) or not.
Additionally, I don't necessarily want to delegate the management of the memory backing the futures to an Io, or pass around a blob of syscalls and an associated runtime, which accesses everything via a vtable. I would prefer to have these things be compile time generic only.
Your preference to have them be compile time generic shouldn’t come at the cost of those that would want runtime virtualisation.
As the article concludes, you get the best of both worlds here, where the result is effectively compile time generic if you only use one io implementation in your program. In theory it’d also partially compile time generic if you exclusively use one io for one set of libraries/functions and a different io for another set of libraries/functions.
I see this as the objectively correct design based on the existing design decisions in Zig. It follows from the allocator interface decision.
Yes, I understand that the designers prefer the Allocator situation and that Reader and Writer being anytype was downstream of the difficulty of using async readers and writers otherwise. So the intention was always to go with the design that I do not prefer. One reason I do not prefer it is that the Reader and Writer interfaces were already staggeringly inefficient, despite the lack or virtualization. We have avoided the issue by reimplementing a bunch of their API in some specific readers and writers and modifying the stdlib Reader and Writer to dispatch to these methods if they are present.
To be honest, I just do not have much faith in the commitment to optimality, when it seems like the team has not spent time doing things like profiling a program that spends a lot of time formatting integers as decimal syrings, and noticing that the vast majority of that formatting runtime is UTF-8 validation. I am happy to continue using the language, because it makes it easy enough to fix these issues oneself.
The only aspect that may not be recoverable by the end user is the "am I async/is this async" reflection issue, though a core team member has clarified in this comment section that the code in the article is a sketch and the design of stackless coroutines is far from done, so we may yet get this.
Some other philosophical point is, like, lua's coroutine.create/resume/yield/clone are control flow primitives for use within a single thread of execution. It's fine to ship an async runtime, which embodies the view they they are not control flow primitives for use within a single thread of execution, for doing I/O. But focusing the primitives for creating and switching between execution contexts too narrowly on the async runtime use case is liable to he harmful to other use cases for these operations. Ideally, we would be able to write things like a prominent SNES emulator that uses stack switching to ensure the simulation of different components proceeds in an order known to be more correct than other orders, and we would be able to do it using native language features, which would compile down to something a bit cheaper than dumping all of our registers onto the stack. Ideally when we do this we would not be asked by the language to consider what it would mean to "cancel" the execution context managing one of the components, in the same way that we do not need to consider what it means to cancel an arbitrary struct, or the function which is calling the function currently executing.
So you have to do:
io.async(saveFile, .{io, data, "saveA.txt"}).await(io);
Considering there is very little use case for mix and matching different Ios, any chance of having some kind of default / context IO to avoid all these ?
If you're going to immediately await it, you can just do
saveFile(io, data, "saveA.txt");
io.async(saveFile, .{io, data, "saveA.txt"}).await(io);
So this would be valid:
var save_future = io.async(saveFile, .{io, data, "saveA.txt"});
save_future.await(io);
const save_future = io.async(saveFile, .{io, data, "saveA.txt"});
save_future.await(io); // compile errorI wish Zig had not done async/await. CPS (like you have in Go) is way, way better, and is lower level, making it possible to do you own "async/await" if you really want to.
Read the article, the new Zig async/await interface doesn't imply the typical async/await state-machine code transformation. You can write a simple blocking runtime, or a green-thread implementation, or a thread-pool, or the state-machine approach via stackless coroutines (but AFAIK this needs a couple of language builtins which then must be implemented in an IO implementation).
Async/await tend to be IO bound, in zigs case hiw can i know what to use and when? Say i do a db call, thats clearly IO, and later do heavy CPU thing, now how do i know that the CPU thing does not block inside my async io thing?
I guess its pure luck if the io implementation can handle both, but who knows?
This was my point. With Go it does not matter, i can do IO or CPU both with Gos concurrency (CSP), but with async/await i usully cannot, i assume this is not something Zig is planning on, as it seems to be all about IO.
Because it allows multiple topologies of producers and consumers.
I believe async/await means you have a single consumer (caller) and a single producer (callee) and only a single value will be produced (resolved).
With CPS you may send and receive many times over to whomever and from whomever you like.
In JavaScript you may write..
const fetchData = async () => {
// snip
return data;
}
const data = await fetchData();
channel := make(chan int);
go func() {
// snip
channel <- data;
}()
data := <-channel
go func() {
for { // Infinite loop:
// snip
channel1 <- data;
channel2 <- data;
channel3 <- data;
}
}()Love the no function coloring solution! I am so looking forward to Zig 1.0. Finally, a system programming language that I can actually read and understand without putting in heavy labor. Hell, I could fully follow this blog post, without actually knowing anything much about Zig. Broke my head on async Rust several times before throwing in the towel.
As the author of a semi-famous post about how Zig has function colors [1], I decided to read up on this.
I see that blocking I/O is an option:
> The most basic implementation of `Io` is one that maps to blocking I/O operations.
So far, so good, but blocking I/O is not async.
There is a thread pool that uses blocking I/O. Still good so far, but blocking I/O is still not async.
Then there's green threads:
> This implementation uses `io_uring` on Linux and similar APIs on other OSs for performing I/O combined with a thread pool. The key difference is that in this implementation OS threads will juggle multiple async tasks in the form of green threads.
Okay, they went the Go route on this one. Still (sort of) not async, but there is an important limitation:
> This implementation requires having the ability to perform stack swapping on the target platform, meaning that it will not support WASM, for example.
But still no function colors, right?
Unfortunately not:
> This implementation [stackless coroutines] won’t be available immediately like the previous ones because it depends on reintroducing a special function calling convention and rewriting function bodies into state machines that don’t require an explicit stack to run.
(Emphasis added.)
And the function colors appear again.
Now, to be fair, since there are multiple implementation options, you can avoid function colors, especially since `Io` is a value. But those options are either:
* Use blocking I/O.
* Use threads with blocking I/O.
* Use green threads, which Rust removed [2] for good reasons [3]. It only works in Go because of the garbage collector.
In short, the real options are:
* Block (not async).
* Use green threads (with their problems).
* Function colors.
It doesn't appear that the function colors problem has been defeated. Also, it appears to me that the Zig team decided to have every concurrency technique in the hope that it would appear innovative.
[1]: https://gavinhoward.com/2022/04/i-believe-zig-has-function-c...
[2]: https://github.com/aturon/rfcs/blob/remove-runtime/active/00...
[3]: https://www.open-std.org/JTC1/SC22/WG21/docs/papers/2018/p13...
It’s hard to judge before stackless coroutines are reintroduced. But I think you’re entirely wrong that the next version of it will have colored functions, even according to your definition.
It has been mentioned that it’s possible that the default for debug builds is that every single function is compiled as an async function. I.e. there is canonically only one function color. Changing function color could become an optimisation for release builds. This is really not much different from inlining functions or other tricks the compiler can do with the calling convention if it has perfect knowledge of all callers.
> it appears to me that the Zig team decided to have every concurrency technique in the hope that it would appear innovative.
That’s a really bad take. It’s not much different from what they did to make allocators explicit. It’s an excellent approach for what Zig is supposed to be. Different concurrency models have different performance tradeoffs, just like with allocators. If the can support different IO models without making the language complicated, that’s a huge win, and they seem to be achieving that.
I find this approach the opposite of “appear innovative”. They’ve moved away from designing in a bunch of fancy syntax that locks users into one particular concurrency model, and gone for a more explicit and boring design which puts power in the hands of the user. It may not be right for everyone, but for what Zig is setting out to do it’s perfect. A disciplined decision in my opinion.
Getting stackless coroutines right for a low level language like Zig would be somewhat innovative. But not in a way that’s flashy or super interesting.
> It has been mentioned that it’s possible that the default for debug builds is that every single function is compiled as an async function. I.e. there is canonically only one function color.
But then, for those that choose to only use blocking I/O or green threads, they still pay the penalty of async merely existing.
> That’s a really bad take. It’s not much different from what they did to make allocators explicit.
I mean, Zig explicit allocators are really the same thing is Go interfaces, just dressed up as an innovative feature by a specific use case. This is what I mean by "appearing" innovative: they are taking old and tested ideas and presenting them in new ways to make them appear new.
Also, Zig could have had explicit allocators without needing to pass them to every function [1].
> They’ve moved away from designing in a bunch of fancy syntax that locks users into one particular concurrency model, and gone for a more explicit and boring design which puts power in the hands of the user.
Except that if every function is made async, they have actually removed from users the power to choose to not use async.
Their bet seems to be that they can transparently implement real async inside an IO implementation using compiler magic. But then it means if you use that IO instance with the magic then your function gets transformed into a state machine?
Then this whole thing is useless for implementing cooperative scheduling async like in rust?
> But then it means if you use that IO instance with the magic then your function gets transformed into a state machine?
This was essentially like the old async/await implementation in Zig already worked. The same function gets the state-machine treatment if it was called in an async context, otherwise it's compiled as a 'regular' sequential function.
E.g. at runtime there may be two versions of a function, but not in the code base. Not sure though how that same idea would be implemented with the new IO interface, but since Zig strictly uses a single-compilation-unit model the compiler might be able to trace the usage of a specific IO implementation throughout the control flow?
> it depends on reintroducing a special function calling convention
This is an internal implementation detail rather than a fact which is usually exposed to the user. This is essentially just explaining that the Zig compiler needs to figure out which functions are async and lower them differently.
We do have an explicit calling convention, `CallingConvention.async`. This was necessary in the old implementation of async functions in order to make runtime function pointer calls work; the idea was that you would cast your `fn () void` to a `fn () callconv(.async) void`, and then you could call the resulting `*const fn () callconv(.async) void` at runtime with the `@asyncCall` builtin function. This was one of the biggest flaws in the design; you could argue that it introduced a form of coloring, but in practice it just made vtables incredibly undesirable to use, because (since nobody was actually doing the `@asyncCall` machinery in their vtable implementations) they effectively just didn't support async.
We're solving this with a new language feature [0]. The idea here is that when you have a virtual function -- for a simple example, let's say `alloc: *const fn (usize) ?[*]u8` -- you instead give it a "restricted function pointer type", e.g. `const AllocFn = @Restricted(*const fn (usize) ?[*]u8);` with `alloc: AllocFn`. The magic bit is that the compiler will track the full set of comptime-known function pointers which are coerced to `AllocFn`, so that it can know the full set of possible `alloc` functions; so, when a call to one is encountered, it knows whether or not the callee is an async function (in the "stackless async" sense). Even if some `alloc` implementations are async and some are not, the compiler can literally lower `vtable.alloc(123)` to `switch (vtable.alloc) { impl1 => impl1(123), impl2 => impl2(123), ... }`; that is, it can look at the pointer, and determine from that whether it needs to dispatch a synchronous or async call.
The end goal is that most function pointers in Zig should be used as restricted function pointers. We'll probably keep normal function pointers around, but they ideally won't be used at all often. If normal function pointers are kept, we might keep `CallingConvention.async` around, giving a way to call them as async functions if you really want to; but to be honest, my personal opinion is that we probably shouldn't do that. We end up with the constraint that unrestricted pointers to functions where the compiler has inferred the function as async (in a stackless sense) cannot become runtime-known, as that would lead to the compiler losing track of the calling convention it is using internally. This would be a very rare case provided we adequately encourage restricted function pointers. Hell, perhaps we'd just ban all unrestricted default-callconv function pointers from becoming runtime-known.
Note also that stackless coroutines do some with a couple of inherent limitations: in particular, they don't play nicely with FFI (you can't suspend across an FFI boundary; in other words, a function with a well-defined calling convention like the C calling convention is not allowed to be inferred as async). This is a limitation which seems perfectly acceptable, and yet I'm very confident that it will impact significantly more code than the calling convention thing might.
TL;DR: depending on where the design ends up, the "calling convention" mentioned is either entirely, or almost entirely, just an implementation detail. Even in the "almost entirely" case, it will be exceptionally rare for anyone to write code which could be affected by it, to the point that I don't think it's a case worth seriously worrying about unless it proves itself to actually be an issue in practice.
From my experience, the calling convention was, in 0.9.x, just an implementation detail, until it wasn't. I think I may still reserve judgment for when async is fully implemented. Then I'll torture it again.
Yeah, but why is that a problem? Zig doesn't promise any stability before 1.0, and it's not like we don't need to change code in other language ecosystem frequently for all sorts of reasons (e.g. bumping a dependency version, or a new minor C/C++ compiler implementing new warnings).
Large breaking change:
Does this mean that as a side effect, it'll now be possible to enforce functions are pure/deterministic in Zig by not passing in an Io?
Not quite:
* Global variables still exist and can be stored to / loaded from by any code
* Only convention stops a function from constructing its own `Io`
* Only convention stops a function from reaching directly into low-level primitives (e.g. syscalls or libc FFI)
However, in practice, we've found that such conventions tend to be fairly well-respected in most Zig code. I anticipate `Io` being no different. So, if you see a function which doesn't take `Io`, you can be pretty confident (particularly if it's in a somewhat reputable codebase!) that it's not interacting with the system (e.g. doing filesystem accesses, opening sockets, sleeping the thread).
What about random number generation; is that something that will also fall under Io?
I feel that I have to point this out once again, because the article goes so far as to state that:
> With this last improvement Zig has completely defeated function coloring.
I disagree with this. Let's look at the 5 rules referenced in the famous "What color is your function?" article referenced here.
> 1. Every function has a color
Well, you don't have async/sync/red/blue anymore, but you now have IO and non-IO functions.
> 2. The way you call a function depends on its color.
Now, technically this seems to be solved, but you still need to provide IO as a parameter. Non-IO functions don't need/take it.
It looks like a regular function call, but there's no real difference.
> 3. You can only call a red function from within another red function
This still applies. You can only call IO functions from within other IO functions.
Technically you could pass in a new executor, but is that really what you want? Not to mention that you can also do this in languages that don't claim to solve the coloring problem.
> 4. Red functions are more painful to call
I think the spirit still applies here.
> 5. Some core library functions are red
This one is really about some things being only possible to implement in the language and/or stdlib. I don't think this applies to Zig, but it doesn't apply to Rust either for instance.
Now, I think these rules need some tweaking, but the general problem behind function coloring is that of context. Your function needs some context (an async executor, auth information, an allocator, ...). In order to call such a function you also need to provide the context. Zig hasn't really solved this.
That being said, I don't think Zig's implementation here is bad. If anything, it does a great job at abstracting the usage from the implementation. This is something Rust fails at spectacularly.
However, the coloring problem hasn't really been defeated.