HN Reader

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

https://sans-io.readthedocs.io/

https://news.ycombinator.com/item?id=40872020

Is this in effect introducing algebraic effects by concept? E.g. the io passed in is an effect handler, and it is the effect handler's choice whether to perform stack switching (or other means of non-blocking waiting) to enable asynchronicity?

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...

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 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.

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 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.

> 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.

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.

This is very well written, and very exciting! I especially love the implications for WebAssembly -- WASI in userspace? Bring your own IO? Why not both!

I like the IO interface simply for the fact that it would allow me to create language level vfs

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

Ok, they are implementing an effect system. Is there any acknowledgement that they are going down an established path?

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.

What about timers? That's the other resource an event loop must offer, besides I/O.

If you are using I/O with green threads but sleep the OS thread, you block other green threads. Likewise, if you have stackless coroutines (when they exist in Zig) but sleep the OS thread, you block other coroutines.

So is there an io.sleep?

Also - does any of this use io_uring on Linux?

This is a good time to implement "context", a way to pass down the call stack parameters instead of having to add a io argument to every function

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.

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).

So you have to do:

    io.async(saveFile, .{io, data, "saveA.txt"}).await(io);

That is 3 references to `io` in a single call.

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 ?

I'm a bit concerned when library authors only test it with blocking Io and the consuming app with a different kind. Wouldn't this potentially lead to bugs?

Et tu, Zig?

I don't know Zig, but wouldn't such a change be a major breaking change where all prior Zig code doing Io wouldn't work anymore if upgraded?

Speaking of colors… I like the color scheme of the code snippets, is it a standard scheme available in VSCode?

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?

I 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.

So is that zig becoming a type AND effect system?

Interesting. This is a bit reminiscent of how OCaml handles async these days.

The discussion around function color misses the distinction between the typical await approach and other solutions.

For example, consider a library that implements the C preprocessor; it implements a single function that takes a string to be processed and applies the C pre-processing algorithm to it and returns the preprocessed string.

     c_preprocessor_v1(body: string) -> string

The C preprocessor has includes operations, so it might need to (recursively) open additional files. Instead of making assumptions about what's the include path is or even the existence of a filesystem, the designer of the c_preprocessor decided, in v2, to delegates the file opening to a separate function [1]:

     c_preprocessor_v2(file: path, loader : path->string) -> string

c_preprocessor_v2 will incrementally call loader as it discover new include statements, possibly from the output of loader itself.

_v1 can of course be implemented in term of _v2 given a default loader definition.

Now you want to implement a preprocessor-as-a-service. It provides a rich API for the user to submit an initial file to your service and for the service to ask the user to submit the additional files on demand. And of course you want to use the c_preprocessor library. You expect your service to have to server hundreds of thousands of concurrent requests, so you want to make it async, in particular you want to make the loading async.

If you are using JS I believe you are screwed: you can't use the library as is: c_preprocessor_v2 and the async loader live in separate worlds: red (async) functions can call blue (sync) functions, but not vice versa; you need to ask the maintainer for a new async c_preprocessor_v3 that takes an async loader.

In some other languages (rust, c#, python) can wrap your async loader with a wrapper that blocks (in a way, closing over the async-ness of the function), but this is hardly ideal, the resulting call to c_preprocessor_v2 would not be async and prevent you from scaling to hundreds of thousands of requests. You might play around with offloading to thread pools, but as the bulk of the work is inside the c_preprocessor function it is never going to work well. In practice your blue functions can call red functions, but the resulting function is blue.

There is a third class of languages that allow you to combine blue and red functions producing red ones (Go, lua, scheme, and I believe this new Zig proposal); in these languages the caller can sandwich calls to sync functions across async domains, while still allowing suspending the whole call stack.

One disadvantage of the third class is that, as side effects are often unrestricted, if c_preprocessor relies on hidden global state, it might not be able to handle reentrancy correctly.

There is then a fourth class of languages where, not only side effects are always explicit (Haskel, some effectful programming languages), it is possible, and indeed idiomatic to be able to abstract over it. So c_processor_v2 might not only be able to call synchronous or asynchronous loaders transparently, but the idiomatic implementation might even be able to extract additional concurrency by not imposing dependencies unless necessary. One interpretation is that in these languages functions are always red, but I think that's reductive and not useful.

[1] this example uses higher order functions, but an OOP example would be of course completely equivalent.

Damn that’s some ugly async syntax.