> Doing that is an instant performance hit. Not to mention annoying to do.

The cost of virtual dispatch on IO path is almost always negligible. It is literally one conditional vs syscall. I doubt it you can even measure the difference.

>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.

> You can't pass around "async/await" as a value attached to another object

Sure you can? You can just pass e.g. a Task around in C# without awaiting it, it's when you need a result from a task that you must await it.

Sure you can. An `async` function in Javascript is essentially a completely normal function that returns a promise. The `async`/`await` syntax is a convenient syntax sugar for working with promises, but the issue would still exist if it didn't exist.

More to the point, the issue would still exist even if promises didn't exist — a lot of Node APIs originally used callbacks and a continuation-passing style approach to concurrency, and that had exactly the same issues.

Other commenters have already provided examples for other languages, and it's the same for Rust: async functions are just regular functions that return an impl Future type. As a sync function, you can call a bunch of async functions and return the futures to your caller to handle, or you can block your current thread with the block_on function typically available through a handle (similar to the Io object here) provided by your favorite async runtime [0].

In other words, you don't need such an Io object upfront: You need it when you want to actually drive its execution and get the result. From this perspective, the Zig approach is actually less flexible than Rust.

[0]: https://docs.rs/tokio/latest/tokio/runtime/struct.Handle.htm...

Conceptually, there's not much of a difference.

If you have a sync/non-IO function that now needs to do IO, it becomes async/IO. And since IO and async are viral, it's callers must also now be IO/async and call it with IO/await. All the way up the call stack.

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.

Sure it is a function coloring. Just in a different form. `async` in other languages is something like an implicit parameter. In zig they made this implicit parameter explicit. Is that more better/more ergonomic? I don't know yet. The sugar is different, but the end result the same. Unless you can show me concrete example of things that the approach zig has taken can do that is not possible in say, rust. Than I don't buy that it's not just another form of function coloring.

> Unless you can show me concrete example

add io to a struct and let the struct keep track of its own io.

You can also carry around a runtime and dispatch async function in non-async functions.

It’s more like adding a runtime handle to the struct.

Modulo that I’m not sure any langage with a sync/async split has an “async” runtime built entirely out of sync operations. So a library can’t take a runtime for a caller and get whatever implementation the caller decided to use.

> I’m not sure any langage with a sync/async split has an “async” runtime built entirely out of sync operations.

You get into hairy problems of definition, but you can definitely create an "async" runtime out of "sync" operations: implement an async runtime with calls to C. C doesn't have a concept of "async", and more or less all async runtime end up like this.

I've implemented Future (Rust) on a struct for a Windows operation based only on C calls into the OS. The struct maintains everything needed to know the state of the IO, and while I coupled the impl to the runtime for efficiency (I've written it too), it's not strictly necessary from memory.

> You get into hairy problems of definition, but you can definitely create an "async" runtime out of "sync" operations: implement an async runtime with calls to C. C doesn't have a concept of "async", and more or less all async runtime end up like this.

While C doesn't have async OS generally provide APIs which are non-blocking, and that is what async runtimes are implemented on top of.

By sync operations I mean implementing an "async" runtime entirely atop blocking operations, without bouncing them through any sort of worker threads or anything.

I've been pondering this, it feels like a brain teaser. Is it possible to create an "async" runtime out of only blocking operations?

It feels like it turns purely on what "blocking operations" are (does setting a lock bit and returning count as non-blocking?)

It's funny, but I do actually like it. It's just that it walks like a duck, swims like a duck and quacks like a duck.

I don't have a problem with IO conceptually (but I do have a problem with Zig ergonomics, allocator included). I do have a problem with claiming you defeated function coloring.

Like, look. You didn't even get rid of await ...

> try a_future.await(io);

I do want to say that I regretted that comment as nonconstructive after it was too late to edit it. Others in the thread are representing my argument better than I can or care to.

I mean... you use `await` if you've used `async`. It's your choice whether or not you do; and if you don't want to, your callers and callees can still freely `async` and `await` if they want to. I don't understand the point you're trying to make here.

To be clear, where many languages require you to write `const x = await foo()` every time you want to call an async function, in Zig that's just `const x = foo()`. This is a key part of the colorless design; you can't be required to acknowledge that a function is async in order to use it. You'll only use `await` if you first use `async` to explicitly say "I want to run this asynchronously with other code here if possible". If you need the result immediately, that's just a function call. Either way, your caller can make its own choice to call you or other functions as `async`, or not to; as can your callees.

> in Zig that's just ...

Well, no. In zig that's `const x = foo(io)`.

The moment you take or even know about an io, your function is automatically "generic" over the IO interface.

Using stackless coroutines and green threads results in a completely different codegen.

I just noticed this part of the article:

> Stackless Coroutines > > This implementation 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. > > This execution model is compatible with WASM and other platforms where stack swapping is not available or desireable.

I wonder what will happen if you try to await a future created with a green thread IO using a stackless coroutine IO.

> Well, no. In zig that's `const x = foo(io)`.

If `foo` needs to do IO, sure. Or, more typically (as I mentioned in a different comment), it's something like `const x = something.foo()`, and `foo` can get its `Io` instance from `something` (in the Zig compiler this would be a `Compilation` or a `Zcu` or a `Sema` or something like that).

> Using stackless coroutines and green threads results in a completely different codegen.

Sure, but that's abstracted away from you. To be clear, stackless coroutines are the only case where the codegen of callers is affected, which is why they require a language feature. Even if your application uses two `Io` implementations for some reason, one of which is based on stackless coroutines, functions using the API are not duplicated.

> I wonder what will happen if you try to await a future created with a green thread IO using a stackless coroutine IO.

Mixing futures from any two different `Io` implementations will typically result in Illegal Behavior -- just like passing a pointer allocated with one `Allocator` into the `free` of a different `Allocator` does. This really isn't a problem. Even with allocators, it's pretty rare for people to mess this up, and with allocators you often do have multiple of them available in one place (e.g. a gpa and an arena). In contrast, it will be extraordinarily rare to have more than one `Io` lying around. Even if you do mess it up, the IB will probably just trip a safety check, so it shouldn't take you too long to realise what you've done.

I find these two statements to be contradictory

> Sure, but that's abstracted away from you

> Mixing futures from any two different `Io` implementations will typically result in Illegal Behavior

Thinking about it more, you've possibly added even more colors. Each executor adds a different color and while each function is color-agnostic (but not colorless) futures aren't.

> it will be extraordinarily rare to have more than one `Io`

Will it? I can immediately think of a use case where a program might want to block for files on disk, but defer fetching from network to some background async executor.

but that's not even the case, because it's certainly possible to write a function that receives an object that holds onto an io (and uses it in its vtable calls) that equally well receives an object that doesn't have anything to do with io [0]. The consumers of those objects don't have to care, so there's no coloring.

[0] and this isn't even really a theoretical matter, having colorblind object passing is extremely useful for say, mocking. Oh, I have a database lookup/remote API call, which obviously requires io, but i want fast tests and I can mock it with an object with preseeded values/expects -- hey, that doesn't require IO.

I think in practice the caller still needs to know.

If I call `a.foo()` but `a` has and is using a stackless coroutine IO but the caller is being executed from a green thread IO then as was said before, I'm hitting UB.

But, I do like that you could skip/mock IO for instance. That's pretty neat.

here is example code. you wont "use the wrong io".

    const VTable = struct {
      f: &fn (*VTable) void,
    };

    const A = struct {
      io: IO,
      v: VTable = .{ .f = &A.uses_io },
      fn uses_io(this: *VTable) void {
        const self: *A = @fieldParentPtr(.v, this);
        self.io.some_io_fn(...);
      }
    };

    const B = struct{v: VTable = .{.f = &void_fn}};
    fn void_fn(_: *VTable) void {}

    pub fn calls_vtable(v: VTable) {
      v.f()
    }

> 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 can call an async function from a function that is not async by passing in a global runtime (/ event loop).

As a trivial example the main entry point in rust is never async. How’d you suppose you’d bootstrap the runtime that you’d eventually need.

This is pretty much like every other langage.

and yet... there are libraries that were written twice, once for async and once for not.

Which should be a pretty big hint that you've misidentified the issue.

People in software really do have poor abilities to see the forest for the trees don't they lol

>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?

How will that work with code mixing different Io implementations? Say a library pulled in uses a global Io instance while the calling code is using another.

I guess this can just be shot down with "don't do that" but it feels like a new kind of pitfall get get into.

> Say a library pulled in uses a global Io instance while the calling code is using another.

it'll probably carry a stigma like using unsafe does.

Zig already has an Allocator interface that gets passed around, and the convention is that libraries don't select an Allocator. Only provide APIs that accept allocators. If there's a certain process that works best with an Arena, then the API may wrap a provided function in an Arena, but not decide on their own underlying allocator for the user.

For Zig users, adopting this same mindset for Io is not really anything new. It's just another parameter that occasionally needs to be passed into an API.

while not really idiomatic, as long as you let the user define the Io instance (eg with some kind of init function), then it doesn't really matter how that value is accessed within the library itself.

that's why this isn't really the same as async "coloring"