[hardwaresofton]

See also: monads and in particular the Free(r) monad, and effects systems[0].

The idea of separating logic from execution is a whole thing, well trodden by the Haskell ecosystem.

[EDIT] Also, they didn't mention how they encapsulated the `tokio::select!` call that shows up when they need to do time-related things -- are they just carrying around a `tokio::Runtime` that they use to make the loop code async without requiring the outside code to be async?

[EDIT2] Maybe they weren't trying to show an encapsulated library doing that, but rather to show that the outside application can use the binding in an async context...

I would have been more interested in seeing how they could implement an encapsulated function in the sans-IO style that had to do something like wait on an action or a timer -- or maybe the answer they're expecting there is just busy-waiting, or carrying your own async runtime instance (that can essentially do the busy waiting for you, with something like block_in_place.

[0]: https://okmij.org/ftp/Computation/free-monad.html

[wh33zle]

> I would have been more interested in seeing how they could implement an encapsulated function in the sans-IO style that had to do something like wait on an action or a timer

The "encapsulated function" is the `StunBinding` struct. It represents the functionality of a STUN binding. It isn't a single function you can just call, instead it requires an eventloop.

The point though is, that `StunBinding` could live in a library and you would be able to use it in your application by composing it into your program's state machine (assuming you are also structuring it in a sans-IO style).

The linked `snownet` library does exactly this. Its domain is to combine ICE + WireGuard (without doing IO) which is then used by the `connlib` library that composes ACLs on top of it.

Does that make sense?

EDIT: There is no busy-waiting. Instead, `StunBinding` has a function that exposes, what it is waiting for using `poll_timeout`. How the caller (i.e. eventloop) makes that happen is up to them. The appropriate action will happen once `handle_timeout` gets called with the corresponding `Instant`.

[hardwaresofton]

> The "encapsulated function" is the `StunBinding` struct. It represents the functionality of a STUN binding. It isn't a single function you can just call, instead it requires an eventloop. > > The point though is, that `StunBinding` could live in a library and you would be able to use it in your application by composing it into your program's state machine (assuming you are also structuring it in a sans-IO style).

What I was thinking was that the functionality being executed in main could just as easily be in a library function -- that's what I meant by encapsulated function, maybe I should have said "encapsulated functionality".

If the thing I want to do is the incredibly common read or timeout pattern, how do I do that in a sans-IO way? This is why I was quite surprised to see the inclusion of tokio::select -- that's not very sans IO, but is absolutely the domain of a random library function that you might want to expose.

It's a bit jarring to introduce the concept as not requiring choices like async vs not, then immediately require the use of async in the event loop (required to drive the state machine to completion).

Or is the point that the event loop should be async? That's a reasonable expectation for event loops that are I/O bound -- it's the whole point of a event loop/reactor pattern. Maybe I'm missing some example where you show an event loop that is not async, to show that you can drive this no matter whether you want or don't want async?

So if I to try to condense & rephrase:

If I want to write a function that listens or times out in sans-IO style, should I use tokio::select? If so, where is the async runtime coming from, and how will the caller of the function be able to avoid caring?

[wh33zle]

> If I want to write a function that listens or times out in sans-IO style, should I use tokio::select? If so, where is the async runtime coming from, and how will the caller of the function be able to avoid caring?

To "time-out" in sans-IO style means that your state machine has an `Instant` internally and, once called at a specific point in the future, compares the provided `now` parameter with the internal timeout and changes its state accordingly. See [0] for an example.

> but is absolutely the domain of a random library function that you might want to expose.

That entire `main` function is _not_ what you would expose as a library. The event loop should always live as high up in the stack as possible, thereby deferring the use of blocking or non-blocking IO and allowing composition with other sans-IO components.

You can absolutely write an event loop without async. You can set the read-timeout of the socket to the value of `poll_timeout() - Instant::now` and call `handle_timeout` in case your `UdpSocket::recv` call errors with a timeout. str0m has an example [1] like that in their repository.

> It's a bit jarring to introduce the concept as not requiring choices like async vs not, then immediately require the use of async in the event loop (required to drive the state machine to completion).

All the event loops you see in the post are solely there to ensure we have a working program but are otherwise irrelevant, esp. implementation details like using `tokio::select` and the like. Perhaps I should have made that clearer.

[0]: https://github.com/firezone/firezone/blob/1e7d3a40d213c9524a... [1]: https://github.com/algesten/str0m/blob/5b100e8a675cd8838cdd8...

[hardwaresofton]

> To "time-out" in sans-IO style means that your state machine has an `Instant` internally and, once called at a specific point in the future, compares the provided `now` parameter with the internal timeout and changes its state accordingly. See [0] for an example.

This part of the post was clear -- I didn't ask any clarifications about that, my point was about what I see as "read or timeout", a reasonable functionality to expose as a external facing function.

The question is still "If I want to read or timeout, from inside a function I expose in a library that uses sans-IO style, how do I do that?".

It seems like the answer is "if you want to accomplish read or timeout at the library function level, you either busy wait or pull in an async runtime, but whatever calls your state machine has to take care of that at a higher level".

You see how this doesn't really work for me? Now I have to decide if my read_or_timeout() function exposed is either the default sync (and I have to figure out how long to wait, etc), or async.

It seems in sans-IO style read_or_timeout() would be sync, and do the necessary synchronous waiting internally, without the benefit of being able to run other tasks from unrelated state machines in the meantime.

> That entire `main` function is _not_ what you would expose as a library.

Disagree -- it's entirely reasonable to expose "read your public IP via STUN" as a library function. I think we can agree to disagree here.

> The event loop should always live as high up in the stack as possible, thereby deferring the use of blocking or non-blocking IO and allowing composition with other sans-IO components.

Sure... but that means the code you showed me should never be made into a library (we can agree to disagree there), and I think it's reasonable functionality for a library...

What am I missing here? From unrelated code, I want to call `get_ip_via_stun_or_timeout(hostnames: &[String], timeout: Duration) -> Option<String>`, is what I'm missing that I need to wrap this state machine in another to pass it up to the level above? That I need to essentially move the who-must-implement-the-event-loop one level up?

> You can absolutely write an event loop without async. You can set the read-timeout of the socket to the value of `poll_timeout() - Instant::now` and call `handle_timeout` in case your `UdpSocket::recv` call errors with a timeout. str0m has an example [1] like that in their repository.

Didn't say you couldn't!

What you've described is looping with a operation-supported timeout, which requires timeout integration at the function call level below you to return control. I get that this is a potential solution (I mentioned it in my edits on the first comment), but not mentioning it in the article was surprising to me.

The code I was expecting to find in that example is like the bit in strom:

https://github.com/algesten/str0m/blob/5b100e8a675cd8838cdd8...

Clearly (IMO evidenced by the article using this method), the most ergonomic way to do that is with a tokio::select, and that's what I would reach for as well -- but I thought a major point was to do it sans IO (where "IO" here basically means "async runtime").

Want to note again, this is not to do with the state machine (it's clear how you would use a passed in Instant to short circuit), but more about the implications of abstracting the use of the state machine.

> All the event loops you see in the post are solely there to ensure we have a working program but are otherwise irrelevant, esp. implementation details like using `tokio::select` and the like. Perhaps I should have made that clearer.

I personally think it exposes a downside of this method -- while I'm not a fan of simply opting in to either async (and whichever runtime smol/tokio/async-std/etc) or sync, what it seems like this pattern will force me to:

- Write all code as sync - Write sync code that does waiting based on operations that yielding back control early - Hold my own tokio runtime so I can do concurrent things (this, you argue against)

Async certainly can be hard to use and have many footguns, but this approach is certainly not free either.

At this point if I think I want to write a library that supports both sync and async use cases it feels like feature flags & separate implementations might produce an easier to understand outcome for me -- the sync version can even start as mostly `tokio::Runtime::block_on`s, and graduate to a more performant version with better custom-tailored efficiency (i.e. busy waiting).

Of course, I'm not disparaging the type state pattern here/using state machines -- just that I'd probably just use that from inside an async/sync-gated modules (and be able to share that code between two impls).

[wh33zle]

> What am I missing here? From unrelated code, I want to call `get_ip_via_stun_or_timeout(hostnames: &[String], timeout: Duration) -> Option<String>`, is what I'm missing that I need to wrap this state machine in another to pass it up to the level above? That I need to essentially move the who-must-implement-the-event-loop one level up?

Essentially yes! For such a simple example as STUN, it may appear silly because the code that is abstracted away in a state machine is almost shorter than the event loop itself.

That very quickly changes as the complexity of your protocol increases though. The event loop is always roughly the same size yet the protocol can be almost arbitrarily nested and still reduces down to an API of `handle/poll_timeout`, `handle_input` & `handle_transmit`.

For example, we've been considering adding a QUIC stack next to the WireGuard tunnels as a control protocol in `snownet`. By using a sans-IO QUIC implementation like quinn, I can do that entirely as an implementation detail because it just slots into the existing state machine, next to ICE & WireGuard.

> At this point if I think I want to write a library that supports both sync and async use cases it feels like feature flags & separate implementations might produce an easier to understand outcome for me -- the sync version can even start as mostly `tokio::Runtime::block_on`s, and graduate to a more performant version with better custom-tailored efficiency (i.e. busy waiting).

> Of course, I'm not disparaging the type state pattern here/using state machines -- just that I'd probably just use that from inside an async/sync-gated modules (and be able to share that code between two impls).

This is what quinn does: It uses tokio + async to expose an API that uses `AsyncRead` and `AsyncWrite` and thus fully buys into the async ecosystem. The actual protocol implementation however - quinn-proto - is sans-IO.

The way I see this is that you can always build more convenience layers, whether or not they are in the same crate or not doesn't really matter for that. The key thing is that they should be optional. The problems of function colouring only exist if you don't focus on building the right thing: an IO-free implementation of your protocol. The protocol implementation is usually the hard bit, the one that needs to be correct and well-tested. Integration with blocking or non-blocking IO is just plumbing work that isn't difficult to write.

[hardwaresofton]

Ahh thanks for clarifying this! Makes a ton of sense now -- I need to try writing some of these style of programs (in the high perf Rust style) to see how they feel.

> For example, we've been considering adding a QUIC stack next to the WireGuard tunnels as a control protocol in `snownet`. By using a sans-IO QUIC implementation like quinn, I can do that entirely as an implementation detail because it just slots into the existing state machine, next to ICE & WireGuard.

Have you found that this introduces a learning curve for new contributors? Being able to easily stand up another transport is pretty important, and I feel like I can whip together an async-required interface for a new protocol very easily (given I did a decent job with the required Traits and used the typestate pattern) where as sans-IO might be harder to reason about.

Thanks for pointing out quinn-proto (numerous times at this point) as well -- I'll take a look at the codebase and see what I can learn from it (as well as str0m).

[EDIT]

> The problems of function colouring only exist if you don't focus on building the right thing: an IO-free implementation of your protocol. The protocol implementation is usually the hard bit, the one that needs to be correct and well-tested.

The post, in a couple lines!

[EDIT2] Any good recommendations of a tiny protocol that might be a good walk through intro to this?

Something even simpler than Gopher or SMTP? Would be nice to have a really small thing to do a tiny project in.