Make the output channel generic

[vincent-herlemont]

Currently, event listening via the watch command does not support generic channel management. The user is forced to choose between the tokio feature or the standard Rust channel (see: src/watch/mod.rs#L38-L46). The purpose of this issue is to investigate whether it is possible to elegantly support any type of channel.

TODO

• Investigate

[lu-zero]

The less than elegant solution is to rely on flume or another crate supporting both sync and async at the same time. If you think it is acceptable I can provide a PR

[vincent-herlemont]

@lu-zero this could save a feature. However, the goal here is to find a way to be compatible with all types of channels to avoid using multiple types of channels in a crate.

[lu-zero]

There isn't a common trait across all the channel implementation so there isn't a way even to support alternative implementations of sync channels like flume vs crossbeam vs std or async channels such as flume vs tokio vs async-channel short of providing the trait, assume that there is a sender and take a generic over it (and still have to differentiate between sync and async).

[vincent-herlemont]

Yes, you are right, there won't be any generics in terms of Rust declarations. However, we could instead find a way to make it "generic" by using a Fn (that takes the event as an argument), which will be defined by the users, as a parameter and is called every time an event is sent. In this case, users will be able to use these events as they wish, with channels or something else if they want.

[lu-zero]

Storing Fn requires a generic and tends to be annoying in my experience though...

[vincent-herlemont]

Storing Fn requires a generic

@lu-zero How does this imply a generic declaration? Here's what I imagined, what's your point of view?

// /!\ Pseudo code
let (sender, receiver) = mpsc::channel();
db.watch(move |event| {
    // Send the event message into the channel
   sender.send(event);
});

As you can see, you can put whatever you want in the closure. This does not imply a generic.

[lu-zero]

Fn/FnMut are traits and closures are always unique.

[vincent-herlemont]

@lu-zero Can you provide a detailed example of a situation where this would pose a problem?

[lu-zero]

It is easier to write down the signature of your pseudocode I guess :)

// `clo` is generic over anything implementing `FnMut`
fn watch(&mut self, clo: impl FnMut(EventTy)) {

// and if we want to save it someplace we have to box it
self.closure = Box::new(clo);

...
}  

This should be a way to make it work for sync, for async it gets more annoying since we do not have yet async closures.

[vincent-herlemont]

@lu-zero Yes, that's exactly the idea! And indeed, for async, we'll have to wait a bit ;)

BTW If you need support for flume or something else in the meantime, you can add it to the two existing channel supports, and I will merge it.

[lu-zero]

I do not need it myself, but since it is just few minutes to implement it using flume, it could solve the use-case of keeping the same API across features.