A perfect pet project to learn Rust

I have been willing to learn Rust for a while. I wanted to understand if all the hype around this language was deserved, how much of the supposedly Haskell influence is actually present and if it is as difficult as people say. But I didn’t have any project at hand to try it out.

For me the best way to learn a new language is to actually build something and to make the experience even better it is somehow of important that it solves a concrete problem that I have.

Recently I found the perfect pet project to learn Rust.

Context

A bit of context first, I have been happy LMS user for a while now. LMS stands for logitech media server. It is an open source music server first developed by Logitech along great sounding hardware called Squeezeboxes. They have done different models such as SqueezeBox Radio or SqueezeBox Touch to name the one I own. At some point, Logitech lost interest in these audio devices and abandoned the project.

But it has been taking over by the open source community. It still works fine on many platforms including the Raspberry PI and it has regularly new features implemented. The community also developed a bunch of great third party software such as an Android and IPhone apps. There is also a software player called squeezelite.

This is the software I use to listen to my music library from my desktop computer. It works perfectly fine but it is missing an important feature for me: an MPRIS interface.

What is MPRIS ?

MPRIS is a DBUS interface specification to interact with media players. For my use case, I use playerctl, an MPRIS client, to control the audio players running on my system:

  • to show the current track being played
  • to toggle the playing status between play and pause
  • to move to the next or previous track

I put that into a polybar module using a super simple script. That way, on whatever desktop I am, I can see what is currently playing and change the track if I feel like it.

I find it super useful and the cool thing is that it works whatever the media player being used: VLC, chrome and so on. So I really need squeezelite to have it as well.

I first looked at adding that interface to squeezelite itself. I quickly had a POC working but in order to get the track currently played, I’d need to talk to the server over a socket using either the HTTP or the CLI API. This is not particularly difficult, but not particularly exciting either. I know this kind of stuff can be tricky to get right in C (squeezelite is written in C) and I don’t have that much time to spend on it. See where it is going: it would be cool to implement that project in a higher level and safer language.

So let’s do this in Rust.

The project

Here is the project: instead of adding an MPRIS interface to squeezelite I could add squeezelite to an MPRIS interface. That’s what mprisqueeze does. It starts squeezelite in the background while exposing an MPRIS interface to control it. The commands are sent to LMS using the HTTP API.

This project has the perfect requirements for me to learn a new language:

  • it is something I need, which brings motivation
  • it is not too ambitious I can get something working pretty fast
  • but it’s not trivial either

Here what I need to implement in mprisqueeze:

  • parse the command line arguments
  • start and monitor the squeezelite process in the background
  • implement a DBUS interface
  • discover the LMS server on the network
  • pass the commands to the LMS server using the HTTP API

The implementation was a matter of a couple of weeks working in my free time.

Feedback on Rust

Overall it was a super smooth experience. Rust is very well documented and the tooling, cargo and the language server, are great.

I can see some Haskell influence there but not that much actually. Rust is still a very imperative language, purity or higher-kinded types are nowhere to be found here. I would say this is more a general functional programming influence with the usual features: immutability by default, sum types and pattern matching.

This was not really that difficult for me to get into it. Having worked for years with C++ and all its footguns, I know that copying is costly and I understand the need for move semantics and ownership.

The thing that I really enjoyed is definitely error handling. I like the fact that the errors are returned as values. It makes it easy to follow the compiler and handle errors when appropriate or letting them bubble up in the stack. Unlike exceptions in Haskell, there is no hidden control flow. Also to have one simple, yet ergonomic way to handle errors, consistent across the whole ecosystem is really amazing.

Conclusion

The project is published on crates.io and the sources can be found on github. If you are a LMS user, try it out and let me know how you like it.

Related work

slimpris2 has almost the same goals. It implements an MPRIS interface for one or all players connected to an LMS server. I have found in its code the way to discover the LMS server on the network.

June 1, 2023