Are you a user? Check out Xest's website at https://jhgarner.github.io/Xest-Window-Manager/
Are you a developer? Keep reading!
Xest is a window manager written in Haskell. The goal of this codebase is to be at least partially accessible to other Haskell users who are familiar with some language extensions. Since I've been the only contributor though, there is almost no way that goal has been met. I used this codebase as a testing ground for new practices which means a lot of the code will be filled with strange patterns and obtuse logic. If you're trying to understand the code and something doesn't make sense, let me know or open up an issue.
I'm always happy to answer questions about Xest no matter how silly they may seem. Feel free to open up issues, join the Matrix chat, or check out my profile for how to contact me if you have any questions. I'm also always happy to hear what ideas you might have about making the code more readable, robust, or performent.
If you've never seen the Haskell language before, jumping into this codebase is probably a bad idea. If you have a little bit of experience, I would recommend taking a look at some of the files in the src/Tiler/ folder. All of the functions in there are "pure" in that they only modify internal data structures and don't require knowing much about Xlib.
The weirdest things you will find in those files are the uses of "Fix" and "cata". I would highly recommend reading https://blog.sumtypeofway.com/posts/introduction-to-recursion-schemes.html if you haven't seen them before. That blog series fundamentally changed how I thought about recursion and ADTs. I likely would have given up on this project if I hadn't found that way of organizing the code.
If you want to look at the other parts of the codebase, you'll need a basic understanding of what the X11 protocol is. I would recommend https://magcius.github.io/xplain/article/ and https://jichu4n.com/posts/how-x-window-managers-work-and-how-to-write-one-part-ii/ as good descriptions of X11 and how window managers work. The Haskell X11 library is pretty close to the original xlib so any docs related to xlib will probably help as well. I found myself using the Tronche website a lot for xlib documentation as well as the XMonad source code.
ICCCM and EWMH are protocols built on top of X11 and tend to get sprinkled around the src/XEvents.hs code. The official documentations for those protocols are probably the best resources for learning about them.
Xest currently uses freer-simple as its effect system. If you've never seen
free monad based effects before then it might look a little weird. Luckily, a
lot of people have written about free monads and freer-simple so there should
be good tutorials online for it. You can avoid most of the gross effect code by
staying out of the base/DoAll.hs file. You'll see stuff like
Members '[Thing, Thing2, etc] m in a few different files. Basically, that lets
you operate in a monad that has the capabilities defined in the list.
X11 is a bit of a mess and Wayland is almost certainly better. I chose not to make a Wayland compositor because the Haskell tooling is extremely immature. This was my first foray into Linux window management and having to deal with all sorts of new technologies and lacking documentation would have been awful.
The other reason I didn't pick Wayland is that I expect my window manager to be fairly flashy. On X11 I can use Compton to get really nice blurring, fading, vsync, and shadows. On Wayland I would have to write all of that code myself. Since I have Nvidia cards in my devices, I also have to deal with all of the problems they've created.
For those reasons, Wayland seemed like too much to tackle. With that said, I am happy to work on making Xest more backend agnostic and adopting Wayland when it looks like there's a path forward for the tooling on Haskell. If someone knows that path, please share it with me.