LSP for XMonad on NixOS

Configuring XMonad can be a bit of a pain in the ass. Since it's configured in haskell, LSP can be enabled for XMonad configurations. I'll try to explain how I set up LSP for my config.

Prerequisites

This setup uses a shell.nix file, direnv, and an editor with direnv integration (emacs in my case, vscode and (n)vim also have it).

Setup

Firstly, clone the xmonad and xmonad-contrib repositories in your XMonad config directory.

shell.nix

We'll use direnv for this. If you haven't heard of it, please read about it. Setting up direnv is pretty easy, just create your shell.nix or flake.nix file and create the .envrc with the content:

use nix # use flake if you are using `flake.nix` file

This is the shell.nix file:

{ pkgs ? import <nixpkgs> {} }:
with pkgs;
mkShell {
  buildInputs = [
    haskellPackages.cabal-install
    haskellPackages.haskell-language-server
    haskellPackages.hlint
    haskellPackages.ghcid
    haskellPackages.ormolu
    haskellPackages.implicit-hie
    haskellPackages.X11

    pkg-config

    xorg.libX11
    xorg.libX11.dev

    xorg.libXft
    xorg.libXext
    xorg.libXrandr
    xorg.libXrender
    xorg.libXinerama
    xorg.libXScrnSaver
  ];
}

implicit-hie is the important program here. We'll use it to generate hie.yaml file which is used by haskell-language-server.

By this point, our XMonad config directory should look like this:

❯ ls
drwxr-xr-x   - mzanic 27 Nov 22:55 xmonad
drwxr-xr-x   - mzanic 27 Nov 22:56 xmonad-contrib
.rw-r--r--  22 mzanic 27 Nov 22:56 .envrc
.rw-r--r-- 463 mzanic 27 Nov 22:56 shell.nix
.rw-r--r--   0 mzanic 27 Nov 22:54 xmonad.hs

Cabal

After setting up shell.nix and direnv, we need to setup the xmonad-conf.cabal and cabal.project files. We are doing this so that we can use gen-hie to generate the hie.yaml file.

Here is how our xmonad-conf.cabal:

cabal-version:      2.4
name:               xmonad-conf

executable xmonad-conf
    main-is: xmonad.hs
    build-depends:     X11
                     , base
                     , hostname
                     , containers
                     , utf8-string
                     , xmonad
                     , xmonad-contrib

  default-language:    Haskell2010
  default-extensions:  LambdaCase
  ghc-options:         -Wall

and cabal.project files look like:

packages: . */*.cabal

gen-hie

We can generate the hie.yaml file by typing in the command gen-hie > hie.yaml. Here what gen-hie output looks like:

cradle:
  cabal:
    - path: "xmonad/src"
      component: "lib:xmonad"

    - path: "xmonad/./Main.hs"
      component: "xmonad:exe:xmonad"

    - path: "xmonad/tests"
      component: "xmonad:test:properties"

    - path: "xmonad-contrib/./"
      component: "lib:xmonad-contrib"

    - path: "xmonad-contrib/tests"
      component: "xmonad-contrib:test:tests"

    - path: "xmonad-contrib/."
      component: "xmonad-contrib:test:tests"

After generating hie.yaml file, we need to append this to it:

    - path: "./xmonad.hs"
      component: "xmonad-conf:exe:xmonad-conf"

The structure of the component part is:

<name in the .cabal file>:exe:<name of the executable>

in our case, both xmonad-conf.

Now, all that is left is for you to open up the editor and enable lsp. Haskell-language-server should read the config from hie.yaml file.

Run cabal build . if you got an error, it's a good first step for debugging.

You can test the lsp with this xmonad.hs file:

import XMonad
import XMonad.Hooks.DynamicLog
import XMonad.Hooks.ManageDocks
import XMonad.Util.Run(spawnPipe)
import XMonad.Util.EZConfig(additionalKeys)
import System.IO


main = do
    xmonad $ defaultConfig

Here's how my editor with HLS looks like:

Final remarks

I configured the HLS for my xmonad config about 6 months ago, and I tried to do the same for my xmobar config. While doing that, I got stuck and could hardly remember what I did the last time.

So this post is a guide for my future self and for all XMonad users who want LSP and don't know where to start (I was pretty frustrated while configuring HLS for XMonad because of the lack of resources online). Hope this helps!

If you have any questions, contact me at mislavzanic3@gmail.com.