xmonad: add libFiles option and build type compilation

The `libFiles` option allows Home Manager to manage additional files
for xmonad.

Also compile xmonad during configuration build time. This avoids the
need to compile the configuration during activation.

modules/services/window-managers/xmonad.nix

@@ -75,27 +75,86 @@ in {
           by Home Manager.
         '';
       };
+
+      libFiles = mkOption {
+        type = types.attrsOf (types.oneOf [ types.path ]);
+        default = { };
+        example = literalExample ''
+          {
+            "Tools.hs" = pkgs.writeText "Tools.hs" '''
+               module Tools where
+               screenshot = "scrot"
+             ''';
+          }
+        '';
+        description = ''
+          Additional files that will be saved in
+          <filename>~/.xmonad/lib/</filename> and included in the configuration
+          build. The keys are the file names while the values are paths to the
+          contents of the files.
+        '';
+      };
     };
   };
 
-  config = mkIf cfg.enable (mkMerge [
+  config = let
+
+    xmonadBin = "${
+        pkgs.runCommandLocal "xmonad-compile" {
+          nativeBuildInputs = [ xmonad ];
+        } ''
+          mkdir -p $out/bin
+
+          export XMONAD_CONFIG_DIR="$(pwd)/xmonad-config"
+          export XMONAD_DATA_DIR="$(pwd)/data"
+          export XMONAD_CACHE_DIR="$(pwd)/cache"
+
+          mkdir -p "$XMONAD_CONFIG_DIR/lib" "$XMONAD_CACHE_DIR" "$XMONAD_DATA_DIR"
+
+          cp ${cfg.config} xmonad-config/xmonad.hs
+
+          declare -A libFiles
+          libFiles=(${
+            concatStringsSep " "
+            (mapAttrsToList (name: value: "['${name}']='${value}'")
+              cfg.libFiles)
+          })
+          for key in "''${!libFiles[@]}"; do
+            cp "''${libFiles[$key]}" "xmonad-config/lib/$key";
+          done
+
+          xmonad --recompile
+
+          # The resulting binary name depends on the arch and os
+          # https://github.com/xmonad/xmonad/blob/56b0f850bc35200ec23f05c079eca8b0a1f90305/src/XMonad/Core.hs#L565-L572
+          mv "$XMONAD_DATA_DIR/xmonad-${pkgs.hostPlatform.system}" $out/bin/
+        ''
+      }/bin/xmonad-${pkgs.hostPlatform.system}";
+
+  in mkIf cfg.enable (mkMerge [
     {
       home.packages = [ (lowPrio xmonad) ];
-      xsession.windowManager.command = "${xmonad}/bin/xmonad";
+      xsession.windowManager.command = xmonadBin;
     }
 
     (mkIf (cfg.config != null) {
       home.file.".xmonad/xmonad.hs".source = cfg.config;
-      home.file.".xmonad/xmonad.hs".onChange = ''
+      home.file.".xmonad/xmonad-${pkgs.hostPlatform.system}" = {
-        echo "Recompiling xmonad"
+        source = xmonadBin;
-        $DRY_RUN_CMD ${config.xsession.windowManager.command} --recompile
+        onChange = ''
-
           # Attempt to restart xmonad if X is running.
           if [[ -v DISPLAY ]] ; then
             echo "Restarting xmonad"
             $DRY_RUN_CMD ${config.xsession.windowManager.command} --restart
           fi
         '';
+      };
     })
+
+    {
+      home.file = mapAttrs' (name: value:
+        attrsets.nameValuePair (".xmonad/lib/" + name) { source = value; })
+        cfg.libFiles;
+    }
   ]);
 }