nixos/xmonad: enableConfiguredRecompile

Commit 9a5b5d9fe858f33f7f5ce0870be2b8a38516a1d4 added Haskell
dependencies (GHC and packages) to the xmonad binary's environment even
if xmonad had been preconfigured (via the "config" option). The intent
was to enable one-off recompiling using a local config file (e.g.
~/.config/xmonad/xmonad.hs), so the user can get quick feedback while
developing their config.

While this works, it may not be a common use-case, and it requires some
careful crafting in xmonad.hs itself. On top of that, it significantly
increases the size of the closure.

Given all that, commit b69d9d3c23 removed
GHC and packages from the binary's environment.

But there are still those among us who want to be able to recompile from
a preconfigured xmonad, so let's provide a way to opt-into configured
recompilation.

nixos/modules/services/x11/window-managers/xmonad.nix

@@ -2,7 +2,7 @@
 
 with lib;
 let
-  inherit (lib) mkOption mkIf optionals literalExpression;
+  inherit (lib) mkOption mkIf optionals literalExpression optionalString;
   cfg = config.services.xserver.windowManager.xmonad;
 
   ghcWithPackages = cfg.haskellPackages.ghcWithPackages;
@@ -26,11 +26,14 @@ let
     in
       pkgs.runCommandLocal "xmonad" {
         nativeBuildInputs = [ pkgs.makeWrapper ];
-      } ''
+      } (''
         install -D ${xmonadEnv}/share/man/man1/xmonad.1.gz $out/share/man/man1/xmonad.1.gz
         makeWrapper ${configured}/bin/xmonad $out/bin/xmonad \
+      '' + optionalString cfg.enableConfiguredRecompile ''
+          --set NIX_GHC "${xmonadEnv}/bin/ghc" \
+      '' + ''
           --set XMONAD_XMESSAGE "${pkgs.xorg.xmessage}/bin/xmessage"
-      '';
+      '');
 
   xmonad = if (cfg.config != null) then xmonad-config else xmonad-vanilla;
 in {
@@ -95,12 +98,14 @@ in {
           xmonad from PATH. This allows e.g. switching to the new xmonad binary
           after rebuilding your system with nixos-rebuild.
           For the same reason, ghc is not added to the environment when this
-          option is set.
+          option is set, unless <option>enableConfiguredRecompile</option> is
+          set to <literal>true</literal>.
 
           If you actually want to run xmonad with a config specified here, but
           also be able to recompile and restart it from a copy of that source in
-          $HOME/.xmonad on the fly, you will have to implement that yourself
-          using something like "compileRestart" from the example.
+          $HOME/.xmonad on the fly, set <option>enableConfiguredRecompile</option>
+          to <literal>true</literal> and implement something like "compileRestart"
+          from the example.
           This should allow you to switch at will between the local xmonad and
           the one NixOS puts in your PATH.
         '';
@@ -135,6 +140,16 @@ in {
         '';
       };
 
+      enableConfiguredRecompile = mkOption {
+        default = false;
+        type = lib.types.bool;
+        description = ''
+          Enable recompilation even if <option>config</option> is set to a
+          non-null value. This adds the necessary Haskell dependencies (GHC with
+          packages) to the xmonad binary's environment.
+        '';
+      };
+
       xmonadCliArgs = mkOption {
         default = [];
         type = with lib.types; listOf str;