From df6b40c980f5d0e7d000ce43e69f212e6149df5b Mon Sep 17 00:00:00 2001
From: "L. S. Leary" <LSLeary@users.noreply.github.com>
Date: Thu, 25 Aug 2022 19:37:06 +1200
Subject: [PATCH] flake.nix:   * Avoid mangling unused parts of pkgs.haskell in
 `fromHOL`.   * Create 'defComp', specifying the default compiler,
 customisable     through ./comp.nix.   * Include `modernise` module as a
 workaround for NIX_GHC -> XMONAD_GHC. .gitignore:   * Include nix 'result'
 symlink.

---
 .gitignore |  3 +++
 flake.nix  | 37 +++++++++++++++++++++++++++++++------
 2 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/.gitignore b/.gitignore
index ec7ede5..bf13c11 100644
--- a/.gitignore
+++ b/.gitignore
@@ -27,3 +27,6 @@ tags
 /cabal.sandbox.config
 /dist-newstyle/
 /dist/
+
+# nix artifacts
+result
diff --git a/flake.nix b/flake.nix
index fa8a0e1..bb88faf 100644
--- a/flake.nix
+++ b/flake.nix
@@ -15,17 +15,26 @@
        else [ "haskell" "packages" compiler ]
       );
     fromHOL = hol: comp: final: prev: with prev.lib; with attrsets;
-      setAttrByPath (hpath comp)
-        ((getAttrFromPath (hpath comp) prev).override (old: {
+      let
+        path   = hpath comp;
+        root   = head path;
+        branch = tail path;
+        hpkgs' = (getAttrFromPath path prev).override (old: {
           overrides = composeExtensions (old.overrides or (_: _: {}))
             (hol final prev);
-        }));
+        });
+      in {
+        ${root} = recursiveUpdate prev.${root} (setAttrByPath branch hpkgs');
+      };
     hoverlay = final: prev: hself: hsuper:
       with prev.haskell.lib.compose; {
         xmonad = hself.callCabal2nix "xmonad"
           (git-ignore-nix.lib.gitignoreSource ./.) { };
       };
-    overlay = fromHOL hoverlay { };
+    defComp = if builtins.pathExists ./comp.nix
+      then import ./comp.nix
+      else { };
+    overlay = fromHOL hoverlay defComp;
     overlays = [ overlay ];
     nixosModule = { config, pkgs, lib, ... }: with lib; with attrsets;
       let
@@ -64,12 +73,28 @@
     nixosModules = [ nixosModule ];
   in flake-utils.lib.eachDefaultSystem (system:
   let pkgs = import nixpkgs { inherit system overlays; };
+      hpkg = pkgs.lib.attrsets.getAttrFromPath (hpath defComp) pkgs;
   in
   rec {
-    devShell = pkgs.haskellPackages.shellFor {
+    devShell = hpkg.shellFor {
       packages = p: [ p.xmonad ];
     };
-    defaultPackage = pkgs.haskellPackages.xmonad;
+    defaultPackage = hpkg.xmonad;
+    # An auxiliary NixOS module that modernises the standard xmonad NixOS module
+    # and wrapper script used, replacing them with versions from unstable.
+    # Currently, due to the NIX_GHC --> XMONAD_GHC env var change, this is
+    # necessary in order for Mod-q recompilation to work out-of-the-box.
+    modernise =
+      let
+        xmonadModFile = "services/x11/window-managers/xmonad.nix";
+        unpkgs = import unstable { inherit system; };
+        replaceWrapper = _: _:
+          { xmonad-with-packages = unpkgs.xmonad-with-packages; };
+      in {
+        disabledModules = [ xmonadModFile ];
+        imports = [ (unstable + "/nixos/modules/" + xmonadModFile) ];
+        nixpkgs.overlays = [ replaceWrapper ];
+      };
   }) // {
     inherit hoverlay overlay overlays nixosModule nixosModules;
     lib = { inherit hpath fromHOL; };