Use nixos-unified to scaffold toplevel flake

flake.nix

@@ -2,23 +2,16 @@
   description = "Nix template for Haskell projects";
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
-    systems.url = "github:nix-systems/default";
     flake-parts.url = "github:hercules-ci/flake-parts";
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
+    nixos-unified.url = "github:srid/nixos-unified/mkFlake";
     haskell-flake.url = "github:srid/haskell-flake";
     treefmt-nix.url = "github:numtide/treefmt-nix";
     treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
     fourmolu-nix.url = "github:jedimahdi/fourmolu-nix";
   };
 
   outputs = inputs:
-    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
-      systems = import inputs.systems;
-
-      # See ./nix/modules/*.nix for the modules that are imported here.
-      imports = with builtins;
-        map
-          (fn: ./nix/modules/${fn})
-          (attrNames (readDir ./nix/modules));
-    };
+    inputs.nixos-unified.lib.mkFlake
+      { inherit inputs; root = ./.; };
 }

nix/modules/haskell.nix → nix/modules/flake-parts/haskell.nix

@@ -1,4 +1,4 @@
-{ inputs, ... }:
+{ root, inputs, ... }:
 {
   imports = [
     inputs.haskell-flake.flakeModule
@@ -11,12 +11,12 @@
       # To avoid unnecessary rebuilds, we filter projectRoot:
       # https://community.flake.parts/haskell-flake/local#rebuild
       projectRoot = builtins.toString (lib.fileset.toSource {
-        root = ../..;
+        inherit root;
         fileset = lib.fileset.unions [
-          ../../src
-          ../../haskell-template.cabal
-          ../../LICENSE
-          ../../README.md
+          (root + /src)
+          (root + /haskell-template.cabal)
+          (root + /LICENSE)
+          (root + /README.md)
         ];
       });