Add a flake.nix file

flake.nix

@@ -0,0 +1,26 @@
+{
+  inputs = {
+    flake-utils.url = github:numtide/flake-utils;
+    git-ignore-nix.url = github:IvanMalison/gitignore.nix/master;
+  };
+  outputs = { self, flake-utils, nixpkgs, git-ignore-nix }:
+  let
+    overlay = final: prev: {
+      haskellPackages = prev.haskellPackages.override (old: {
+        overrides = prev.lib.composeExtensions (old.overrides or (_: _: {}))
+        (hself: hsuper: {
+          xmonad = hself.callCabal2nix "xmonad" (git-ignore-nix.gitIgnoreSource ./.) { };
+        });
+      });
+    };
+    overlays = [ overlay ];
+  in flake-utils.lib.eachDefaultSystem (system:
+  let pkgs = import nixpkgs { inherit system overlays; };
+  in
+  rec {
+    devShell = pkgs.haskellPackages.shellFor {
+      packages = p: [ p.xmonad ];
+    };
+    defaultPackage = pkgs.haskellPackages.xmonad;
+  }) // { inherit overlay overlays; } ;
+}