Clean up the Nix flake, recommended practices, provide overlay (#6)

* flake: update description

* flake: add alejandra as formatter

* flake: format nix code

* flake: make systems overrideable

* flake: replace forAllSystems with eachSystem

* flake: inherit lib at top level

* flake: move usages of with to smallest scopes

* flake: introduce overlay

* flake: package: remove needless unpackPase override

* flake: devShell: replace buildInputs with packages

* chore: move alejandra to pre-commit-hook

* chore: inherit from nixpkgs.lib not nixpkgs

* chore: remove some lets add others

* chore: change mkShell back to buildInputs

---------

Co-authored-by: Daylin Morgan <daylinmorgan@gmail.com>

.pre-commit-config.yaml

@@ -12,3 +12,8 @@ repos:
       - id: ruff-format
       - id: ruff
         args: [ --fix ]
+  - repo: https://github.com/kamadorueda/alejandra
+    rev: 3.0.0
+    hooks:
+       # Requires Alejandra to be previously installed in the system
+      - id: alejandra-system

flake.lock

@@ -18,7 +18,23 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "systems": "systems"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
       }
     }
   },

flake.nix

@@ -1,29 +1,35 @@
 {
-  description = "brain";
-  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  description = "A script to patch the MonoLisa font with Nerd Fonts glyphs.";
 
-  outputs = inputs @ {
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    systems.url = "github:nix-systems/default";
+  };
+
+  outputs = {
     self,
     nixpkgs,
+    systems,
   }: let
-    inherit (nixpkgs.lib) genAttrs;
-    forAllSystems = f:
-      genAttrs
-      ["x86_64-linux" "x86_64-darwin" "aarch64-linux" "aarch64-darwin"]
-      (system: f nixpkgs.legacyPackages.${system});
+    inherit (nixpkgs.lib) genAttrs makeBinPath;
+    eachSystem = fn:
+      genAttrs (import systems)
+      (system:
+        fn system
+        (import nixpkgs {
+          localSystem.system = system;
+          overlays = [self.overlays.default];
+        }));
   in {
-    packages = forAllSystems (
-      pkgs:
-      with pkgs; {
-        default = stdenv.mkDerivation {
+    overlays = {
+      default = final: _prev: let
+        pkgs = final;
+      in {
+        monolisa-nerdfont-patch = pkgs.stdenv.mkDerivation {
           name = "monolisa-nerdfont-patch";
           src = ./.;
-        nativeBuildInputs = [ makeWrapper ];
-        buildInputs = [
-          fontforge
-          python3
-        ];
-        unpackPhase = ":";
+          nativeBuildInputs = with pkgs; [makeWrapper];
+          buildInputs = with pkgs; [fontforge python3];
           buildPhase = ":";
           installPhase = ''
             mkdir -p $out/bin
@@ -34,25 +40,21 @@
           '';
           postFixup = ''
             wrapProgram $out/bin/monolisa-nerdfont-patch \
-            --set PATH ${lib.makeBinPath [
-              fontforge
-            ]}
+              --set PATH ${makeBinPath (with final; [fontforge])}
           '';
         };
-    }
-      );
-
-    devShells = forAllSystems (
-      pkgs:
-        with pkgs; {
-          default = mkShell {
-            buildInputs = [
-              fontforge
-              python3
-              pre-commit
-            ];
       };
-        }
-    );
+    };
+
+    packages = eachSystem (system: pkgs: {
+      default = self.packages.${system}.monolisa-nerdfont-patch;
+      monolisa-nerdfont-patch = pkgs.monolisa-nerdfont-patch;
+    });
+
+    devShells = eachSystem (_: pkgs: {
+      default = pkgs.mkShell {
+        buildInputs = with pkgs; [fontforge python3 pre-commit];
+      };
+    });
   };
 }