Focus on stable Nix and use npins for dependencies

- Simplifies the whole thing due to not having to mess with `system`, see NixOS/nix#3843
- Makes the lockfile _way_ smaller, see NixOS/nix#7730

.reuse/dep5

@@ -5,6 +5,10 @@ Copyright: 2019 Serokell <[email protected]>
            2019 Lars Jellema <[email protected]>
 License: MPL-2.0
 
+Files: npins/*
+Copyright: 2024 Silvan Mosberger <[email protected]>
+License: MPL-2.0
+
 Files: test/diff/*
 Copyright: 2024 piegames <[email protected]>
 License: MPL-2.0

default.nix

@@ -1,17 +1,66 @@
-# SPDX-FileCopyrightText: 2020 Serokell <https://serokell.io/>
+# © 2019 Serokell <[email protected]>
+# © 2019 Lars Jellema <[email protected]>
+# © 2024 Silvan Mosberger <[email protected]>
 #
 # SPDX-License-Identifier: MPL-2.0
 
-(import
-  (
-    let
-      lock = builtins.fromJSON (builtins.readFile ./flake.lock);
-    in
-    fetchTarball {
-      url = "https://github.com/edolstra/flake-compat/archive/${lock.nodes.flake-compat.locked.rev}.tar.gz";
-      sha256 = lock.nodes.flake-compat.locked.narHash;
-    }
-  )
-  {
-    src = ./.;
-  }).defaultNix
+{
+  system ? builtins.currentSystem,
+  sources ? import ./npins,
+}:
+let
+  overlay = self: super: {
+    haskell = super.haskell // {
+      packageOverrides = self: super: {
+        nixfmt = self.callCabal2nix "nixfmt" src { };
+      };
+    };
+  };
+
+  pkgs = import sources.nixpkgs {
+    inherit system;
+    overlays = [ overlay (import (sources.serokell-nix + "/overlay")) ];
+    config = {};
+  };
+
+  inherit (pkgs) haskell lib;
+
+  regexes =
+    [ ".*.cabal$" "^src.*" "^main.*" "^Setup.hs$" "^js.*" "LICENSE" ];
+  src = builtins.path {
+    path = ./.;
+    name = "nixfmt-src";
+    filter = path: type:
+      let relPath = lib.removePrefix (toString ./. + "/") (toString path);
+      in lib.any (re: builtins.match re relPath != null) regexes;
+  };
+
+  build = pkgs.haskellPackages.nixfmt;
+
+in
+build // rec {
+  packages = {
+    nixfmt = build;
+    nixfmt-static = haskell.lib.justStaticExecutables packages.nixfmt;
+    nixfmt-deriver = packages.nixfmt-static.cabal2nixDeriver;
+
+    nixfmt-shell = packages.nixfmt.env.overrideAttrs (oldAttrs: {
+      buildInputs = oldAttrs.buildInputs ++ (with pkgs; [
+        # nixfmt: expand
+        cabal-install
+        stylish-haskell
+        shellcheck
+        npins
+      ]);
+    });
+
+    inherit (pkgs) reuse;
+  };
+
+  shell = packages.nixfmt-shell;
+
+  checks = {
+    hlint = pkgs.build.haskell.hlint ./.;
+    stylish-haskell = pkgs.build.haskell.stylish-haskell ./.;
+  };
+}