refactoring: reintroduce nix

.envrc

@@ -1 +1 @@
-use flake -Lv --fallback
+use flake

.github/workflows/nix-check.yml

@@ -0,0 +1,19 @@
+name: "libsodium hs nix check"
+on:
+  pull_request:
+  push:
+    branches: ["main"]
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    continue-on-error: true
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        github_access_token: ${{ secrets.GITHUB_TOKEN }}
+    - uses: cachix/cachix-action@v14
+      with:
+        name: libsodium-hs
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix flake check -Lv --allow-import-from-derivation --fallback

flake.nix

@@ -0,0 +1,124 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-23.11-small";
+    flake-utils.url = "github:numtide/flake-utils";
+    flake-compat = {
+      url = "github:edolstra/flake-compat";
+      flake = false;
+    };
+    pre-commit-hooks = {
+      url = "github:cachix/pre-commit-hooks.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+      inputs.flake-utils.follows = "flake-utils";
+    };
+    gitignore = {
+      url = "github:hercules-ci/gitignore.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  nixConfig = {
+    extra-substituters = [
+      "https://libsodium-hs.cachix.org"
+    ];
+    extra-trusted-public-keys = [
+      "libsodium-hs.cachix.org-1:u/v4XdWrbl+G/fDUoEwB1yvMdlxdKM4al2odCNsrqkg="
+    ];
+    allow-import-from-derivation = true;
+  };
+
+  outputs = inputs@{ nixpkgs, ... }:
+    let
+      # this is to allow running `nix flake check` by using `--impure`
+      systems =
+        if builtins.hasAttr "currentSystem" builtins
+        then [ builtins.currentSystem ]
+        else nixpkgs.lib.systems.flakeExposed;
+    in
+    inputs.flake-utils.lib.eachSystem systems (system:
+      let
+        inherit (inputs.gitignore.lib) gitignoreSource;
+
+        pkgs = import nixpkgs {
+          inherit system;
+          config.allowBroken = true;
+        };
+
+        pre-commit-check = inputs.pre-commit-hooks.lib.${system}.run {
+          src = ./.;
+          hooks = {
+            # nix checks
+            nixpkgs-fmt.enable = true;
+            deadnix.enable = true;
+            statix.enable = true;
+
+            # Haskell checks
+            fourmolu.enable = true;
+            cabal-fmt.enable = true;
+            hlint.enable = true;
+          };
+        };
+
+        hsPkgs = pkgs.haskellPackages.override (_old: {
+          overrides = with pkgs.haskell.lib.compose; hself: hsuper:
+            let
+              commonOverrides = overrideCabal (_drv: {
+                doInstallIntermediates = true;
+                enableSeparateIntermediatesOutput = true;
+                pkg-configDepends = [
+                  pkgs.libsodium
+                ];
+              });
+            in
+            {
+              libsodium-bindings = commonOverrides (hself.callCabal2nix "libsodium-bindings" (gitignoreSource ./libsodium-bindings) { });
+              sel =
+                commonOverrides (hself.callCabal2nix "sel" (gitignoreSource ./sel) {
+                  base16 = hsuper.base16_1_0;
+                  hedgehog = hsuper.hedgehog_1_4;
+                  tasty = hsuper.tasty_1_5;
+                });
+              # text-display = markUnbroken hsuper.text-display;
+            };
+        });
+
+        hsShell = hsPkgs.shellFor {
+          shellHook = ''
+            ${pre-commit-check.shellHook}
+            set -x
+            export LD_LIBRARY_PATH="${pkgs.libsodium}/lib"
+            set +x 
+          '';
+
+          packages = ps: with ps; [
+            libsodium-bindings
+            sel
+          ];
+
+          buildInputs = with hsPkgs; [
+            pkgs.pkg-config
+            pkgs.libsodium.dev
+            cabal-install
+            haskell-language-server
+            hlint
+            cabal-fmt
+            fourmolu
+          ];
+        };
+
+      in
+      {
+        checks = {
+          inherit (hsPkgs) libsodium-bindings sel;
+          shell = hsShell;
+          formatting = pre-commit-check;
+        };
+
+        packages = {
+          inherit (hsPkgs) libsodium-bindings sel;
+        };
+
+        devShells.default = hsShell;
+      }
+    );
+}