Add flake.nix and test on CI

.github/workflows/ci.yml

@@ -5,7 +5,8 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ghc: ['8.4.4', '8.6.5', '8.8.4', '8.10.7', '9.0.2', '9.2.2', '9.4.2', '9.6.3', '9.8.1', '9.10.1', '9.12.1']
+        # Always keep in sync with the tested-with section in the cabal file
+        ghc: ['8.4.4', '8.6.5', '8.8.4', '8.10.7', '9.0.2', '9.2.2', '9.4.2', '9.6.6', '9.8.1', '9.10.1', '9.12.1']
     name: Haskell GHC ${{ matrix.ghc }} cabal
     env:
       cabal_project_freeze: cabal.project.${{ matrix.ghc }}.freeze
@@ -33,3 +34,39 @@ jobs:
       - name: Cabal test
         run: |
           cabal test all --enable-tests --test-show-details=Always --project-file=cabal.project.${{ matrix.ghc }}
+
+  generate-flake-ghc-matrix:
+    name: Generate GHC versions for nix flake build matrix
+    runs-on: ubuntu-latest
+    outputs:
+      versions: ${{ steps.generate-versions.outputs.versions }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+      - name: Generate versions
+        id: generate-versions
+        run: |
+          echo -n "versions=" >> "$GITHUB_OUTPUT"
+          nix eval .#supportedGhcs --json >> "$GITHUB_OUTPUT"
+
+  build-flake:
+    name: Nix Flake
+    needs:
+      - generate-flake-ghc-matrix
+    strategy:
+      matrix:
+        version: ${{ fromJSON(needs.generate-flake-ghc-matrix.outputs.versions) }}
+        os:
+          - ubuntu-latest
+          - macos-latest
+      fail-fast: false # So the cache is still filled
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v30
+      - uses: DeterminateSystems/magic-nix-cache-action@v8
+      - run: nix flake check
+      - run: nix build .#${{ matrix.version }}
+      - run: nix develop .#${{ matrix.version }} -c cabal update
+      - run: nix develop .#${{ matrix.version }} -c cabal test

flake.nix

@@ -0,0 +1,49 @@
+{
+  description = "clay";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable-small";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    let
+      inherit (nixpkgs) lib;
+      supportedGhcs = [ "ghc810" "ghc90" "ghc92" "ghc94" "ghc96" "ghc98" "ghc910" "ghc912" ];
+      haskellPackagesFor = pkgs: lib.genAttrs supportedGhcs (ghc: pkgs.haskell.packages.${ghc}) // {default = pkgs.haskellPackages;};
+      hoverlay = hfinal: hprev: {
+        clay = hfinal.callCabal2nix "clay" ./. {};
+      };
+      overlay = final: prev: {
+        haskell = prev.haskell // {
+          packageOverrides = lib.composeManyExtensions [
+            hoverlay
+            prev.haskell.packageOverrides
+          ];
+        };
+      };
+    in
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        # Always keep in sync with the tested-with section in the cabal file
+        pkgs = nixpkgs.legacyPackages.${system};
+        pkgsOverridden = pkgs.extend overlay;
+        clay-all = pkgsOverridden.linkFarm "clay-all" (lib.mapAttrs (_ghcVersion: haskellPackages: haskellPackages.clay) (haskellPackagesFor pkgsOverridden));
+      in {
+        inherit self nixpkgs;
+
+        packages = lib.mapAttrs (_ghcVersion: haskellPackages: haskellPackages.clay) (haskellPackagesFor pkgsOverridden) // {
+          default = clay-all;
+        };
+        devShells = lib.mapAttrs (_ghcVersion: haskellPackages: haskellPackages.shellFor {
+          packages = hps: with hps; [clay];
+          nativeBuildInputs = with haskellPackages;
+            lib.optional (lib.versionAtLeast haskellPackages.ghc.version "9.2") haskell-language-server;
+        }) (haskellPackagesFor pkgs);
+
+      }
+    ) // {
+      inherit supportedGhcs;
+      overlays.default = overlay;
+    };
+}