Avoid IFD through cabal2nix

cabal.nix

@@ -0,0 +1,19 @@
+{ mkDerivation, aeson, async, base, data-default, directory
+, filepath, lib, mtl, optics-core, profunctors, relude, shower
+, time, with-utf8
+}:
+mkDerivation {
+  pname = "haskell-template";
+  version = "0.1.0.0";
+  src = ./.;
+  isLibrary = false;
+  isExecutable = true;
+  executableHaskellDepends = [
+    aeson async base data-default directory filepath mtl optics-core
+    profunctors relude shower time with-utf8
+  ];
+  homepage = "https://srid.ca/haskell-template";
+  description = "A template for Haskell projects using Nix";
+  license = lib.licenses.mit;
+  mainProgram = "haskell-template";
+}

nix/modules/flake-parts/haskell.nix

@@ -17,6 +17,7 @@
           (root + /haskell-template.cabal)
           (root + /LICENSE)
           (root + /README.md)
+          (root + /cabal.nix)
         ];
       });
 
@@ -25,6 +26,11 @@
 
       # Packages to add on top of `basePackages`
       packages = {
+        # For IFD-less evaluation, you can point to pre-generated `cabal2nix` expression 
+        # here for each package,
+        # haskell-flake will use this file to build the package.
+        # By default haskell-flake refers to `cabal.nix`
+        haskell-template.cabal2NixFile = config.pre-commit.settings.hooks.cabal2nix.settings.outputFilename;
         # Add source or Hackage overrides here
         # (Local packages are added automatically)
         /*

nix/modules/flake-parts/pre-commit.nix

@@ -7,13 +7,21 @@
   perSystem = { config, ... }: {
     pre-commit.settings = {
       hooks = {
-        nixpkgs-fmt.enable = true;
+        nixpkgs-fmt = {
+          enable = true;
+          # cabal2nix, nixpkgs-fmt both modifies default.nix, hence exlude fmt.
+          excludes = [ config.pre-commit.settings.hooks.cabal2nix.settings.outputFilename ];
+        };
         cabal-fmt.enable = true;
         fourmolu = {
           enable = true;
           package = config.fourmolu.wrapper;
         };
         hlint.enable = true;
+        cabal2nix = {
+          enable = true;
+          settings.outputFilename = "cabal.nix";
+        };
       };
     };