feat: cabal2nix integration for IFD-less evalutation

nix/build-haskell-package.nix

@@ -22,15 +22,37 @@ let
       name = "${name}";
       inherit src;
     };
+
 in
 
-name: root:
+name: root: cabal2NixFile:
 lib.pipe root
   [
     # Avoid rebuilding because of changes in parent directories
     (mkNewStorePath "source-${name}")
     (x: log.traceDebug "${name}.mkNewStorePath ${x.outPath}" x)
 
-    (root: self.callCabal2nix name root { })
-    (x: log.traceDebug "${name}.cabal2nixDeriver ${x.cabal2nixDeriver.outPath}" x)
+    (root:
+      let path = "${root}/${cabal2NixFile}";
+      in
+      # Check if cached cabal2nix generated nix expression is present,
+        # if present use it with callPackage
+        # to avoid IFD
+      if builtins.pathExists path
+      then
+        (log.traceDebug "${name}.callPackage[cabal2nix] ${path}")
+          self.callPackage
+          path
+          { }
+      else
+        let
+          pkg =
+            (log.traceDebug "${name}.callCabal2nix ${root}")
+              self.callCabal2nix
+              name
+              root
+              { };
+        in
+        (log.traceDebug "${name}.cabal2nixDeriver ${pkg.cabal2nixDeriver.outPath}" pkg)
+    )
   ]

nix/modules/project/packages/default.nix

@@ -61,8 +61,7 @@ in
           getOrMkPackage = name: cfg:
             if lib.types.path.check cfg.source
             then
-              log.traceDebug "${name}.callCabal2nix ${cfg.source}"
-                (build-haskell-package name cfg.source)
+              (build-haskell-package name cfg.source cfg.cabal2NixFile)
             else
               log.traceDebug "${name}.callHackage ${cfg.source}"
                 (self.callHackage name cfg.source { });

nix/modules/project/packages/package.nix

@@ -23,6 +23,16 @@ in
       '';
     };
 
+    cabal2NixFile = lib.mkOption {
+      type = lib.types.str;
+      description = ''
+        The Nix file which contains cached (pre-generated) `cabal2nix` expressions.
+
+        By default, it refers to `cabal.nix` file.
+      '';
+      default = "cabal.nix";
+    };
+
     cabal.executables = mkOption {
       type = types.nullOr (types.listOf types.str);
       description = ''