feat: cabal2nix integration for IFD-less evalutation

flake.nix

@@ -65,6 +65,13 @@
             };
           };
 
+          test-cabal2nix = {
+            dir = "test/cabal2nix";
+            overrideInputs = {
+              inherit nixpkgs flake-parts haskell-flake;
+            };
+          };
+
           test-with-subdir = {
             dir = "test/with-subdir";
             overrideInputs = {

nix/build-haskell-package.nix

@@ -22,15 +22,30 @@ 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] ${path}")
+          (self.callPackage path { })
+      else
+        lib.pipe (self.callCabal2nix name root { })
+          [
+            (pkg: log.traceDebug "${name}.callCabal2nix root=${root} deriver=${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 = ''

test/cabal2nix/default.nix

@@ -0,0 +1,11 @@
+{ mkDerivation, base, lib, random }:
+mkDerivation {
+  pname = "haskell-flake-test";
+  version = "0.1.0.0";
+  src = ./.;
+  isLibrary = false;
+  isExecutable = true;
+  executableHaskellDepends = [ base random ];
+  license = "unknown";
+  mainProgram = "haskell-flake-test";
+}

test/cabal2nix/flake.nix

@@ -0,0 +1,35 @@
+{
+  # Disable IFD for this test.
+  nixConfig = {
+    allow-import-from-derivation = false;
+  };
+
+  # Since there is no flake.lock file (to avoid incongruent haskell-flake
+  # pinning), we must specify revisions for *all* inputs to ensure
+  # reproducibility.
+  inputs = {
+    nixpkgs = { };
+    flake-parts = { };
+    haskell-flake = { };
+  };
+
+  outputs = inputs@{ self, nixpkgs, flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = nixpkgs.lib.systems.flakeExposed;
+      imports = [
+        inputs.haskell-flake.flakeModule
+      ];
+      debug = true;
+      perSystem = { config, self', pkgs, lib, ... }: {
+        haskellProjects.default = {
+          # If IFD is disabled, 
+          # we need to specify the pre-generated `cabal2nix` expressions
+          # file to haskell-flake for the package, 
+          # otherwise build would fail as it would use `callCabal2nix` function
+          # which uses IFD.
+          packages.haskell-flake-test.cabal2NixFile = "default.nix";
+          settings = { };
+        };
+      };
+    };
+}

test/cabal2nix/haskell-flake-test.cabal

@@ -0,0 +1,20 @@
+cabal-version:   3.0
+name:            haskell-flake-test
+version:         0.1.0.0
+license:         NONE
+author:          Joe
+maintainer:      [email protected]
+build-type:      Simple
+
+common warnings
+    ghc-options: -Wall
+
+executable haskell-flake-test
+    import:           warnings
+    main-is:          Main.hs
+    build-depends:
+        base,
+        random
+    hs-source-dirs:   src
+    default-language: Haskell2010
+

test/cabal2nix/src/Main.hs

@@ -0,0 +1,6 @@
+module Main where
+
+import System.Random
+
+main :: IO ()
+main = putStrLn "Hello, Haskell!"