Add otherOverlays option

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - #210: Add `extraLibraries` to `settings` module.
+- #277: Add `otherOverlays` option to add custom Haskell package overlays.
 - #225: settings: add `removeReferencesTo`
 - #215: Improved debug logging.
 - #216: Remove `debug` option (pass `--trace-verbose` to nix instead)
@@ -30,8 +31,8 @@
 ## 0.3.0 (May 22, 2023)
 
 - #134: Add `autoWire` option to control generation of flake outputs
-    - #138: Add `checks` to `outputs` submodule
-    - #143: Changed `autoWire` to be an enum type, for granular controlling of which outputs to autowire.
+  - #138: Add `checks` to `outputs` submodule
+  - #143: Changed `autoWire` to be an enum type, for granular controlling of which outputs to autowire.
 - #137: Expose cabal executables as flake apps. Add a corresponding `outputs.apps` option, while the `outputs.localPackages` option is renamed to `outputs.packages` (it now contains package metadata, including packages and its executables).
   - #151: Use `lib.getBin` to get the bin output
 - #148: Remove automatic hpack->cabal generation. Use `pre-commit-hooks.nix` instead.

flake.nix

@@ -64,6 +64,13 @@
             inherit nixpkgs flake-parts haskell-flake;
           };
         };
+
+        test-otherOverlays = {
+          dir = "test/otherOverlays";
+          overrideInputs = {
+            inherit nixpkgs flake-parts haskell-flake;
+          };
+        };
       };
   };
 }

nix/modules/project/default.nix

@@ -68,6 +68,17 @@ in
       default = pkgs.haskellPackages;
       defaultText = lib.literalExpression "pkgs.haskellPackages";
     };
+    otherOverlays = lib.mkOption {
+      type = types.listOf (import ../../types/haskell-overlay-type.nix { inherit lib; });
+      description = ''
+        Extra overlays to apply.
+
+        Normally, you would only use `packages.*` and `settings.*` (which
+        translate to overlays), but you can use this option if you want control
+        over the final overlay.
+      '';
+      default = [ ];
+    };
     autoWire =
       let
         outputTypes = [ "packages" "checks" "apps" "devShells" ];

nix/modules/project/outputs.nix

@@ -83,10 +83,10 @@ in
       localPackages =
         lib.filterAttrs (_: cfg: cfg.local.toCurrentProject) config.packages;
 
-      finalOverlay = lib.composeManyExtensions [
+      finalOverlay = lib.composeManyExtensions ([
         config.packagesOverlay
         config.settingsOverlay
-      ];
+      ] ++ config.otherOverlays);
 
       finalPackages = config.basePackages.extend finalOverlay;
 

test/otherOverlays/flake.nix

@@ -0,0 +1,32 @@
+{
+  # 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 = { };
+
+    haskell-multi-nix.url = "github:srid/haskell-multi-nix/d6ac6ccab559f886d1fc7da8cab44b99cb0c2c3d";
+    haskell-multi-nix.inputs.haskell-flake.follows = "haskell-flake";
+    haskell-multi-nix.inputs.nixpkgs.follows = "nixpkgs";
+    haskell-multi-nix.inputs.flake-parts.follows = "flake-parts";
+  };
+  outputs = inputs@{ self, nixpkgs, flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = nixpkgs.lib.systems.flakeExposed;
+      imports = [
+        inputs.haskell-flake.flakeModule
+      ];
+      perSystem = { self', pkgs, ... }: {
+        haskellProjects.default = {
+          otherOverlays = [
+            (self: super: {
+              foo = super.callCabal2nix "foo" "${inputs.haskell-multi-nix}/foo" { };
+            })
+          ];
+        };
+        packages.default = self'.packages.haskell-flake-test;
+      };
+    };
+}

test/otherOverlays/haskell-flake-test.cabal

@@ -0,0 +1,19 @@
+cabal-version:   3.0
+name:            haskell-flake-test
+version:         0.1.0.0
+license:         NONE
+author:          Joe
+maintainer:      joe@example.com
+build-type:      Simple
+
+common warnings
+    ghc-options: -Wall
+
+executable haskell-flake-test
+    import:           warnings
+    main-is:          Main.hs
+    build-depends:    
+        base,
+        foo
+    hs-source-dirs:   src
+    default-language: Haskell2010

test/otherOverlays/src/Main.hs

@@ -0,0 +1,7 @@
+module Main where
+
+import Foo
+
+main :: IO ()
+main = do
+    fooFunc