From 4585d422cd4428e2279e344d7dc84436bab93300 Mon Sep 17 00:00:00 2001
From: Sridhar Ratnakumar <3998+srid@users.noreply.github.com>
Date: Fri, 10 Feb 2023 15:07:06 -0500
Subject: [PATCH] Make `overrides` an overlay type rather than a `functionTo`
 (#67)

---
 CHANGELOG.md     |  2 ++
 flake-module.nix | 57 ++++++++++++++++++++++++++++++++++++++++--------
 test/flake.nix   | 35 ++++++++++++++++++++---------
 3 files changed, 75 insertions(+), 19 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4625452..0a80ba1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@
     - #37: Group `buildTools` (renamed to `tools`), `hlsCheck` and `hlintCheck` under the `devShell` submodule option; and allow disabling them all using `devShell.enable = false;` (useful if you want haskell-flake to produce just the package outputs).
     - #64: Remove hlintCheck (use [treefmt-nix](https://github.com/numtide/treefmt-nix#flake-parts) instead)
     - #52: Expose the final package set as `finalPackages`. Rename `haskellPackages`, accordingly, to `basePackages`. Overlays are applied on top of `basePackage` -- using `source-overrides`, `overrides`, `packages` in that order -- to produce `finalPackages`.
+    - #68: You can now use `imports` inside of `haskellProjects.<name>` to modularize your Haskell project configuration.
+      - #67: `overrides` will be combined using `composeManyExtensions`, however their order is arbitrary. This is an experimental feature, and a warning will be logged.
 
 ## 0.1.0
 
diff --git a/flake-module.nix b/flake-module.nix
index eb7ea8a..742034d 100644
--- a/flake-module.nix
+++ b/flake-module.nix
@@ -99,16 +99,55 @@ in
                     description = ''Package overrides given new source path'';
                     default = { };
                   };
-                  overrides = mkOption {
-                    type = functionTo (functionTo (types.lazyAttrsOf raw));
-                    description = ''
-                      Overrides for the Cabal project
+                  overrides =
+                    let
+                      # WARNING: While the order is deterministic, it is not
+                      # determined by the user. Thus overlays may be applied in
+                      # an unexpected order.
+                      # We need: https://github.com/NixOS/nixpkgs/issues/215486
+                      haskellOverlayType = types.mkOptionType {
+                        name = "haskellOverlay";
+                        description = "An Haskell overlay function";
+                        descriptionClass = "noun";
+                        # NOTE: This check is not exhaustive, as there is no way
+                        # to check that the function takes two arguments, and
+                        # returns an attrset.
+                        check = lib.isFunction;
+                        merge = _loc: defs:
+                          let
+                            logWarning =
+                              if builtins.length defs > 1
+                              then builtins.trace "WARNING[haskell-flake]: Multiple haskell overlays are applied in arbitrary order." null
+                              else null;
+                            overlays =
+                              map (x: x.value)
+                                (builtins.seq
+                                  logWarning
+                                  defs);
+                          in
+                          lib.composeManyExtensions overlays;
+                      };
+                    in
+                    mkOption {
+                      type = haskellOverlayType;
+                      description = ''
+                        Cabal package overrides for this Haskell project
                 
-                      For handy functions, see <https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/lib/compose.nix>
-                    '';
-                    default = self: super: { };
-                    defaultText = lib.literalExpression "self: super: { }";
-                  };
+                        For handy functions, see 
+                        <https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/lib/compose.nix>
+
+                        **WARNING**: When using `imports`, multiple overlays
+                        will be merged using `lib.composeManyExtensions`.
+                        However the order the overlays are applied can be
+                        arbitrary (albeit deterministic, based on module system
+                        implementation).  Thus, the use of `overrides` via
+                        `imports` is not officiallly supported. If you'd like
+                        to see proper support, add your thumbs up to
+                        <https://github.com/NixOS/nixpkgs/issues/215486>.
+                      '';
+                      default = self: super: { };
+                      defaultText = lib.literalExpression "self: super: { }";
+                    };
                   packages = mkOption {
                     type = types.lazyAttrsOf packageSubmodule;
                     description = ''
diff --git a/test/flake.nix b/test/flake.nix
index 6768ad8..9f8c253 100644
--- a/test/flake.nix
+++ b/test/flake.nix
@@ -24,19 +24,34 @@
       ];
       perSystem = { self', pkgs, ... }: {
         haskellProjects.default = {
+          # Multiple modules should be merged correctly.
+          imports =
+            let
+              defaults = {
+                overrides = self: super: {
+                  # This is purposefully incorrect (pointing to ./.) because we
+                  # expect it to be overriden below.
+                  foo = self.callCabal2nix "foo" ./. { };
+                };
+                devShell = {
+                  tools = hp: {
+                    # Setting to null should remove this tool from defaults.
+                    ghcid = null;
+                  };
+                  hlsCheck.enable = true;
+                };
+              };
+            in
+            [ defaults ];
           overrides = self: super: {
-            # Custom library overrides (here, "foo" comes from a flake input)
+            # This overrides the overlay above (in `defaults`), because the
+            # module system merges them in such order. cf. the WARNING in option
+            # docs.
             foo = self.callCabal2nix "foo" (inputs.haskell-multi-nix + /foo) { };
           };
-          devShell = {
-            tools = hp: {
-              # Some buildTools are included by default. If you do not want them,
-              # set them to 'null' here.
-              ghcid = null;
-              # You can also add additional build tools.
-              fzf = pkgs.fzf;
-            };
-            hlsCheck.enable = true;
+          devShell.tools = hp: {
+            # Adding a tool should make it available in devshell.
+            fzf = pkgs.fzf;
           };
         };
         # haskell-flake doesn't set the default package, but you can do it here.