From 7997b6fc5dc2f5beb1bbdbb7a5bbed2e6c530dac Mon Sep 17 00:00:00 2001
From: Sridhar Ratnakumar <3998+srid@users.noreply.github.com>
Date: Mon, 6 Mar 2023 17:37:35 -0500
Subject: [PATCH] Add a default for `haskellFlakeProjectModules` (#106)

---
 CHANGELOG.md         |  1 +
 doc/guide/modules.md | 30 +++++++++++++++++++++++++
 nix/flake-module.nix | 52 +++++++++++++++++++++++++++++++++++---------
 3 files changed, 73 insertions(+), 10 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index aebe1be..a3c32d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@
     - #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.
       - #79: `flake.haskellFlakeProjectModules.<name>` option can be used to set and expose your Haskell project modules to other flakes.
+        - #106: Auto-generate the default modules in 'haskellFlakeProjectModules' for reuse in downstream flakes.
       - #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/doc/guide/modules.md b/doc/guide/modules.md
index c9f5be0..3b2cab5 100644
--- a/doc/guide/modules.md
+++ b/doc/guide/modules.md
@@ -52,6 +52,36 @@ A haskell-flake project module takes the following arguments:
 | `pkgs` | The perSystem's `pkgs` argument |
 | `self` | The flake's `self` |
 
+## Default modules
+
+By default, haskell-flake will generate the following modules for the "default" `haskellProject`:
+
+| Module | Contents |
+| -- | -- |
+| `haskellFlakeProjectModules.input` | Dependency overrides only |
+| `haskellFlakeProjectModules.local` | Local packages only |
+| `haskellFlakeProjectModules.output` | Local packages & dependency overrides |
+
+The idea here being that you can "connect" two Haskell projects such that they depend on one another while reusing the overrides from one place. For example, if you have a project "foo" that depends on "bar" and if "foo"'s flake.nix has "bar" as its input, then in "foo"'s `haskellProject.default` entry you can import "bar" as follows:
+
+```nix
+# foo's flake.nix's perSystem
+{ 
+  haskellProjects.default = {
+    imports = [
+      inputs.bar.haskellFlakeProjectModules.output
+    ];
+    packages = {
+      foo.root = ./.;
+    };
+  };
+}
+```
+
+By importing "bar"'s `output` project module, you automatically get the overrides from "bar" (unless you use the `local` module) as well as the local packages[^bar]. This way you don't have to duplicate the `overrides` and manually specify the `source-overrides` in "foo"'s flake.nix.
+
+[^bar]: Local packages come from the `packages` option. So this is typically the "bar" package itself for single-package projects; or all the local projects if it is a multi-package project.
+
 ## Examples
 
 - https://github.com/srid/nixpkgs-140774-workaround
diff --git a/nix/flake-module.nix b/nix/flake-module.nix
index d74335a..f91468b 100644
--- a/nix/flake-module.nix
+++ b/nix/flake-module.nix
@@ -1,9 +1,10 @@
 # A flake-parts module for Haskell cabal projects.
-{ self, config, lib, flake-parts-lib, ... }:
+{ self, config, lib, flake-parts-lib, withSystem, ... }:
 
 let
   inherit (flake-parts-lib)
-    mkPerSystemOption;
+    mkPerSystemOption
+    mkSubmoduleOptions;
   inherit (lib)
     mkOption
     types;
@@ -226,14 +227,6 @@ in
               type = types.attrsOf projectSubmodule;
             };
 
-            haskellFlakeProjectModules = mkOption {
-              type = types.lazyAttrsOf types.deferredModule;
-              default = { };
-              description = ''
-                An attrset of `haskellProjects.<name>` modules that can be imported in
-                other flakes.
-              '';
-            };
           };
 
           config =
@@ -272,5 +265,44 @@ in
                   config.haskellProjects;
             };
         });
+
+    flake = mkSubmoduleOptions {
+      haskellFlakeProjectModules = mkOption {
+        type = types.lazyAttrsOf types.deferredModule;
+        description = ''
+          An attrset of `haskellProjects.<name>` modules that can be imported in
+          other flakes.
+        '';
+        defaultText = ''
+          Package and dependency information for this project exposed for reuse
+          in another flake, when using this project as a Haskell dependency.
+
+          Typically the consumer of this flake will want to use one of the
+          following modules:
+
+            - output: provides both local package and dependency overrides.
+            - local: provides only local package overrides (ignores dependency
+              overrides in this flake)
+        '';
+        default = rec {
+          # The 'output' module provides both local package and dependency
+          # overrides.
+          output = {
+            imports = [ input local ];
+          };
+          # The 'local' module provides only local package overrides.
+          local = { pkgs, lib, ... }: withSystem pkgs.system ({ config, ... }: {
+            source-overrides =
+              lib.mapAttrs (_: v: v.root)
+                config.haskellProjects.default.packages;
+          });
+          # The 'input' module contains only dependency overrides.
+          input = { pkgs, ... }: withSystem pkgs.system ({ config, ... }: {
+            inherit (config.haskellProjects.default)
+              source-overrides overrides;
+          });
+        };
+      };
+    };
   };
 }