settings: add removeReferencesTo (#225)

This setting helps remove unnecessary dependencies to your haskell package that
bloats up the final closure size

Source: https://github.com/srid/emanote/blob/5623bb3814e382dfea7ffd21d3da25c2d3179001/nix/removeReferencesTo.nix#L9


---------

Co-authored-by: Sridhar Ratnakumar <3998+srid@users.noreply.github.com>
Co-authored-by: Sridhar Ratnakumar <srid@srid.ca>

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - #210: Add `extraLibraries` to `settings` module.
+- #225: settings: add `removeReferencesTo`
 - #215: Improved debug logging.
 - #216: Remove `debug` option (pass `--trace-verbose` to nix instead)
 - Fixes

doc/settings.md

@@ -46,7 +46,18 @@ haskellProjects.default = {
 
 ## Custom settings {#custom}
 
-- [Emanote overrides](https://github.com/srid/emanote/commit/5b24bd04f94e03afe66ee01da723e4a05d854953): demonstrates how to add a *new* setting option (`removeReferencesTo`).
+You can provide custom settings for use in multiple packages (even across multiple repos). For example, see [this Emanote change](https://github.com/srid/emanote/commit/5b24bd04f94e03afe66ee01da723e4a05d854953) which demonstrates how to add a *new* setting option (`removeReferencesTo`).
 
+## Extra settings {#extra}
+
+haskell-flake provides the following settings on top of those provided by [nixpkgs].
+
+### `removeReferencesTo`
+
+Remove references to other packages from this Haskell package. This is useful to eliminate unnecessary data dependencies from your Haskell executable so as to reduce its closure size.
+
+> [!info] For more, see
+> - https://github.com/NixOS/nixpkgs/pull/204675
+> - https://srid.ca/remove-references-to
 
 [nixpkgs]: https://nixos.asia/en/nixpkgs

nix/modules/project/settings/all.nix

@@ -311,6 +311,28 @@ in
       impl = triggerRebuild;
     };
 
+    removeReferencesTo = {
+      type = types.listOf types.package;
+      description = ''
+        Packages to remove references to.
+
+        This is useful to ditch unnecessary data dependencies from your Haskell
+        executable so as to reduce its closure size.
+
+        cf.
+        - https://github.com/NixOS/nixpkgs/pull/204675
+        - https://srid.ca/remove-references-to
+      '';
+      impl = disallowedReferences: drv:
+        drv.overrideAttrs (old: rec {
+          inherit disallowedReferences;
+          postInstall = (old.postInstall or "") + ''
+            ${lib.concatStrings (map (e: "echo Removing reference to: ${e}\n") disallowedReferences)}
+            ${lib.concatStrings (map (e: "remove-references-to -t ${e} $out/bin/*\n") disallowedReferences)}
+          '';
+        });
+    };
+
     # When none of the above settings is suitable:
     custom = {
       type = types.functionTo types.package;