settings.<name>.custom perhaps not working as documented

[raehik]

I need to override a package with a newer one. I could do so with packages.<name>.source, but it needs more Nix-specific configuration, and it turns out nixpkgs has the newer one with all that config already done.

The documentation https://zero-to-flakes.com/haskell-flake/dependency#nixpkgs says I can do this:

haskellProjects.default = {
  settings = { super, ... }: {
    fourmolu.custom = super.fourmolu_0_13_1_0;
  };
};

I use that syntax, but receive the error A definition for option 'custom' is not of type 'null or (function that evaluates to a(n) package)'. PR here: raehik/generic-data-functions#3 , package attempting to override is splitmix.

I've found relevant code, but I'm not sure how to understand it:

haskell-flake/nix/modules/project/settings/all.nix

Lines 307 to 323 in 63a1608

	# When none of the above settings is suitable:
	custom = {
	type = types.functionTo types.package;
	description = ''
	A custom funtion to apply on the Haskell package.
	Use this only if none of the existing settings are suitable.
	The function must take three arguments: self, super and the package being
	applied to.
	Example:
	custom = pkg: builtins.trace pkg.version pkg;
	'';
	impl = f: f;
	};

[srid]

It needs to be a function. See example at #192 where we also discuss the possibility of improving this interface.

haskellProjects.default = {
  settings = {
    fourmolu = { super, ...}: { custom = _: super.fourmolu_0_13_1_0; };
  };
};

[raehik]

Thanks as always! I think the docs on zero-to-flakes wants an update to reflect that syntax.

[srid]

It is briefly mentioned as an example in the docs:

https://community.flake.parts/haskell-flake/dependency#nixpkgs