Expose overrides in flake

[srid]

Rationale: When I want to use emanote as a library, I want to use its overrides (and source-overrides) so as to be able to re-use it without replicating it in the local project (that uses emanote as a dependency).

https://github.com/srid/emanote/blob/6f83b857741fb6607716cf97cee3382c7f029180/flake.nix#L43-L51

[srid]

@locallycompact's https://horizon-haskell.net/ 's overlay could also be exposed in this manner?

[roberth]

https://github.com/srid/emanote/blob/6f83b857741fb6607716cf97cee3382c7f029180/flake.nix#L43-L51

I think these lines could be turned into a small module that can be imported at the project level.

      flake.haskellFlakeProjectModules.default =
        ... those lines ...;
      perSystem = { pkgs, system, inputs', self', ... }: {
        haskellProjects.default = {
          imports = [ self.haskellFlakeProjectModules.default ];

[srid]

Interesting idea. What happens if two modules define the same option, say buildTools? Merge, or replace, or ?

[roberth]

It's determined by the option type.
attrsOf and lazyAttrsOf will merge; listOf appends. Types like package and str will error out.
mkForce and mkOverride replace.

[srid]

I suppose for overrides we ought to create a custom type with composeExtensions-based merge semantics, because the default semantics for types.functionTo just calls the functions and merges the results (attrsets): https://github.com/NixOS/nixpkgs/blob/cd71c681ca4e49de4f65e6a45f686f11f3e1b7d3/lib/types.nix#L620-L621

[srid]

It just occurred to me that this approach (imports) can also solve the problem of decoupling the hardcoded buildTools, so that imports = [ inputs.haskell-flake.defaultSettings ]; for example could set the buildTools defaults (and the user can ignore them if they want to start pristine).

[srid]

Example:

• Using: really decouple the nixpkgs workaround haskell-template#85
• Publishing: https://github.com/srid/nixpkgs-140774-workaround