Idiomatic way to provide a "library" of Nix functions

[srid]

We are using the module system options to expose functions for other modules to use.

For example,

{ self, ... }:
{
  options = {
    mkCheck = lib.mkOption {
      type = lib.types.functionTo lib.types.unspecified;
      default = { package, ... }: 
        abort "TODO: return a derivation that runs the tests for $package";
    };
  };
}

Then we use the mkCheck "library" function from other flake-module.nix modules (there are >10 of them, hence the library function) as:

{ self, ... }: {
  perSystem = { config, ... }: {
    checks.foo = config.mkCheck { ... };
  };
}

Is this the recommended way to use the module system to achieve this?

[roberth]

I don't think there's an idiomatic way yet, but we can think of a good opinionated way to do it.

Some requirements:

• good namespacing
• consider both libraries for internal use and for external use
• consider libraries that depend on system and those that don't
  • system-dependent ones shouldn't have to pass system
    • refs Add moduleWithSystem flake module parameter #48
  • lib flake attribute should be an attrset, but may contain a system-dependent function for constructing a system-dependent library
• any generic support modules should go in flakeModules.<foo> or a separate repo, not the default module list, which is supposed to be unopinionated, capturing only what nix itself hardcodes

Possible design:

Defining and using public libs (<flake>.lib)

• Use flake.lib for putting lib attributes that don't necessarily depend on system
• Use perSystem.lib for lib attributes that do depend on system
• Expose perSystem.lib in flake.lib.libWithSystem = system: ...
• Define inputs'.<name>.lib to be inputs.<name>.lib // inputs.<name>.lib.libWithSystem system (or // {} if not exists)
• Define perSystem module argument libs so that libs.<name> = inputs'.foo.lib

Defining a lib to be used in a <flake>.flakeModule (module to be used in another flake)

• Option a. Define a <flake>.lib, define flakeModule with withSystem or Add moduleWithSystem flake module parameter #48
  • This lib being public is probably a good thing?
• Option b. Something else? We can't rely on inputs' because the consumer flake should be free to have the flake inputs it wants with the names it wants.

Defining a lib to be used locally

• Similar to previous, but libs.self may be ok for non-public flake module. (Similar to how specialArgs should only be used by the final consumer, e.g. <flake>.nixosConfigurations, for this consumer's own purposes only)
  • Need some sort of alias for flake.lib in order not to expose a private lib to the flake.

Does this seem like it would work for you?

[clhodapp]

Defining a lib to be used in someone else's exported flake module is made a lot more challenging by #104