How to make an option that takes in a function?

Help
1

I’m making a custom module for myself, and I’ve found it necessary to define an argument (i.e. mkOption) that takes in a function.

I’ve tried using functionTo, but I couldn’t figure out how to get it to work, and documentation on it is lacking.

How do I do this?

2

As an example that might help, this is how I use it in one of my modules:

build = lib.mkOption {
  type = lib.types.functionTo lib.types.package;
  default = cfg: import ./disk-image-builder.nix {
    inherit lib pkgs;
    config = cfg;
  };
  description = "Function that builds the disk image.";
};
3

There’s no existing type for this. Only function output type is defined.

You can of course make your own custom type.