Forward Across Classes

What forward does

forward resolves an aspect for one class and injects the result into a submodule path of another class. This enables cross-class configuration routing.

The canonical use case: forward homeManager modules into nixos.home-manager.users.<name>.

Arguments

forward {
  each       = [ items ];            # list of items to iterate over
  fromClass  = item: "sourceClass";  # class to resolve from
  intoClass  = item: "targetClass";  # class to inject into
  intoPath   = item: [ "path" ];     # submodule path in targetClass
  fromAspect = item: aspect;         # aspect to resolve
}

Parameter	Signature	Purpose
each	listOf any	Items to iterate. One forward per item.
fromClass	item → string	Source class name to resolve
intoClass	item → string	Target class name to inject into
intoPath	item → listOf string	Attribute path for the submodule in target
fromAspect	item → aspect	The aspect to resolve for fromClass

Return value

forward returns { includes = [ ... ]; } — an aspect with one include per item. Each include:

1. Resolves fromAspect item for fromClass item
2. Wraps the resolved module as { imports = [ module ]; } at intoPath item
3. Places the result under intoClass item

Example

flake.aspects = { aspects, ... }: {
  my-host = {
    targetClass = {
      imports = [ targetSubmodule ];
      targetMod.names = [ "from-target" ];
    };
    sourceClass.names = [ "from-source" ];
    includes = [
      ({ class, aspect-chain }: forward {
        each = [ "source" ];
        fromClass = item: "${item}Class";
        intoClass = _: "targetClass";
        intoPath = _: [ "targetMod" ];
        fromAspect = _: lib.head aspect-chain;
      })
    ];
  };
};

Resolving for "targetClass" merges ["from-target", "from-source"].

Note

Test: forward.nix

Real-world: Home-Manager into NixOS

This is how Den forwards user home-manager configs into host NixOS:

hmSupport = { host }: forward {
  each = host.users;
  fromClass = _user: "homeManager";
  intoClass = _user: "nixos";
  intoPath = user: [ "home-manager" "users" user.userName ];
  fromAspect = user: den.aspects.${user.userName};
};

Each user’s homeManager aspect gets resolved and placed at nixos.home-manager.users.<name>.