Parametric Aspects

Source

nix/lib/default.nix — nix/lib/can-take.nix

What is a Parametric Aspect

A parametric aspect is an aspect defined as a function whose arguments are pipeline context values — host, user, home, or any custom entity kind. Den introspects the function’s argument pattern and only calls it in contexts where all required arguments are available.

# This aspect only activates in {host} contexts
den.aspects.networking = { host, ... }: {
  nixos.networking.hostName = host.name;
};

# This only activates when both {host, user} are present
den.aspects.user-groups = { host, user, ... }: {
  nixos.users.users.${user.userName}.extraGroups = [ "wheel" ];
};

# This only activates for standalone {home} contexts
den.aspects.shell-config = { home, ... }: {
  homeManager.programs.zsh.enable = true;
};

No wrapper needed — a bare function requiring { host, user } is silently skipped in contexts that only have { host }. The argument shape is the condition. No mkIf, no enable flags.

Static attrsets (non-functions) are always included regardless of context:

den.aspects.firewall = {
  nixos.networking.firewall.enable = true;
};

Where Parametric Args Work

Aspect level only

Parametric context args (host, user, home) work at the aspect level — the outer function that wraps the entire aspect definition. They are pipeline values, not NixOS module arguments.

For accessing Den context inside class modules (alongside config, pkgs, etc.), use the flat-form class module pattern instead.

Aspect-level parametric — the function wraps the whole aspect:

# host is a pipeline arg — the whole aspect is parametric
den.aspects.laptop = { host, ... }: {
  nixos.networking.hostName = host.name;
  homeManager.programs.git.userName = host.hostName;
};

Class-level context injection — Den args inside a class module:

# host here is pre-applied by wrapClassModule, not parametric dispatch
den.aspects.laptop = {
  nixos = { host, config, pkgs, ... }: {
    networking.hostName = host.name;
  };
};

Both patterns are valid and composable. Use aspect-level parametric when the context drives the entire aspect (which classes to include, what includes to add). Use class-level injection when only a specific class module needs entity data alongside module-system args.

Avoid anonymous functions in includes

Use named aspects instead of inline anonymous functions. Named aspects produce better error traces and are easier to debug with the diagram library.

# Prefer this
den.aspects.my-laptop.includes = [ den.aspects.ssh-config ];
den.aspects.ssh-config = { host, ... }: { ... };

# Avoid this
den.aspects.my-laptop.includes = [
  ({ host, ... }: { ... })
];

See also

• Aspects & Functors — aspect structure and resolution
• Class Modules — flat-form context injection inside class modules
• Resolution Pipeline — how parametric dispatch fits the pipeline
• den.lib reference — canTake and other library functions
• Migrating from den.ctx — if upgrading from deprecated wrapper APIs (den.lib.parametric, perHost, etc.)