Parametric Aspects

import { Aside } from ‘@astrojs/starlight/components’;

What Is a Parametric Aspect?

A parametric aspect uses a __functor that forwards its received context to functions in .includes. Den provides several parametric functors in den.lib.parametric.

den.lib.parametric (the default)

The most common functor. Alias for parametric.withOwn parametric.atLeast:

den.aspects.foo = den.lib.parametric {
  nixos.networking.hostName = "owned";  # always included
  includes = [
    ({ host, ... }: { nixos.time.timeZone = "UTC"; })
  ];
};

When applied with { host = ...; user = ...; }:

Owned configs (nixos.networking.hostName) are included
Static includes are included
Functions matching atLeast the context args are called

parametric.atLeast

Only dispatches to functions — does not include owned configs:

F = parametric.atLeast { includes = [ a b c ]; };

Applied with { x = 1; y = 2; }:

{ x, ... }: ... → called (has at least x)
{ x, y }: ... → called (has exactly x, y)
{ z }: ... → skipped (needs z)

parametric.exactly

Like atLeast, but only calls functions with exactly matching args:

F = parametric.exactly { includes = [ a b c ]; };

Applied with { x = 1; y = 2; }:

{ x, ... }: ... → skipped (has ...)
{ x, y }: ... → called (exact match)
{ z }: ... → skipped

Use exactly to prevent duplicate configs when the same function would match multiple context stages.

parametric.fixedTo

Replaces the context entirely:

foo = parametric.fixedTo { planet = "Earth"; } {
  includes = [
    ({ planet, ... }: { nixos.setting = planet; })
  ];
};

No matter what context foo receives, its includes always get { planet = "Earth"; }.

parametric.expands

Adds attributes to the received context:

foo = parametric.expands { planet = "Earth"; } {
  includes = [
    ({ host, planet, ... }: {
      nixos.setting = "${host.name}/${planet}";
    })
  ];
};

Applied with { host = ...; }, the includes receive { host = ...; planet = "Earth"; }.

parametric.withOwn

Combinator that adds owned config and static includes on top of any dispatch functor:

parametric.withOwn parametric.atLeast {
  nixos.foo = "owned";       # included always
  includes = [
    { nixos.bar = "static"; }  # included always
    ({ host, ... }: { ... })   # dispatched via atLeast
  ];
}

Matching Rules Summary

Functor	Owned	Statics	Functions
`parametric` (default)	✓	✓	atLeast
`parametric.atLeast`	✗	✗	atLeast
`parametric.exactly`	✗	✗	exactly
`parametric.withOwn F`	✓	✓	uses F
`parametric.fixedTo ctx`	✓	✓	fixed ctx
`parametric.expands ctx`	✓	✓	ctx + received

take.exactly and take.atLeast

For individual functions (not whole aspects), use den.lib.take:

den.default.includes = [
  (den.lib.take.exactly ({ host }: { nixos.x = 1; }))
];

This prevents the function from matching { host, user } contexts, avoiding duplicate config values.