Context-Aware Configurations

From Static to Dynamic

So far, aspects have been static attribute sets. But Den’s power comes from making them functions of context. When an aspect is a function, Den passes it the current context — host, user, platform — and the function decides what to produce.

Step 1: A Simple Context Function

Instead of a static aspect, write a function:

{ den, ... }: {
  den.aspects.igloo.includes = [
    ({ host, ... }: {
      nixos.networking.hostName = host.name;
    })
  ];
}

Den calls this function with { host } containing the host’s attributes. The result sets the hostname dynamically from the host’s name.

Step 2: React to User Context

Functions receiving { host, user, ... } run once per user on each host:

{ den, ... }: {
  den.default.includes = [
    ({ host, user, ... }: {
      ${host.class}.users.users.${user.userName}.description =
        "${user.userName} on ${host.name}";
    })
  ];
}

Notice ${host.class} — this dynamically targets nixos or darwin depending on the host’s platform. The same function works on both.

Step 3: Conditional Configuration

Use standard Nix conditionals inside context functions:

{ den, lib, ... }:
let
  git-for-linux = { user, host, ... }:
    if !lib.hasSuffix "darwin" host.system
    then { homeManager.programs.git.enable = true; }
    else { };
in {
  den.aspects.tux.includes = [ git-for-linux ];
}

Git is enabled only for users on Linux hosts. On Darwin, the function returns an empty set — no effect.

Step 4: Use den.lib.parametric

For aspects that need to forward context to their own includes, wrap them with den.lib.parametric:

{ den, ... }:
let
  workspace = den.lib.parametric {
    nixos.networking.hostName = "from-parametric";
    includes = [
      ({ host, ... }: { nixos.time.timeZone = "UTC"; })
    ];
  };
in {
  den.aspects.igloo.includes = [ workspace ];
}

The parametric functor ensures that:

1. Owned config (nixos.networking.hostName) is always included
2. Functions in includes receive the forwarded context
3. Functions whose parameters don’t match are silently skipped

Step 5: Understand Matching

Den matches context by argument names, not values:

Function signature	{ host }	{ host, user }
{ host, ... }: ...	✓	✓
{ host, user }: ...	✗	✓
{ never, ... }: ...	✗	✗

• atLeast — function is called if context has at least the required args
• exactly — function is called only if context has exactly those args

What You’ve Learned

• Aspects can be functions that receive { host, user, home, ... }
• Context functions produce conditional, platform-aware configurations
• den.lib.parametric forwards context to nested includes
• Unmatched functions are silently skipped — no errors
• ${host.class} makes configs target the right Nix class dynamically

Next

• Bidirectional Dependencies — hosts and users configure each other
• Context System — deep dive into den.ctx