Quirks & Pipes

Quick start: firewall ports

The classic example — multiple aspects declare firewall ports, one aspect opens them.

Declare the quirk

den.quirks.firewall = {
  description = "Firewall port declarations";
};

Produce data

Any aspect can emit data on the firewall key. Producers don’t need to know who consumes it:

den.aspects.nginx = {
  nixos.services.nginx.enable = true;
  firewall = { ports = [ 80 443 ]; };
};

den.aspects.postgres = {
  nixos.services.postgresql.enable = true;
  firewall = { ports = [ 5432 ]; };
};

Consume data

A class module receives all firewall entries as a list by naming the quirk in its function arguments:

den.aspects.networking = {
  nixos = { firewall, lib, ... }: {
    networking.firewall.allowedTCPPorts =
      lib.concatMap (f: f.ports or []) firewall;
  };
};

Include everything

den.aspects.igloo = {
  includes = [
    den.aspects.nginx
    den.aspects.postgres
    den.aspects.networking
  ];
};

Result: networking.firewall.allowedTCPPorts = [ 80 443 5432 ].

No pipe policy needed — same-scope aggregation works out of the box. If no producers emit data, the consumer receives [].

Pipe policies: filter, transform, fold

When you need to process data before consumers see it, use pipe policies. All pipe stages are accessed via den.lib.policy.pipe.

Filtering

Remove entries that don’t match a predicate:

den.policies.tcp-only = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "firewall" [
      (pipe.filter (e: e.proto == "tcp"))
    ])
  ];

den.default.includes = [ den.policies.tcp-only ];

Given producers emitting [{ port = 80; proto = "tcp"; } { port = 53; proto = "udp"; }], consumers see only [{ port = 80; proto = "tcp"; }].

Transforming

Map each entry to a new shape:

den.policies.label-items = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "items" [
      (pipe.transform (i: { label = "x-${i.name}"; }))
    ])
  ];

Folding

Reduce all entries to a single value:

den.policies.sum-nums = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "nums" [
      (pipe.fold (acc: n: acc + n) 0)
    ])
  ];

The consumer receives a single-element list: [ 60 ] for inputs [ 10 20 30 ].

Appending

Add a synthetic entry to the pool:

den.policies.add-default = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "items" [
      (pipe.append { name = "default"; })
    ])
  ];

Replacing with `pipe.for`

Replace the entire list. At most one pipe.for per pipe per scope:

den.policies.reverse-items = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "items" [
      (pipe.for (vals: lib.reverseList vals))
    ])
  ];

Chaining stages

Stages compose left-to-right:

pipe.from "items" [
  (pipe.filter (i: i.keep))
  (pipe.transform (i: { label = "x-${i.name}"; }))
]

Cross-scope flow

Upward: `pipe.expose`

Push child-scope data to the parent. A user’s preferences reach the host:

den.quirks.prefs = { description = "User preferences"; };

den.aspects.tux = {
  prefs = [{ editor = "vim"; }];
};

den.aspects.igloo = {
  includes = [ den.aspects.host-consumer ];
};

den.aspects.host-consumer = {
  nixos = { prefs, ... }: {
    networking.hostName =
      lib.concatMapStringsSep "-" (p: p.editor) prefs;
  };
};

den.policies.expose-prefs = { host, user, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "prefs" [ pipe.expose ]) ];

den.default.includes = [ den.policies.expose-prefs ];

Result: host consumer sees [{ editor = "vim"; }].

Exposed data merges with host-local data. If the host aspect also emits prefs, consumers see both host-local and exposed user entries.

Transform stages run before expose:

pipe.from "items" [
  (pipe.filter (i: i.keep))
  (pipe.transform (i: { label = "x-${i.name}"; }))
  pipe.expose
]

Lateral: `pipe.collect`

Harvest data from sibling scopes. Siblings are scopes sharing the same parent in the scope tree. This is how you do cross-host aggregation:

den.hosts.x86_64-linux.igloo.users.tux = {};
den.hosts.x86_64-linux.iceberg.users.alice = {};

den.quirks.http-backends = {
  description = "HTTP backends";
};

den.aspects.iceberg = {
  http-backends = { addr = "10.0.0.2"; port = 80; };
};

den.aspects.igloo = {
  includes = [ den.aspects.lb-consumer ];
  http-backends = { addr = "10.0.0.1"; port = 8080; };
};

den.aspects.lb-consumer = {
  nixos = { http-backends, lib, ... }: {
    networking.hostName =
      lib.concatMapStringsSep "," (b: "${b.addr}:${toString b.port}")
        http-backends;
  };
};

den.policies.fleet-backends = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "http-backends" [
      (pipe.collect ({ host, ... }: true))
    ])
  ];

den.schema.host.includes = [ den.policies.fleet-backends ];

The predicate in pipe.collect filters which sibling scopes to harvest from. Entity kind matching ensures only scopes of the right kind are included.

Collect with provenance

Track where collected data came from:

pipe.from "http-backends" [
  (pipe.collect ({ host, ... }: true))
  pipe.withProvenance
]

Consumers receive [{ value = <data>; source = <context>; }] where source is the full context attrset ({ host = ...; }) of the source scope.

Collect with filtering

Stages after pipe.collect operate on the combined pool:

pipe.from "http-backends" [
  (pipe.collect ({ host, ... }: true))
  (pipe.filter (b: b.port != 8080))
]

Targeted delivery with `pipe.to`

Route pipe data to specific aspects by identity:

den.policies.route-firewall = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "firewall" [
      (pipe.to [ den.aspects.networking ])
    ])
  ];

Only the named aspects receive the data. Other consumers of the same pipe see the unmodified pool.

Recipe: per-aspect secrets delivery

Combine pipe.filter, pipe.append, pipe.for, and pipe.to to deliver a custom attrset to a specific aspect:

den.quirks.secrets = { description = "Secret paths"; };

den.policies.postgres-secrets = { host, ... }:
  let inherit (den.lib.policy) pipe; in
  [ (pipe.from "secrets" [
      (pipe.filter (_: false))          # discard the pool
      (pipe.append { db-password = "/run/secrets/pg-pass"; })
      (pipe.for lib.mergeAttrsList)     # merge into a single attrset
      (pipe.to [ den.aspects.postgres ])
    ])
  ];

den.schema.host.includes = [ den.policies.postgres-secrets ];

The consumer receives a single merged attrset instead of a list:

den.aspects.postgres = {
  nixos = { secrets, ... }: {
    services.postgresql.settings.password_file = secrets.db-password;
  };
};

This pattern works because pipe.for replaces the list with the fold result, and pipe.to delivers only to the named aspect.

Config-dependent thunks

Quirk values can depend on a host’s NixOS config. Declare a function taking { config, ... } as the quirk value:

den.aspects.my-service = {
  nixos.services.my-service.enable = true;
  firewall = { config, ... }: {
    ports = [ config.services.my-service.port ];
  };
};

Local thunks are resolved lazily inside evalModules. Cross-host thunks (via pipe.collect) are resolved eagerly against the source host’s config.