The cycle form

Each key of an aspect gives one element. For 0 to n elements, or to ask the tree for data, write a component under cycle. As in Cycle.js, a component is a function from sources to sinks. Its sources are tree, scope and one source for each sink. It returns sinks by name, and tree requests.

modules/network.nix

# A component: it reads sources (the tree, the sinks) and writes sinks. It gives 0 to n elements.
{ yen, ... }:
{
  yen.aspects.vm-network = {
    at = yen.lib.has yen.kinds.guest; # every host with a guest
    cycle =
      { tree, ... }:
      {
        tree = [
          {
            category = "vms";
            children = yen.kinds.guest;
          }
        ]; # a request: the guest children
        bridges = map (g: "br-${g.name}") (builtins.head (tree.select "vms")); # one element per guest
        errors = [ ];
      };
  };
}

The host has two guests, so the component gives two bridges:

result

{
  server = [
    "br-ci"
    "br-db"
  ];
}

Tree requests

A request has a category and one question. tree.select category gives the answers.

Request	Answer
nearest = yen.kinds.environment	the nearest scope of that kind
parent = true	the parent scope
children = predicate	the children where the predicate holds
scopesOf = instance	every scope of an instance
holds = predicate	true or false at this scope

When to use it

Most aspects never need cycle. Use it when the number of elements depends on the tree, or when a component reports its own errors.

Run this example

Every block on this page is a file of the template cycle. Copy it and run it:

nix flake init -t github:vic/yen#cycle

• flake.nix
• Directorymodules/

  • fleet.nix
  • network.nix
  • sinks.nix