Patterns

A pattern is an arrangement of Blocks an author inserts as one: a section that opens with a heading, a hero with its button. The Catalog holds it as a tree, in the form a Document stores, under ids of its own:

const Site = Catalog.make({
  blocks: [Heading, Section],
  roots: [Content.Section],
  patterns: [
    {
      name: 'Intro',
      description: 'A section that opens with a heading',
      tree: {
        root: 'intro',
        nodes: {
          intro: { block: 'Section', props: { tone: 'plain' }, regions: { body: ['title'] } },
          title: { block: 'Heading', props: { text: 'Welcome', level: 1 }, regions: {} },
        },
      },
    },
  ],
})

const { Op, root } = Composition
Composition.apply(
  Site,
  page,
  Op.usePattern({
    pattern: 'Intro',
    ids: {
      [NodeId.make('intro')]: NodeId.make('intro-2'),
      [NodeId.make('title')]: NodeId.make('title-2'),
    },
    at: root(1),
  }),
)

• Checked where the Catalog is made: a tree that does not decode, does not hold together, or names a Block, a prop or a Region the Catalog refuses throws there, not when an author first uses it. Where it may go is checked by each use, as any insert is.
• usePattern carries a new id for each of its nodes, by the id it has in the pattern, like duplicate, so a replay makes the same nodes. A pattern the Catalog lacks is composition:unknown-pattern.
• An agent's tool takes it too. operationSchema has one UsePattern per pattern, its name a literal and its ids a struct of exactly its nodes' ids, described by its label and description: an agent inserts structure the Catalog vouches for without being trusted with an arbitrary tree.
• Catalog.pattern(catalog, name) finds one (an Option); its words are its label, by default its name spaced, and its description.

Undo#

Undo is the editor's state, not the page's, so this package has none. The Builder keeps the page in an undo history from foldkit-primitives/state, whose present is the Document, and records each applied Operation in the same transition that makes it. A snapshot shares every node the edit did not change.