In @unified-myst/core-parse it has internally mapped dependencies: https://github.com/executablebooks/unified-myst/blob/d77a67e1a87104910a51fe0599cc738bc419f356/packages/core-parse/package.json#L56-L62

It then has a rollup config for building: https://github.com/executablebooks/unified-myst/blob/d77a67e1a87104910a51fe0599cc738bc419f356/packages/core-parse/rollup.config.js#L1

Firstly, when this gets published (va lerna) as an ESM module, these internal mappings are not "transformed" in any way. This means if one simply runs npm install @unified-myst/core-parse, it will not install any of these dependencies.

Secondly, trying to build a "standalone" bundle, I haven't yet worked out how to get @rollup/plugin-node-resolve to resolve these internal mappings.

Finally both lerna run build/lerna publish and npm run build -ws seem to not do any resolution of the order of building/publishing, e.g. that core-parses internal dependencies must be built before building core-parse itself

This extension adds the propagateTargets transform, to propagate target labels to the next viable node. For example:

import { Processor } from '@unified-myst/core-parse'
import * as ext from '@unified-myst/core-parse/extensions'
import { removePosition } from 'unist-util-remove-position'
import { dump } from 'js-yaml'

const parser = new Processor()
    .use(ext.syntaxMystExtension)
    .use(ext.figureExtension)
    .use(ext.targetExtension)

let text = `
(target1)=
% comment
(target2)=
# header

- (target3)=
  paragraph

(target4)=
\`\`\`{image} a.png
:name: other
\`\`\`

(target5)=
`

let ast = parser.toAst(text).ast
ast = removePosition(ast, true)

console.log(dump(ast))

Results in:

type: root
children:
  - type: mystTarget
    label: target1
  - type: mystComment
    value: ' comment'
  - type: mystTarget
    label: target2
  - type: heading
    depth: 1
    children:
      - type: text
        value: header
    mystIds:
      - target1
      - target2
  - type: list
    ordered: false
    start: null
    spread: false
    children:
      - type: listItem
        spread: false
        checked: null
        children:
          - type: mystTarget
            label: target3
          - type: paragraph
            children:
              - type: text
                value: paragraph
            mystIds:
              - target3
  - type: mystTarget
    label: target4
  - type: mystDirective
    name: image
    args:
      - a.png
    options:
      name: other
    value: ''
    bodyOffset: 2
    children:
      - type: image
        url: a.png
        label: other
        mystIds:
          - other
          - target4
  - type: mystTarget
    label: target5

and produces the warning:

{
  message: 'No node found to propagate target label to: target5',
  level: 'warning',
  position: {
    start: { line: 14, column: 1, offset: 102 },
    end: { line: 14, column: 11, offset: 112 }
  },
  type: 'propagateTargets'
}

The logger is passed to all role/directive processors and event hooks.

Any logger object can be initialized on the processor, which defines the logger interface, and the logger can have attached observers.

Extract all definitions and replace linkReference with their definitions. Example:

import { Processor } from '@unified-myst/core-parse'
import * as ext from '@unified-myst/core-parse/extensions'
import { removePosition } from 'unist-util-remove-position'
import { dump } from 'js-yaml'

const parser = new Processor().use(ext.linkReferenceExtension)
let text = `
[*Hi*][a]

[a]: https://example1.com
[a]: https://example2.com
`
let ast = parser.toAst(text).ast
ast = removePosition(ast, true)

console.log(dump(ast))

Generates:

type: root
children:
  - type: paragraph
    children:
      - type: link
        url: https://example1.com
        title: null
        children:
          - type: emphasis
            children:
              - type: text
                value: Hi

and logs warning:

{
  message: 'Duplicate definition identifier: a',
  level: 'warning',
  position: {
    start: { line: 6, column: 1, offset: 39 },
    end: { line: 6, column: 26, offset: 64 }
  },
  type: 'definition'
}

import { Processor } from '@unified-myst/core-parse'
import * as ext from '@unified-myst/core-parse/extensions'
import { removePosition } from 'unist-util-remove-position'
import { dump } from 'js-yaml'

const parser = new Processor()
    .use(ext.syntaxMystExtension)
    .use(ext.tableExtension)

let text = `
\`\`\`{table} This is a caption
:name: test
:class: a

| a | b |
| - | - |
| c | d |
\`\`\`
`

let ast = parser.toAst(text).ast
ast = removePosition(ast, true)

console.log(dump(ast))

Creates:

type: root
children:
  - type: mystDirective
    name: table
    args:
      - This is a caption
    options:
      name: test
      class:
        - a
    value: |-
      | a | b |
      | - | - |
      | c | d |
    bodyOffset: 4
    children:
      - type: container
        kind: table
        children:
          - type: caption
            children:
              - type: text
                value: This is a caption
          - type: table
            align:
              - null
              - null
            children:
              - type: tableRow
                children:
                  - type: tableCell
                    children:
                      - type: text
                        value: a
                  - type: tableCell
                    children:
                      - type: text
                        value: b
              - type: tableRow
                children:
                  - type: tableCell
                    children:
                      - type: text
                        value: c
                  - type: tableCell
                    children:
                      - type: text
                        value: d
        classes:
          - a
        label: test
        mystIds:
          - test

Realize that the sphinx world abbreviation has "explanation", in mdast as well as HTML title is more common? https://github.com/executablebooks/unified-myst/blob/main/packages/core-parse/src/extensions/inlineMarkup.js#L47

Myst node is currently defined here: https://executablebooks.github.io/myst-spec/myst.schema.html#abbreviation

I am looking to contribute an extension here to get a feel for how it works, which should I choose?

There is a decent list of features here: https://github.com/executablebooks/mystjs#mystjs-features