Answer architecture round 1: splitTaskList groups a list into items, bulletBlocks returns the split blocks itself, isTaskItem lives in adf/, a property reads generated task lists through portableMarkdownToAdf, and items 56 and 93 name today's code

src/adf/block-nodes.ts

@@ -1,3 +1,4 @@
+import type { AdfNode } from './document.ts'
 import type { AttributeVocabulary } from './attribute-vocabulary.ts'
 
 export type BlockNodeModel = {
@@ -88,6 +89,10 @@ export function blockNodeModel(type: string): BlockNodeModel | undefined {
   return isBlockType(type) ? blockNodes[type] : undefined
 }
 
+export function isTaskItem(node: AdfNode | undefined): boolean {
+  return node?.type === 'taskItem' || node?.type === 'blockTaskItem'
+}
+
 function isBlockType(type: string): type is BlockType {
   return Object.hasOwn(blockNodes, type)
 }

src/conformance/markdown-property.test.ts

@@ -9,10 +9,11 @@ import type { JsonValue } from '../json-value.ts'
 import type { Result } from '../result.ts'
 import { adfDocument, attributes, jsonKey, jsonValue, markdownPieces, propertyRuns, propertyTimeout, textOf } from './property-harness.ts'
 import { adfToLosslessMarkdown } from '../markdown/emit/adf-to-markdown.ts'
+import { adfToPortableMarkdown } from '../markdown/portable/adf-to-portable-markdown.ts'
 import { blockArgument, listBreakName, marksAttribute } from '../markdown/block-directive.ts'
 import { blockNodes } from '../adf/block-nodes.ts'
 import { carryName } from '../markdown/opaque-carry.ts'
-import { commonMarkToAdf, losslessMarkdownToAdf } from '../markdown/parse/markdown-to-adf.ts'
+import { commonMarkToAdf, losslessMarkdownToAdf, portableMarkdownToAdf } from '../markdown/parse/markdown-to-adf.ts'
 import {
   directivePrefix,
   spellAttributes,
@@ -409,6 +410,24 @@ const document = fc
   .tuple(markdown, fc.oneof({ arbitrary: fc.constant('\n'), weight: 8 }, { arbitrary: fc.constantFrom('\r\n', '\r'), weight: 1 }), fc.constantFrom('', '', '\n', '  \n'))
   .map(([held, ending, trailing]) => `${held}${trailing}`.replaceAll('\n', ending))
 
+function taskListOf(bodies: Arbitrary<string>): Arbitrary<string> {
+  return fc
+    .array(fc.tuple(fc.constantFrom('[ ]', '[x]'), fc.constantFrom('', 'Done\n\n', 'Done\n'), bodies), { maxLength: 3, minLength: 1 })
+    .map((items) => items.map(([marker, lead, body]) => prefixLines(`${lead}${body}`, `- ${marker} `, () => '      '.slice(0, marker.length + 3))).join('\n'))
+}
+
+const taskMarkdown = taskListOf(fc.oneof(cleanMarkdown, hostileMarkdown, taskListOf(cleanMarkdown)))
+
+function blockTaskItemChildren(document: AdfDocument): string[] {
+  const types: string[] = []
+  const pending = [...nodeContent(document)]
+  for (let node = pending.pop(); node !== undefined; node = pending.pop()) {
+    if (node.type === 'blockTaskItem') for (const child of nodeContent(node)) types.push(child.type)
+    pending.push(...nodeContent(node))
+  }
+  return types
+}
+
 function holdsDirectiveShape(document: AdfDocument): boolean {
   const pending = [...nodeContent(document)]
   for (let node = pending.pop(); node !== undefined; node = pending.pop()) {
@@ -458,3 +477,19 @@ test('generated markdown read as CommonMark refuses, or its document reads back
     propertyRuns(gateRuns),
   )
 })
+
+test('generated task lists read as portable markdown refuse, or build block task items of paragraphs alone and write a fixpoint', { timeout: propertyTimeout }, () => {
+  fc.assert(
+    fc.property(taskMarkdown, (input) => {
+      const parsed = portableMarkdownToAdf(input)
+      if (!parsed.ok) return
+      assert.deepEqual(blockTaskItemChildren(parsed.value).filter((type) => type !== 'paragraph'), [], `reading ${JSON.stringify(input)}`)
+      const written = adfToPortableMarkdown(parsed.value)
+      assert.ok(written.ok, written.ok ? '' : `${written.error.code}: ${written.error.message} — spelling ${JSON.stringify(input)}`)
+      const read = portableMarkdownToAdf(written.value)
+      assert.ok(read.ok, read.ok ? '' : `${read.error.code}: ${read.error.message} — reading ${JSON.stringify(written.value)}`)
+      assert.deepEqual(adfToPortableMarkdown(read.value), written, `reading ${JSON.stringify(written.value)}`)
+    }),
+    propertyRuns(gateRuns),
+  )
+})

src/markdown/emit/adf-to-markdown.ts

@@ -4,7 +4,7 @@ import type { WrittenFlavour } from '../portable/conventions.ts'
 import { adfDocumentFault, holdsOnlyAttributes, isUnmarkedSpellableText, nodeAttrs, nodeContent } from '../../adf/document.ts'
 import { alertMarker, flavourClaims, foldedAlertMarker, leadingMarker, readAlertMarker, readTaskMarker, taskMarker } from '../portable/conventions.ts'
 import { blockDirectiveForm, documentSpelling, listBreakSpelling } from '../block-directive.ts'
-import { blockNodeModel, blockNodes } from '../../adf/block-nodes.ts'
+import { blockNodeModel, blockNodes, isTaskItem } from '../../adf/block-nodes.ts'
 import { carriedBlock } from '../opaque-carry.ts'
 import { emitInlineLine } from './inline-line.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
@@ -167,7 +167,7 @@ function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writi
   let headroom = largestNesting - depth - 1
   // A child other than a task nests in the task before it.
   for (const [index, child] of nodeContent(node).entries()) {
-    const task = child.type === 'taskItem' || child.type === 'blockTaskItem'
+    const task = isTaskItem(child)
     const walk = task ? taskBlocks(child, [...path, 'content', index], depth + 1, writing) : placedBlock(child, [...path, 'content', index], depth + 1, writing)
     if (!walk.ok) return walk
     headroom = Math.min(headroom, walk.value.headroom)

src/markdown/parse/markdown-to-adf.ts

@@ -81,10 +81,15 @@ function readBlocks(blocks: readonly ReadBlock[], reading: Reading, path: Conver
       if (fault !== undefined) return positioned(faulted(fault, nodePath), block.position)
       continue
     }
+    if (block.kind === 'bulletList' && reading.claims.taskMarkers) {
+      const listed = positioned(bulletBlocks(block.items, reading, nodePath, depth), block.position)
+      if (!listed.ok) return listed
+      for (const node of listed.value) content.push(node)
+      continue
+    }
     const node = positioned(readBlock(block, reading, nodePath, depth), block.position)
     if (!node.ok) return node
-    if (node.value.type !== 'taskList' || block.kind !== 'bulletList') content.push(node.value)
-    else for (const split of splitTaskList(node.value)) content.push(split)
+    content.push(node.value)
   }
   return success(content)
 }
@@ -116,7 +121,7 @@ function readBlock(block: ReadBlock, reading: Reading, path: ConvertErrorPath, d
     case 'blockquote':
       return reading.claims.alerts ? quoteNode(block.blocks, reading, path, depth) : containerNode({ type: 'blockquote' }, block.blocks, reading, path, depth)
     case 'bulletList':
-      return reading.claims.taskMarkers ? bulletNode(block.items, reading, path, depth) : listNode({ type: 'bulletList' }, block.items, reading, path, depth)
+      return listNode({ type: 'bulletList' }, block.items, reading, path, depth)
     case 'code':
       return codeBlockNode(block.language, block.text, reading, path, depth)
     case 'directive':
@@ -198,11 +203,14 @@ function filledNode(node: AdfNode, content: Result<AdfNode[]>): Result<AdfNode>
   return success({ ...node, content: content.value.length === 0 ? [{ type: 'paragraph' }] : content.value })
 }
 
-function bulletNode(items: readonly Block[][], reading: Reading, path: ConvertErrorPath, depth: number): Result<AdfNode> {
+function bulletBlocks(items: readonly Block[][], reading: Reading, path: ConvertErrorPath, depth: number): Result<AdfNode[]> {
   const led = []
   for (const [first, ...others] of items) {
     const marked = markerLed(first, readTaskMarker)
-    if (marked === undefined) return listNode({ type: 'bulletList' }, items, reading, path, depth)
+    if (marked === undefined) {
+      const list = listNode({ type: 'bulletList' }, items, reading, path, depth)
+      return list.ok ? success([list.value]) : list
+    }
     led.push({ ...marked, others })
   }
   const tasks: AdfNode[] = []
@@ -213,7 +221,7 @@ function bulletNode(items: readonly Block[][], reading: Reading, path: ConvertEr
     if (!read.ok) return read
     tasks.push({ attrs: { state: marker.state }, content: read.value, type: 'blockTaskItem' })
   }
-  return success({ content: tasks, type: 'taskList' })
+  return success(splitTaskList({ content: tasks, type: 'taskList' }))
 }
 
 function directiveNode(block: DirectiveBlock, reading: Reading, path: ConvertErrorPath, depth: number): Result<AdfNode> {

src/markdown/portable/adf-to-portable-markdown.ts

@@ -1,6 +1,6 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
 import { adfDocumentFault, nodeAttrs, nodeContent } from '../../adf/document.ts'
-import { blockNodeModel } from '../../adf/block-nodes.ts'
+import { blockNodeModel, isTaskItem } from '../../adf/block-nodes.ts'
 import { centeredImage, externalMedia } from '../external-image.ts'
 import { commonMarkSpelling, largestListMarker, writeMarkdown, type SpellingMemo } from '../emit/adf-to-markdown.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
@@ -179,7 +179,7 @@ function tasksAsText(list: AdfNode): AdfNode {
   if (list.type !== 'taskList') return list
   const items: AdfNode[] = []
   for (const child of nodeContent(list)) {
-    const previous = isTask(child) || child.type === 'listItem' ? undefined : items.pop()
+    const previous = isTaskItem(child) || child.type === 'listItem' ? undefined : items.pop()
     items.push(itemOf(previous === undefined ? taskAsText(child) : mergedLists([...nodeContent(previous), child])))
   }
   return { content: items, type: 'bulletList' }
@@ -285,27 +285,28 @@ function blankedLines(code: readonly AdfNode[]): AdfNode[] {
   return blanked === '' ? [] : [text(blanked)]
 }
 
-// A task list opening with a task and holding tasks and task lists alone keeps its spelling, a list nesting in the task before it; any other keeps its markers as text. A child reducing to nothing counts for neither.
+// A task list opening with a task and holding tasks and task lists alone keeps its spelling; any other keeps its markers as text. A child reducing to nothing counts for neither.
 function reduceTaskList(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
   const kept: { blocks: AdfNode[]; child: AdfNode }[] = []
   for (const [index, child] of nodeContent(node).entries()) {
     const at = childReduction(reduction, index)
-    const reduced = isTask(child) ? reduceTask(child, at) : reduceStanding(child, at)
+    const reduced = isTaskItem(child) ? reduceTask(child, at) : reduceStanding(child, at)
     if (!reduced.ok) return reduced
     if (reduced.value.length > 0) kept.push({ blocks: reduced.value, child })
   }
-  const regular = isTask(kept[0]?.child) && kept.every(({ child }) => isTask(child) || child.type === 'taskList')
+  const regular = isTaskItem(kept[0]?.child) && kept.every(({ child }) => isTaskItem(child) || child.type === 'taskList')
   const tasks: AdfNode[] = []
   for (const { blocks, child } of kept) {
-    const standsAlone = !regular && !isTask(child) && child.type !== 'taskList'
+    const standsAlone = !regular && !isTaskItem(child) && child.type !== 'taskList'
     for (const block of standsAlone ? [{ content: blocks, type: 'listItem' }] : blocks) tasks.push(block)
   }
-  if (regular) return success(splitTaskList({ content: tasks, type: 'taskList' }).map((block) => (block.type === 'taskList' ? { ...block, content: mergedLists(nodeContent(block)) } : block)))
+  if (regular) return success(splitTasks({ content: tasks, type: 'taskList' }))
   return success(listOf(nodeContent(tasksAsText({ content: tasks, type: 'taskList' })), 'bulletList'))
 }
 
-function isTask(node: AdfNode | undefined): boolean {
-  return node?.type === 'taskItem' || node?.type === 'blockTaskItem'
+// The split sets a task's nested lists beside the list after it, and adjacent lists read back as one.
+function splitTasks(list: AdfNode): AdfNode[] {
+  return splitTaskList(list).map((block) => (block.type === 'taskList' ? { ...block, content: mergedLists(nodeContent(block)) } : block))
 }
 
 function reduceTask(task: AdfNode, at: Reduction): Result<AdfNode[]> {

src/markdown/portable/task-list.ts

@@ -1,36 +1,55 @@
 import type { AdfNode } from '../../adf/document.ts'
+import { isTaskItem } from '../../adf/block-nodes.ts'
 import { nodeContent } from '../../adf/document.ts'
 
-// Atlassian's schema holds a blockTaskItem to paragraphs and extensions, and nests a task list beside its item.
+// Atlassian's schema holds a blockTaskItem to paragraphs and extensions, and the portable flavour spells no extension: an item keeps its paragraphs, then the task lists nested in it, and the rest of it stands after the list, which resumes at the next task.
 export function splitTaskList(list: AdfNode): AdfNode[] {
   const blocks: AdfNode[] = []
   let tasks: AdfNode[] = []
-  let standing = false
-  for (const child of nodeContent(list)) {
-    const isTask = child.type === 'taskItem' || child.type === 'blockTaskItem'
-    if (isTask) standing = false
-    let stands: readonly AdfNode[] = standing || !(isTask || child.type === 'taskList') ? [child] : []
-    if (child.type === 'blockTaskItem') {
-      const content = nodeContent(child)
-      let held = 0
-      while (content[held]?.type === 'paragraph') held += 1
-      let nested = held
-      while (content[nested]?.type === 'taskList') nested += 1
-      tasks.push(heldTask(child, content.slice(0, held)))
-      for (const taskList of content.slice(held, nested)) tasks.push(taskList)
-      stands = content.slice(nested)
-    } else if (stands.length === 0) tasks.push(child)
-    if (stands.length === 0) continue
+  for (const item of itemsOf(nodeContent(list))) {
+    const { kept, rest } = itemParts(item)
+    for (const node of kept) tasks.push(node)
+    if (rest.length === 0) continue
     if (tasks.length > 0) blocks.push({ ...list, content: tasks })
     tasks = []
-    for (const block of stands) blocks.push(block)
-    standing = true
+    for (const block of rest) blocks.push(block)
   }
   if (tasks.length > 0) blocks.push({ ...list, content: tasks })
   return blocks
 }
 
-function heldTask(task: AdfNode, paragraphs: readonly AdfNode[]): AdfNode {
+// An item is a task and the children up to the next one.
+function itemsOf(children: readonly AdfNode[]): AdfNode[][] {
+  const items: AdfNode[][] = []
+  for (const child of children) {
+    const item = items.at(-1)
+    if (item === undefined || isTaskItem(child)) items.push([child])
+    else item.push(child)
+  }
+  return items
+}
+
+function itemParts(item: readonly AdfNode[]): { kept: AdfNode[]; rest: AdfNode[] } {
+  const [task] = item
+  if (task === undefined || !isTaskItem(task)) return { kept: [], rest: [...item] }
+  const content = task.type === 'blockTaskItem' ? nodeContent(task) : []
+  const paragraphs = runEnd(content, 0, 'paragraph')
+  const nested = runEnd(content, paragraphs, 'taskList')
+  const unheld = content.slice(nested)
+  const after = item.slice(1)
+  const beside = unheld.length === 0 ? runEnd(after, 0, 'taskList') : 0
+  const head = task.type === 'blockTaskItem' ? taskOfParagraphs(task, content.slice(0, paragraphs)) : task
+  return { kept: [head, ...content.slice(paragraphs, nested), ...after.slice(0, beside)], rest: [...unheld, ...after.slice(beside)] }
+}
+
+function runEnd(nodes: readonly AdfNode[], start: number, type: string): number {
+  let end = start
+  while (nodes[end]?.type === type) end += 1
+  return end
+}
+
+// One paragraph or none reads as a taskItem.
+function taskOfParagraphs(task: AdfNode, paragraphs: readonly AdfNode[]): AdfNode {
   const [only] = paragraphs
   if (paragraphs.length > 1) return { ...task, content: [...paragraphs] }
   return { ...task, content: nodeContent(only ?? {}).slice(), type: 'taskItem' }

todo.md

@@ -91,10 +91,10 @@ of one character from each of its characters.
 
 ### 93. Report a refusal's `path` where the node would stand after `portableMarkdownToAdf` moves it out of a task item.
 
-`readBlocks` fixes a block's path before `splitTaskList` and `bulletNode` move blocks beside or
-after their task, so `- [x] A\n\n  ```\n  x\n  ```\n\n  <div>x</div>\n` reports
-`/content/0/content/0/content/2` for a block that would stand at `/content/2`. The README says
-`path` is the node's place from the document root.
+`bulletBlocks` fixes a block's path before `splitTaskList` moves it beside or after its task, so
+`- [x] A\n\n  ```\n  x\n  ```\n\n  <div>x</div>\n` reports `/content/0/content/0/content/2`
+for a block that would stand at `/content/2`. The README says `path` is the node's place from the
+document root.
 
 ### 86. Write an image whose `media` only a link marks, and one captioned with plain text, in the CommonMark spelling a writer panel picks, in `adfToLosslessMarkdown`.
 
@@ -354,7 +354,7 @@ functions write `pending`, `pieces`, `deactivatedBefore` and `openingSpellableLi
 `mintTaskIds` (`portable/task-ids.ts`), which writes `attrs` on the document `readDocument` built;
 `takeFallback` (`emit/inline-line.ts`), which writes the record the next `lineSegments` pass reads;
 `writeUnpaired`, `markPairings` and `markHighlights` (`inline-content.ts`), whose order changes the
-output; and `nestIn` (`portable/adf-to-portable-markdown.ts`), which pushes into its caller's array.
+output.
 Once `Scan`'s pieces have owners, the bracket resolution — `linkedAlt` through `linkTo`,
 `holdsLink`, and `closeBracket` through `altText` — can leave `inline-content.ts` (about 690 lines)
 for a module of its own.