README.md

@@ -147,12 +147,17 @@ read replaces mentions, attachments and macros with text.
   `_(image not included)_`, `_(jira-issues-table not included)_`, `_(synced block not included)_`,
   `_(link card not included)_`, `_(extension not included)_`.
 - `backgroundColor`, `code`, `em`, `link`, `strike` and `strong` stay; every other mark drops,
-  keeping its text, and so does a mark CommonMark cannot spell where it stands.
+  keeping its text, and so does a mark the flavour cannot spell where it stands.
 - A newline in text is a hard break and in an expand's title a space, edge whitespace outside a
   link or code span is trimmed, carriage returns and null characters are removed, and an empty
   paragraph drops.
 - An ordered list numbered past `999999999`, or adjacent ordered lists whose numbering does not
   continue, is one bullet list keeping its numbers as text.
+- A task list beside a bullet or decision list, or holding a block other than a task, joins one
+  bullet list keeping its states as text: `- \[x] Done`.
+- Text that would read as a marker takes a backslash: `==` wherever it could open or close a
+  highlight, `[!…]` opening a quote, and `[x]` or `[ ]` opening any list item, since GitHub reads
+  that marker per item.
 - A node read back carries no `localId`, which Atlassian's schema requires on `taskList`,
   `taskItem` and `blockTaskItem`: mint one where the receiving site requires it.
 

spec/flavour.md

@@ -67,9 +67,7 @@ normalizes to it through the round-trip.
   matching below, which is what lets the emitter decide its own pairings.
 - Blocks separated by one blank line at document level, inside a blockquote and between CommonMark
   blocks; inside a directive container a pair holding a directive block takes none. No trailing
-  whitespace outside a code
+  whitespace outside a code block's content, single trailing newline; a document with no blocks is the empty string.
-  block's
-  content, single trailing newline; a document with no blocks is the empty string.
 
 ## Directives
 

src/conformance/adf-property.test.ts

@@ -2,9 +2,10 @@ import assert from 'node:assert/strict'
 import fc from 'fast-check'
 import test from 'node:test'
 
+import type { AdfNode } from '../adf/document.ts'
 import { adfDocument, propertyRuns, propertyTimeout } from './property-harness.ts'
 import { adfToMarkdown } from '../markdown/emit/adf-to-markdown.ts'
-import { adfToPlainMarkdown } from '../markdown/emit/plain-reduction.ts'
+import { adfToPlainMarkdown, reduceToPlain } from '../markdown/emit/plain-reduction.ts'
 import { directivePrefix } from '../markdown/directive-syntax.ts'
 import { markdownToAdf, plainMarkdownToAdf } from '../markdown/parse/markdown-to-adf.ts'
 import { toEditorNormal } from '../adf/editor-normal.ts'
@@ -19,6 +20,20 @@ function readsNoDirective(markdown: string): boolean {
   return read.ok && renamed.ok && JSON.stringify(read.value).replaceAll(directivePrefix, renamedPrefix) === JSON.stringify(renamed.value)
 }
 
+// Each block's text, an expand's title and an image's alt and url, in document order: what the plain pair keeps.
+function shownText(nodes: readonly AdfNode[]): string[] {
+  const shown: string[] = []
+  for (const node of nodes) {
+    const attrs = node.attrs ?? {}
+    if ((node.type === 'expand' || node.type === 'nestedExpand') && typeof attrs['title'] === 'string') shown.push(attrs['title'])
+    if (node.type === 'media') shown.push(`${JSON.stringify(attrs['alt'] ?? '')} ${JSON.stringify(attrs['url'])}`)
+    const content = node.content ?? []
+    if (content.some((child) => child.type === 'text' || child.type === 'hardBreak')) shown.push(content.map((child) => child.text ?? '\n').join(''))
+    else for (const text of shownText(content)) shown.push(text)
+  }
+  return shown.filter((text) => text !== '')
+}
+
 test('a generated document refuses to emit, or its markdown reads back to it', { timeout: propertyTimeout }, () => {
   fc.assert(
     fc.property(adfDocument, (document) => {
@@ -32,7 +47,7 @@ test('a generated document refuses to emit, or its markdown reads back to it', {
   )
 })
 
-test('a generated document writes plain markdown refusing only what the guard refuses, and that markdown reads back to itself', { timeout: propertyTimeout }, () => {
+test('a generated document writes plain markdown refusing only what the guard refuses, and that markdown reads back to its text and to itself', { timeout: propertyTimeout }, () => {
   fc.assert(
     fc.property(adfDocument, (document) => {
       const written = adfToPlainMarkdown(document)
@@ -40,6 +55,8 @@ test('a generated document writes plain markdown refusing only what the guard re
       assert.ok(readsNoDirective(written.value), `a directive in ${JSON.stringify(written.value)}`)
       const read = plainMarkdownToAdf(written.value)
       assert.ok(read.ok, read.ok ? '' : `${read.error.code}: ${read.error.message} — reading ${JSON.stringify(written.value)}`)
+      const reduced = reduceToPlain(document)
+      assert.deepEqual(shownText(read.value.content ?? []), reduced.ok ? shownText(reduced.value.content ?? []) : reduced, `reading ${JSON.stringify(written.value)}`)
       assert.deepEqual(adfToPlainMarkdown(read.value), written, `reading ${JSON.stringify(written.value)}`)
     }),
     propertyRuns(gateRuns),

src/conformance/property-harness.ts

@@ -23,7 +23,7 @@ export const propertyTimeout = 600000
 const depthIdentifier = fc.createDepthIdentifier()
 const emptyCell: AdfNode = { content: [{ type: 'paragraph' }], type: 'tableCell' }
 const flatCommonMarkShapeWeight = 4
-export const markdownPieces = fc.constantFrom(...'aZ09 \t\n!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~é\xa0🎉', 'ab:', 'http://', directivePrefix, `${directivePrefix}a[`, `${directivePrefix}a{`)
+export const markdownPieces = fc.constantFrom(...'aZ09 \t\n!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~é\xa0🎉', '==', '[!NOTE]', '[x]', 'ab:', 'http://', directivePrefix, `${directivePrefix}a[`, `${directivePrefix}a{`)
 const nestingCommonMarkShapeWeight = 21
 const spelledTypes = new Set(['text', ...Object.keys(blockNodes), ...Object.keys(inlineNodes), ...Object.keys(markAttributes)])
 
@@ -80,6 +80,7 @@ function pipeTable({ body, header }: { body: AdfNode[][]; header: AdfNode[] }):
 
 const mark: Arbitrary<AdfMark> = fc.oneof(
   { arbitrary: fc.oneof(...Object.entries(markAttributes).map(([type, vocabulary]) => attributes(vocabulary).map((attrs) => ({ attrs, type })))), weight: 9 },
+  { arbitrary: attributes({ color: 'string' }).map((attrs) => ({ attrs, type: 'backgroundColor' })), weight: 2 },
   { arbitrary: fc.record({ attrs: fc.dictionary(jsonKey, jsonValue, { maxKeys: 2, noNullPrototype: true }), type: unknownType }), weight: 1 },
 )
 const marks = fc.uniqueArray(mark, { maxLength: 3, selector: (held) => held.type })
@@ -135,8 +136,12 @@ const positions = fc.letrec<Positions>((tie) => {
     paragraph,
     fc.record({ body: fc.array(fc.array(cell('tableCell'), { maxLength: 3 }), { maxLength: 2 }), header: fc.array(cell('tableHeader'), { maxLength: 3, minLength: 1 }) }).map(pipeTable),
   ]
+  const task = (type: string, content: Arbitrary<AdfNode[]>) =>
+    fc.record({ content, state: fc.constantFrom('DONE', 'TODO') }).map(({ content: held, state }): AdfNode => ({ attrs: { state }, content: held, type }))
+  const taskItem = fc.oneof({ arbitrary: task('taskItem', inlineContent), weight: 3 }, { arbitrary: task('blockTaskItem', blockContent), weight: 1 })
   const nestingCommonMarkShapes = [
     blockContent.map((content): AdfNode => ({ content, type: 'blockquote' })),
+    fc.array(fc.oneof({ arbitrary: taskItem, weight: 3 }, { arbitrary: tie('block'), weight: 1 }), { depthIdentifier, maxLength: 3, minLength: 1 }).map((content): AdfNode => ({ content, type: 'taskList' })),
     listItems.map((content): AdfNode => ({ content, type: 'bulletList' })),
     fc
       .record({ content: listItems, order: fc.oneof({ arbitrary: fc.integer({ max: 3, min: 0 }), weight: 4 }, { arbitrary: fc.integer({ max: 999999999, min: 0 }), weight: 1 }) })

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

@@ -1,6 +1,8 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
 import type { BlockNodeModel } from '../../adf/block-nodes.ts'
+import type { Flavour } from '../plain-conventions.ts'
 import { adfDocumentFault, carriesOnly, nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
+import { alertMarker, foldedAlertMarker, leadingMarker, readAlertMarker, readTaskMarker, taskMarker } from '../plain-conventions.ts'
 import { blockDirectiveForm, listBreakSpelling } from '../block-directive.ts'
 import { blockNodeModel, blockNodes } from '../../adf/block-nodes.ts'
 import { carriedBlock } from '../opaque-carry.ts'
@@ -24,28 +26,33 @@ type PlacedBlock = Omit<EmittedBlock, 'headroom'> & { node: AdfNode }
 export type SpellingMemo = Map<AdfNode, KeptSpelling>
 type Walk = { blocks: readonly PlacedBlock[]; headroom: number }
 type WalkedItem = { node: AdfNode; walk: Walk }
+export type Writing = { flavour: Flavour; memo: SpellingMemo | undefined }
 
 export const largestListMarker = 999999999
 // Bare because tryList admits no item carrying attributes, marks or text.
 const listItemOpener = spellDirectiveOpener('listItem', undefined, '')
 
 export function adfToMarkdown(document: AdfDocument): Result<string> {
+  return writeMarkdown(document, 'lossless')
+}
+
+export function writeMarkdown(document: AdfDocument, flavour: Flavour): Result<string> {
   const fault = adfDocumentFault(document)
   if (fault !== undefined) return faulted(fault, [])
   if (document.version !== 1) return failure('unsupported-document-version', `no markdown spelling carries ADF version ${document.version}`, [])
-  const walk = walkBlocks(nodeContent(document), [], 0, undefined)
+  const walk = walkBlocks(nodeContent(document), [], 0, { flavour, memo: undefined })
   if (!walk.ok) return walk
   const text = joinBlocks(walk.value.blocks, 'document')
   return success(text === '' ? '' : `${text}\n`)
 }
 
 // headroom: the least slack any depth guard below the walk has.
-function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<Walk> {
+function walkBlocks(nodes: readonly AdfNode[], path: ConvertErrorPath, depth: number, writing: Writing): Result<Walk> {
   let headroom = largestNesting - depth
   if (headroom < 0) return tooDeep(path)
   const blocks: PlacedBlock[] = []
   for (const [index, node] of nodes.entries()) {
-    const block = emitBlock(node, [...path, 'content', index], depth, memo)
+    const block = emitBlock(node, [...path, 'content', index], depth, writing)
     if (!block.ok) return block
     headroom = Math.min(headroom, block.value.headroom)
     blocks.push({ ...block.value, node })
@@ -81,51 +88,120 @@ function separationBetween(previous: PlacedBlock, next: PlacedBlock, container:
 
 function interruptsParagraph(node: AdfNode): boolean {
   const items = nodeContent(node)
-  const empty = items[0] === undefined || nodeContent(items[0]).length === 0
+  const empty = node.type !== 'taskList' && (items[0] === undefined || nodeContent(items[0]).length === 0)
   if (node.type !== 'orderedList') return markerInterruptsParagraph(undefined, empty)
   return markerInterruptsParagraph(listStart(node, items.length) ?? 0, empty)
 }
 
-function emitBlock(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<EmittedBlock> {
+function emitBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> {
   const model = blockNodeModel(node.type)
   if (model === undefined) return commonMarkLine(carriedBlock(node, path, depth))
-  const readable = readableBlock(node, path, depth, memo)
+  const readable = readableBlock(node, path, depth, writing)
   if (readable !== undefined) return readable
-  return emitDirectiveBlock(node, model, path, depth, () => walkBlocks(nodeContent(node), path, depth + 1, memo))
+  return emitDirectiveBlock(node, model, path, depth, () => walkBlocks(nodeContent(node), path, depth + 1, writing))
 }
 
-export function commonMarkSpelling(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo): Result<null> | undefined {
+export function commonMarkSpelling(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<null> | undefined {
-  const readable = readableBlock(node, path, depth, memo)
+  const readable = readableBlock(node, path, depth, writing)
   if (readable === undefined) return undefined
   if (!readable.ok) return readable
   return readable.value.spelling === 'directive' ? undefined : success(null)
 }
 
-function readableBlock(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<EmittedBlock> | undefined {
+function readableBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
+  const { memo } = writing
   const kept = memo?.get(node)
   if (kept !== undefined) {
     if (kept.block === undefined) return undefined
     // A read below the fill would skip the depth guards the walk it replaces runs (docs/decisions.md §The spelling memo).
     if (depth <= kept.depth) return success({ ...kept.block, headroom: kept.block.headroom + kept.depth - depth })
   }
-  const spelled = spellReadableBlock(node, path, depth, memo)
+  const spelled = spellReadableBlock(node, path, depth, writing)
   if (spelled === undefined) memo?.set(node, { block: undefined, depth })
   else if (spelled.ok) memo?.set(node, { block: spelled.value, depth })
   return spelled
 }
 
-function spellReadableBlock(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<EmittedBlock> | undefined {
+function spellReadableBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
-  if (node.type === 'blockquote') return tryBlockquote(node, path, depth, memo)
+  const plain = writing.flavour === 'plain' ? spellPlainBlock(node, path, depth, writing) : undefined
-  if (node.type === 'bulletList' || node.type === 'orderedList') return tryList(node, path, depth, memo)
+  if (plain !== undefined) return plain
+  if (node.type === 'blockquote') return tryBlockquote(node, path, depth, writing)
+  if (node.type === 'bulletList' || node.type === 'orderedList') return tryList(node, path, depth, writing)
   if (node.type === 'codeBlock') return tryCodeBlock(node, path)
-  if (node.type === 'heading') return tryHeading(node, path)
+  if (node.type === 'heading') return tryHeading(node, path, writing.flavour)
   if (node.type === 'mediaSingle') return readableText(tryImage(node, path))
-  if (node.type === 'paragraph') return tryParagraph(node, path)
+  if (node.type === 'paragraph') return tryParagraph(node, path, writing.flavour)
   if (node.type === 'rule') return readableText(tryRule(node))
-  if (node.type === 'table') return readableText(tryPipeTable(node, path))
+  if (node.type === 'table') return readableText(tryPipeTable(node, path, writing.flavour))
   return undefined
 }
 
+// The plain flavour's nodes, in the shapes the plain reduction leaves them.
+function spellPlainBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
+  if (node.type === 'panel') return quotedUnder(alertMarker(nodeAttrs(node)['panelType']), node, path, depth, writing)
+  if (node.type === 'taskList') return tryTaskList(node, path, depth, writing)
+  if (node.type !== 'expand' && node.type !== 'nestedExpand') return undefined
+  const title = nodeAttrs(node)['title']
+  if (typeof title !== 'string') return quotedUnder(foldedAlertMarker, node, path, depth, writing)
+  // The reader takes a title as lossless inline text.
+  const line = emitInlineLine([{ text: title, type: 'text' }], 'paragraph', path, 'lossless')
+  return line.ok ? quotedUnder(`${foldedAlertMarker} ${line.value}`, node, path, depth, writing) : line
+}
+
+function quotedUnder(head: string, node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> {
+  const inner = walkBlocks(nodeContent(node), path, depth + 1, writing)
+  if (!inner.ok) return inner
+  const body = joinBlocks(inner.value.blocks, 'document')
+  return success(commonMarkText(quoted(body === '' ? head : `${head}\n\n${body}`), inner.value.headroom))
+}
+
+function quoted(text: string): string {
+  return text
+    .split('\n')
+    .map((line) => (line === '' ? '>' : `> ${line}`))
+    .join('\n')
+}
+
+function tryTaskList(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> {
+  const items: PlacedBlock[][] = []
+  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 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)
+    const previous = items.at(-1)
+    if (task || previous === undefined) items.push([...walk.value.blocks])
+    else for (const block of walk.value.blocks) previous.push(block)
+  }
+  const lines = items.map((blocks) => tryListItemLines(joinBlocks(blocks, 'list-item'), '- '))
+  // The plain reduction leaves no task a list item cannot hold: a directive here would break the flavour.
+  return lines.includes(undefined) ? failure('unsupported-node-shape', 'a task holds blocks no list item spells', path) : success({ headroom, spelling: 'list', text: lines.join('\n') })
+}
+
+function placedBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<Walk> {
+  const block = emitBlock(node, path, depth, writing)
+  return block.ok ? success({ blocks: [{ ...block.value, node }], headroom: block.value.headroom }) : block
+}
+
+// The marker leads the first paragraph, or stands as one where the blocks open with another.
+function taskBlocks(task: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<Walk> {
+  const marker = taskMarker(nodeAttrs(task)['state'])
+  const markerBlock: PlacedBlock = { node: { type: 'paragraph' }, spelling: 'commonmark', text: marker }
+  if (task.type === 'taskItem') {
+    const content = nodeContent(task)
+    const line = content.length === 0 ? success('') : emitInlineLine(content, 'paragraph', path, writing.flavour)
+    if (!line.ok) return line
+    return success({ blocks: [{ ...markerBlock, text: line.value === '' ? marker : `${marker} ${line.value}` }], headroom: Number.POSITIVE_INFINITY })
+  }
+  const walk = walkBlocks(nodeContent(task), path, depth, writing)
+  if (!walk.ok) return walk
+  const [first, ...rest] = walk.value.blocks
+  const blocks = first?.node.type === 'paragraph' ? [{ ...first, text: `${marker} ${first.text}` }, ...rest] : [markerBlock, ...walk.value.blocks]
+  return success({ blocks, headroom: walk.value.headroom })
+}
+
 function readableText(text: string | undefined): Result<EmittedBlock> | undefined {
   return text === undefined ? undefined : success(commonMarkText(text))
 }
@@ -156,7 +232,7 @@ function emitDirectiveBlock(node: AdfNode, model: BlockNodeModel, path: ConvertE
 function emitDirectiveBody(node: AdfNode, model: BlockNodeModel, opener: string, path: ConvertErrorPath, walkBody: () => Result<Walk>): Result<EmittedBlock> {
   if (blockDirectiveForm(node.type) === 'leaf') return success({ headroom: Number.POSITIVE_INFINITY, spelling: 'directive', text: opener })
   if (model.contentModel === 'inline') {
-    const line = emitInlineLine(nodeContent(node), 'paragraph', path)
+    const line = emitInlineLine(nodeContent(node), 'paragraph', path, 'lossless')
     if (!line.ok) return line
     return success(directivePair(node, opener, line.value))
   }
@@ -165,15 +241,13 @@ function emitDirectiveBody(node: AdfNode, model: BlockNodeModel, opener: string,
   return success(directivePair(node, opener, joinBlocks(walk.value.blocks, 'directive'), walk.value.headroom))
 }
 
-function tryBlockquote(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<EmittedBlock> | undefined {
+function tryBlockquote(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
   if (!carriesOnly(node, [])) return undefined
-  const inner = walkBlocks(nodeContent(node), path, depth + 1, memo)
+  const inner = walkBlocks(nodeContent(node), path, depth + 1, writing)
   if (!inner.ok) return inner
   const text = joinBlocks(inner.value.blocks, 'document')
-    .split('\n')
+  const alert = writing.flavour === 'plain' && leadingMarker(text, readAlertMarker) !== undefined
-    .map((line) => (line === '' ? '>' : `> ${line}`))
+  return success(commonMarkText(quoted(alert ? `\\${text}` : text), inner.value.headroom))
-    .join('\n')
-  return success(commonMarkText(text, inner.value.headroom))
 }
 
 function tryCodeBlock(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {
@@ -216,19 +290,19 @@ function codeBlockText(node: AdfNode, path: ConvertErrorPath): Result<string> {
   return success(text)
 }
 
-function tryHeading(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {
+function tryHeading(node: AdfNode, path: ConvertErrorPath, flavour: Flavour): Result<EmittedBlock> | undefined {
   if (!carriesOnly(node, ['level'])) return undefined
   const level = nodeAttrs(node)['level']
   if (typeof level !== 'number' || !Number.isInteger(level) || level < 1 || level > 6) return undefined
   const hashes = '#'.repeat(level)
   const content = nodeContent(node)
   if (content.length === 0) return success(commonMarkText(hashes))
-  const line = emitInlineLine(content, 'heading', path)
+  const line = emitInlineLine(content, 'heading', path, flavour)
   if (!line.ok) return line
   return success(commonMarkText(`${hashes} ${line.value}`))
 }
 
-function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, memo: SpellingMemo | undefined): Result<EmittedBlock> | undefined {
+function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
   const ordered = node.type === 'orderedList'
   if (!carriesOnly(node, ordered ? ['order'] : [])) return undefined
   const items = nodeContent(node)
@@ -238,14 +312,17 @@ function tryList(node: AdfNode, path: ConvertErrorPath, depth: number, memo: Spe
   const walked: WalkedItem[] = []
   let headroom = Number.POSITIVE_INFINITY
   for (const [offset, item] of items.entries()) {
-    const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1, memo)
+    const walk = walkBlocks(nodeContent(item), [...path, 'content', offset], depth + 1, writing)
     if (!walk.ok) return walk
     headroom = Math.min(headroom, walk.value.headroom)
     walked.push({ node: item, walk: walk.value })
   }
   const lines: string[] = []
   for (const [offset, item] of walked.entries()) {
-    const line = tryListItemLines(item.walk.blocks, ordered ? `${start + offset}. ` : '- ')
+    const inner = joinBlocks(item.walk.blocks, 'list-item')
+    // GitHub reads a task marker opening any item's first paragraph as a checkbox, whatever its siblings hold.
+    const escaped = writing.flavour === 'plain' && leadingMarker(inner, readTaskMarker) !== undefined ? `\\${inner}` : inner
+    const line = tryListItemLines(escaped, ordered ? `${start + offset}. ` : '- ')
     if (line === undefined) {
       // The directive form spends a level the walk did not count.
       if (headroom < 1) return tooDeep(path)
@@ -267,8 +344,7 @@ function listStart(node: AdfNode, items: number): number | undefined {
   return start + items - 1 > largestListMarker ? undefined : start
 }
 
-function tryListItemLines(blocks: readonly PlacedBlock[], marker: string): string | undefined {
+function tryListItemLines(inner: string, marker: string): string | undefined {
-  const inner = joinBlocks(blocks, 'list-item')
   if (inner === '') return marker.trimEnd()
   const body = inner.split('\n')
   if (body.some((line) => line !== '' && isBlankLine(line))) return undefined
@@ -278,10 +354,10 @@ function tryListItemLines(blocks: readonly PlacedBlock[], marker: string): strin
   return lines.join('\n')
 }
 
-function tryParagraph(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {
+function tryParagraph(node: AdfNode, path: ConvertErrorPath, flavour: Flavour): Result<EmittedBlock> | undefined {
   const content = nodeContent(node)
   if (content.length === 0 || !carriesOnly(node, [])) return undefined
-  const line = emitInlineLine(content, 'paragraph', path)
+  const line = emitInlineLine(content, 'paragraph', path, flavour)
   if (!line.ok) return line
   return success(commonMarkText(line.value))
 }

src/markdown/emit/inline-line.ts

@@ -1,4 +1,5 @@
 import type { AdfMark, AdfNode } from '../../adf/document.ts'
+import type { Flavour } from '../plain-conventions.ts'
 import type { InlineNodeModel } from '../../adf/inline-nodes.ts'
 import type { LineContainer } from '../line-container.ts'
 import { assembleInlineLine, isSyntax, type InlineEscaping, type InlineSegment, type MarkRun, type NodeRange } from './line-escaping.ts'
@@ -7,6 +8,7 @@ import { claimsLine, holdsNullCharacter, trimTrailingSpace } from '../commonmark
 import { commonMarkLink, linkHref, markSpelling, spellMarkAttributes } from '../mark-spellings.ts'
 import { escapeUnbalanced, spellDestination } from '../commonmark/link-syntax.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
+import { highlightDelimiter } from '../plain-conventions.ts'
 import { inlineNodeModel } from '../../adf/inline-nodes.ts'
 import { largestNesting } from '../../nesting.ts'
 import { longestBacktickRun } from '../commonmark/backtick-runs.ts'
@@ -24,6 +26,7 @@ type InlineContext = {
   atBlockEnd: boolean
   bracketed: boolean
   carried: ReadonlySet<number>
+  flavour: Flavour
   openingLinkAsDirective: boolean
   path: ConvertErrorPath
   spansLines: boolean
@@ -33,32 +36,32 @@ type InlineRun = { index: number; kind: 'marked'; mark: AdfMark; nodes: AdfNode[
 
 type LineAttempt = { fallback: NodeRange | 'opening-link'; line?: undefined } | { fallback?: undefined; line: string }
 
-type LineFallbacks = { carried: Set<number>; openingLinkAsDirective: boolean }
+type LineFallbacks = { carried: Set<number>; flavour: Flavour; openingLinkAsDirective: boolean }
 
-export type PlainLineFallback = { kind: 'claimed-line'; line: number; text: string } | { kind: 'opening-link' } | { kind: 'unspellable-run'; run: MarkRun }
+export type PlainLineFallback = { kind: 'claimed-line'; line: number; text: string } | { kind: 'opening-link' } | { kind: 'unspellable-run'; runs: [MarkRun, ...MarkRun[]] }
 
-export function emitInlineLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath): Result<string> {
+export function emitInlineLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<string> {
-  const emitted = emitLine(nodes, container, path)
+  const emitted = emitLine(nodes, container, path, flavour)
   if (!emitted.ok) return emitted
   return success(emitted.value.line)
 }
 
 export function openingLinkTakesDirective(nodes: readonly AdfNode[], path: ConvertErrorPath): Result<boolean> {
-  const emitted = emitLine(nodes, 'paragraph', path)
+  const emitted = emitLine(nodes, 'paragraph', path, 'lossless')
   if (!emitted.ok) return emitted
   return success(emitted.value.openingLinkAsDirective)
 }
 
 export function plainLineFallback(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath): Result<PlainLineFallback | undefined> {
-  const emission = lineSegments(nodes, container, path, { carried: new Set(), openingLinkAsDirective: false })
+  const emission = lineSegments(nodes, container, path, { carried: new Set(), flavour: 'plain', openingLinkAsDirective: false })
   if (!emission.ok) return emission
   if (emission.value.carry !== undefined) return failure('unsupported-node-shape', 'an inline node on a plain line has no spelling but the carry', path)
-  const verdict = lineVerdict(emission.value.segments, container)
+  const verdict = lineVerdict(emission.value.segments, container, 'plain')
   return success(verdict.kind === 'line' ? undefined : verdict)
 }
 
-export function tryPipeCell(nodes: readonly AdfNode[], path: ConvertErrorPath): string | undefined {
+export function tryPipeCell(nodes: readonly AdfNode[], path: ConvertErrorPath, flavour: Flavour): string | undefined {
-  const emitted = emitLine(nodes, 'table-cell', path)
+  const emitted = emitLine(nodes, 'table-cell', path, flavour)
   if (!emitted.ok) return undefined
   if (emitted.value.segments.some((segment) => isSyntax(segment.escaping) && segment.text.includes('|'))) return undefined
   return emitted.value.line
@@ -69,12 +72,12 @@ export function tryImageLine(alt: string | undefined, href: string, path: Conver
   const destination = spellDestination(href)
   if (destination === undefined) return undefined
   const description: InlineSegment[] = alt === undefined ? [] : [{ escaping: 'bracketed', text: alt }]
-  const attempt = attemptLine([syntax('!['), ...description, syntax(`](${destination})`)], 'paragraph', path)
+  const attempt = attemptLine([syntax('!['), ...description, syntax(`](${destination})`)], 'paragraph', path, 'lossless')
   return attempt.ok ? attempt.value.line : undefined
 }
 
-function emitLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath): Result<EmittedLine> {
+function emitLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<EmittedLine> {
-  const fallbacks: LineFallbacks = { carried: new Set(), openingLinkAsDirective: false }
+  const fallbacks: LineFallbacks = { carried: new Set(), flavour, openingLinkAsDirective: false }
   // Terminates because takeFallback refuses a pass that took no new fallback.
   for (;;) {
     const emission = lineSegments(nodes, container, path, fallbacks)
@@ -84,7 +87,7 @@ function emitLine(nodes: readonly AdfNode[], container: LineContainer, path: Con
       if (!taken.ok) return taken
       continue
     }
-    const attempt = attemptLine(emission.value.segments, container, path)
+    const attempt = attemptLine(emission.value.segments, container, path, flavour)
     if (!attempt.ok) return attempt
     if (attempt.value.line !== undefined) {
       return success({ line: attempt.value.line, openingLinkAsDirective: fallbacks.openingLinkAsDirective, segments: emission.value.segments })
@@ -114,19 +117,20 @@ function lineSegments(nodes: readonly AdfNode[], container: LineContainer, path:
   return success({ segments: carryStrippedWhitespace(emission.value.segments) })
 }
 
-function attemptLine(segments: readonly InlineSegment[], container: LineContainer, path: ConvertErrorPath): Result<LineAttempt> {
+function attemptLine(segments: readonly InlineSegment[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<LineAttempt> {
-  const verdict = lineVerdict(segments, container)
+  const verdict = lineVerdict(segments, container, flavour)
   if (verdict.kind === 'opening-link') return success({ fallback: 'opening-link' })
-  if (verdict.kind === 'unspellable-run') return success({ fallback: verdict.run })
+  if (verdict.kind === 'unspellable-run') return success({ fallback: verdict.runs[0] })
   if (verdict.kind === 'claimed-line') return failure('unspellable-line-start', `block parsing would claim the emitted line ${JSON.stringify(verdict.text)}`, path)
   return success({ line: verdict.text })
 }
 
 // The fallbacks in the order a line takes them, or the line where it takes none.
-function lineVerdict(segments: readonly InlineSegment[], container: LineContainer): PlainLineFallback | { kind: 'line'; text: string } {
+function lineVerdict(segments: readonly InlineSegment[], container: LineContainer, flavour: Flavour): PlainLineFallback | { kind: 'line'; text: string } {
-  const assembled = assembleInlineLine(segments, container)
+  const assembled = assembleInlineLine(segments, container, flavour)
   if (assembled.openingLinkAsDirective) return { kind: 'opening-link' }
-  if (assembled.unspellableRun !== undefined) return { kind: 'unspellable-run', run: assembled.unspellableRun }
+  const [run, ...others] = assembled.unspellableRuns
+  if (run !== undefined) return { kind: 'unspellable-run', runs: [run, ...others] }
   const lines = assembled.line.split('\n')
   const claimed = container === 'paragraph' ? lines.findIndex((single, index) => claimsLine(single, index === 0 ? 'first' : 'later')) : -1
   return claimed === -1 ? { kind: 'line', text: assembled.line } : { kind: 'claimed-line', line: claimed, text: lines[claimed] ?? '' }
@@ -270,6 +274,7 @@ function emitText(node: AdfNode, context: InlineContext, index: number, path: Co
 function emitMarkedRun(nodes: readonly AdfNode[], mark: AdfMark, depth: number, index: number, context: InlineContext): Result<Emission> {
   const path = nodePath(context, index)
   const range: NodeRange = { first: index, last: index + nodes.length - 1 }
+  if (mark.type === 'backgroundColor' && context.flavour === 'plain') return emitHighlight(nodes, depth, range, context)
   const spelling = markSpelling(mark.type)
   if (spelling === undefined) return success({ carry: range })
   const attributes = spellMarkAttributes(mark, spelling.attributes)
@@ -298,6 +303,15 @@ function emitEmphasis(nodes: readonly AdfNode[], spelling: string, depth: number
   })
 }
 
+function emitHighlight(nodes: readonly AdfNode[], depth: number, range: NodeRange, context: InlineContext): Result<Emission> {
+  const inner = emitRun(nodes, depth + 1, range.first, context)
+  if (!inner.ok || inner.value.carry !== undefined) return inner
+  const run = { ...range, depth }
+  return success({
+    segments: [{ escaping: 'none', highlight: 'open', nodes: run, text: highlightDelimiter }, ...inner.value.segments, { escaping: 'none', highlight: 'close', nodes: run, text: highlightDelimiter }],
+  })
+}
+
 function emitCodeSpan(nodes: readonly AdfNode[], depth: number, range: NodeRange, path: ConvertErrorPath): Result<Emission> {
   let text = ''
   for (const node of nodes) {

src/markdown/emit/line-escaping.ts

@@ -1,13 +1,15 @@
+import type { Flavour } from '../plain-conventions.ts'
 import type { LineContainer } from '../line-container.ts'
 import { backslashEscape, escapesLineClaim, inlineHtmlConstruct, opensBracketedAutolink, opensEmailAutolink, type LinePosition } from '../commonmark/grammar.ts'
 import { backtickRun, closingBacktickRun } from '../commonmark/backtick-runs.ts'
 import { claimsDirectivePrefix } from '../directive-syntax.ts'
 import { delimiterFlags, isWordCharacter, matchEmphasis, runLength } from '../commonmark/emphasis-matching.ts'
+import { highlightDelimiter, highlightFlanking } from '../plain-conventions.ts'
 import { isBareDelimiterRow } from '../pipe-table-syntax.ts'
 import { opensLinkDefinition } from '../commonmark/link-reference-definitions.ts'
 import { readEntityReference } from '../commonmark/entity-references.ts'
 
-export type EmphasisRole = 'close' | 'open'
+export type DelimiterRole = 'close' | 'open'
 
 export type InlineEscaping = 'backslash' | 'bracketed' | 'bracketed-link-target' | 'none'
 
@@ -16,11 +18,12 @@ export type NodeRange = { first: number; last: number }
 export type MarkRun = NodeRange & { depth: number }
 
 export type InlineSegment =
-  | { emphasis: EmphasisRole; escaping: 'none'; nodes: MarkRun; text: string }
+  | { emphasis: DelimiterRole; escaping: 'none'; highlight?: undefined; nodes: MarkRun; text: string }
-  | { emphasis?: undefined; escaping: 'none'; nodes: NodeRange; text: string }
+  | { emphasis?: undefined; escaping: 'none'; highlight: DelimiterRole; nodes: MarkRun; text: string }
-  | { emphasis?: undefined; escaping: InlineEscaping; nodes?: undefined; text: string }
+  | { emphasis?: undefined; escaping: 'none'; highlight?: undefined; nodes: NodeRange; text: string }
+  | { emphasis?: undefined; escaping: InlineEscaping; highlight?: undefined; nodes?: undefined; text: string }
 
-export type AssembledLine = { line: string; openingLinkAsDirective?: true; unspellableRun: MarkRun | undefined }
+export type AssembledLine = { line: string; openingLinkAsDirective?: true; unspellableRuns: MarkRun[] }
 
 type ScanLine = { position: LinePosition; start: number; text: string }
 
@@ -32,8 +35,8 @@ const delimiters = ['*', '_', '`', '~']
 
 const followsLinkText = /[([]/
 
-export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
+export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer, flavour: Flavour): AssembledLine {
-  return escape(resolveEmphasis(segments), container)
+  return escape(resolveEmphasis(segments), container, flavour === 'plain')
 }
 
 function resolveEmphasis(segments: readonly InlineSegment[]): InlineSegment[] {
@@ -65,11 +68,11 @@ function resolveEmphasis(segments: readonly InlineSegment[]): InlineSegment[] {
   return resolved
 }
 
-function escape(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
+function escape(segments: readonly InlineSegment[], container: LineContainer, highlights: boolean): AssembledLine {
   const scan = segments.map((segment) => segment.text).join('')
   const escapings: InlineEscaping[] = []
   for (const segment of segments) for (let index = 0; index < segment.text.length; index += 1) escapings.push(segment.escaping)
-  const escaped = escapeClosedRuns(scan, escapings, escapeClaims(scan, escapings, container))
+  const escaped = escapeClosedRuns(scan, escapings, escapeClaims(scan, escapings, container, highlights))
   const placements: number[] = []
   let output = ''
   for (let index = 0; index < scan.length; index += 1) {
@@ -78,34 +81,45 @@ function escape(segments: readonly InlineSegment[], container: LineContainer): A
     output += scan.charAt(index)
   }
   if (container === 'paragraph' && opensLinkDefinition(output)) {
-    if (segments[0]?.nodes !== undefined) return { line: output, openingLinkAsDirective: true, unspellableRun: undefined }
+    if (segments[0]?.nodes !== undefined) return { line: output, openingLinkAsDirective: true, unspellableRuns: [] }
-    return { line: `\\${output}`, unspellableRun: unspellableRun(segments, output, placements) }
+    return { line: `\\${output}`, unspellableRuns: unspellableRuns(segments, output, placements) }
   }
-  return { line: output, unspellableRun: unspellableRun(segments, output, placements) }
+  return { line: output, unspellableRuns: unspellableRuns(segments, output, placements) }
 }
 
-function escapeClaims(scan: string, escapings: readonly InlineEscaping[], container: LineContainer): ReadonlySet<number> {
+function escapeClaims(scan: string, escapings: readonly InlineEscaping[], container: LineContainer, highlights: boolean): ReadonlySet<number> {
   const escaped = new Set<number>()
   const linkClose = lastLinkClose(scan, escapings)
   let line = scanLine(scan, 0)
   let afterEscape = false
+  // Whether the `=` before opens a `==` the reader takes whole, so this one starts nothing.
+  let pairsEquals = false
   for (let index = 0; index < scan.length; index += 1) {
     if (index > line.start + line.text.length) line = scanLine(scan, line.start + line.text.length + 1)
     const escaping = escapings[index]
     const escapable = escaping === 'backslash' || escaping === 'bracketed'
+    const opensEquals: boolean = highlights && !pairsEquals && scan.startsWith(highlightDelimiter, index)
     const claimed: boolean =
       (escapable &&
-        (claimsLineStart(line, index, container) ||
+        ((opensEquals && claimsHighlight(scan, index)) ||
+          claimsLineStart(line, index, container) ||
           mergesWithSyntax(scan, escapings, index) ||
           opensConstruct(scan, linkClose, index, escaping === 'bracketed', container, afterEscape))) ||
       (escaping === 'bracketed-link-target' &&
         ((scan.charAt(index) === '`' && opensCodeSpan(scan, index, afterEscape)) || claimsDirectivePrefix(scan, index)))
     if (claimed) escaped.add(index)
     afterEscape = claimed
+    pairsEquals = opensEquals && !claimed
   }
   return escaped
 }
 
+// Like an emphasis run, a `==` in text escapes where the reader can open or close with it.
+function claimsHighlight(scan: string, index: number): boolean {
+  const flanking = highlightFlanking(scan, index)
+  return flanking.opens || flanking.closes
+}
+
 // CommonMark reads no escape inside a code span, so a backtick string an escape forms or splits off still closes one an earlier bare run opens.
 function escapeClosedRuns(scan: string, escapings: readonly InlineEscaping[], claimed: ReadonlySet<number>): ReadonlySet<number> {
   const escaped = new Set(claimed)
@@ -134,10 +148,27 @@ function escapeClosedRuns(scan: string, escapings: readonly InlineEscaping[], cl
   return escaped
 }
 
-function unspellableRun(segments: readonly InlineSegment[], output: string, placements: readonly number[]): MarkRun | undefined {
+// One emphasis run, the innermost, or every highlight run the line cannot spell.
+function unspellableRuns(segments: readonly InlineSegment[], output: string, placements: readonly number[]): MarkRun[] {
   const { nodes, runs } = emittedRuns(segments, placements, output)
   const pair = misflanked(runs) ?? unpaired(runs)
-  return pair === undefined ? undefined : nodes[pair]
+  const run = pair === undefined ? undefined : nodes[pair]
+  return run === undefined ? unreadHighlights(segments, output, placements) : [run]
+}
+
+// No `==` in text can open or close, and highlights never nest, so a pair reads back where each delimiter flanks.
+function unreadHighlights(segments: readonly InlineSegment[], output: string, placements: readonly number[]): MarkRun[] {
+  const unread: MarkRun[] = []
+  let cursor = 0
+  for (const segment of segments) {
+    const start = placements[cursor] ?? 0
+    cursor += segment.text.length
+    if (segment.highlight === undefined) continue
+    const flanking = highlightFlanking(output, start)
+    const flanks = segment.highlight === 'open' ? flanking.opens : flanking.closes
+    if (!flanks && unread.at(-1) !== segment.nodes) unread.push(segment.nodes)
+  }
+  return unread
 }
 
 function misflanked(runs: readonly EmittedRun[]): number | undefined {

src/markdown/emit/pipe-table.ts

@@ -1,10 +1,11 @@
 import type { AdfNode } from '../../adf/document.ts'
 import type { ConvertErrorPath } from '../../result.ts'
+import type { Flavour } from '../plain-conventions.ts'
 import { carriesOnly, nodeContent } from '../../adf/document.ts'
 import { spellPipeDelimiter, spellPipeRow } from '../pipe-table-syntax.ts'
 import { tryPipeCell } from './inline-line.ts'
 
-export function tryPipeTable(node: AdfNode, path: ConvertErrorPath): string | undefined {
+export function tryPipeTable(node: AdfNode, path: ConvertErrorPath, flavour: Flavour): string | undefined {
   const rows = pipeRows(node)
   if (rows === undefined) return undefined
   const lines: string[] = []
@@ -12,7 +13,7 @@ export function tryPipeTable(node: AdfNode, path: ConvertErrorPath): string | un
     const cells: string[] = []
     for (const [cellIndex, paragraph] of row.entries()) {
       const content = nodeContent(paragraph)
-      const line = content.length === 0 ? '' : tryPipeCell(content, [...path, 'content', rowIndex, 'content', cellIndex, 'content', 0])
+      const line = content.length === 0 ? '' : tryPipeCell(content, [...path, 'content', rowIndex, 'content', cellIndex, 'content', 0], flavour)
       if (line === undefined) return undefined
       cells.push(line)
     }

src/markdown/emit/plain-inline.ts

@@ -1,8 +1,8 @@
 import type { AdfAttributes, AdfMark, AdfNode } from '../../adf/document.ts'
 import type { LineContainer } from '../line-container.ts'
+import type { MarkRun } from './line-escaping.ts'
 import { blockNodeModel } from '../../adf/block-nodes.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
-import { highlightDelimiter } from '../plain-conventions.ts'
 import { largestNesting } from '../../nesting.ts'
 import { mergeAdjacentText, sameMark } from '../../adf/editor-normal.ts'
 import { nodeAttrs, nodeContent, nodeMarks } from '../../adf/document.ts'
@@ -10,6 +10,7 @@ import { plainLineFallback, type PlainLineFallback } from './inline-line.ts'
 import { spellDestination, spellLinkTarget } from '../commonmark/link-syntax.ts'
 
 const highlight = 'backgroundColor'
+const highlightMark: AdfMark = { type: highlight }
 const edgeStrippingMarks: readonly string[] = [highlight, 'em', 'strike', 'strong']
 const keptMarks: readonly string[] = [...edgeStrippingMarks, 'code', 'link']
 
@@ -130,7 +131,7 @@ function textLeaf(text: string, marks: readonly AdfMark[]): AdfNode {
   return marks.length === 0 ? { text, type: 'text' } : { marks: [...marks], text, type: 'text' }
 }
 
-// A highlight goes outermost so its run is one run at depth 0, and code innermost, the only place its spelling holds.
+// A highlight goes first, where `highlighted` looks for it, and code last, the only place its spelling holds.
 function plainMarks(marks: readonly AdfMark[], container: LineContainer, text: string): AdfMark[] {
   const kept: AdfMark[] = []
   for (const mark of marks) {
@@ -140,7 +141,9 @@ function plainMarks(marks: readonly AdfMark[], container: LineContainer, text: s
     if (plain !== undefined) kept.push(plain)
   }
   const rank = (mark: AdfMark): number => (mark.type === highlight ? 0 : mark.type === 'code' ? 2 : 1)
-  return kept.sort((first, second) => rank(first) - rank(second))
+  // The reader highlights no code, as Atlassian's schema allows none.
+  const code = kept.some((mark) => mark.type === 'code')
+  return kept.filter((mark) => !code || mark.type !== highlight).sort((first, second) => rank(first) - rank(second))
 }
 
 function plainLink(mark: AdfMark, container: LineContainer): AdfMark | undefined {
@@ -163,7 +166,7 @@ export function writableHref(href: string): string {
   return written
 }
 
-// The delimiters carry the marks the whole run shares, so they open and close inside them.
+// The marks a whole highlight run shares go outside the highlight, so its delimiters open and close inside them.
 function highlighted(leaves: readonly AdfNode[]): AdfNode[] {
   const spelled: AdfNode[] = []
   let run: AdfNode[] = []
@@ -172,11 +175,11 @@ function highlighted(leaves: readonly AdfNode[]): AdfNode[] {
     const marks = nodeMarks(leaf)
     if (marks[0]?.type === highlight) {
       const held = marks.slice(1)
-      shared = run.length === 0 ? held.filter((mark) => mark.type !== 'code') : shared.filter((mark) => held.some((other) => sameMark(other, mark)))
+      shared = run.length === 0 ? held : shared.filter((mark) => held.some((other) => sameMark(other, mark)))
-      run.push(withMarks(leaf, held))
+      run.push(leaf)
       continue
     }
-    if (run.length > 0) for (const held of [textLeaf(highlightDelimiter, shared), ...run, textLeaf(highlightDelimiter, shared)]) spelled.push(held)
+    for (const held of run) spelled.push(withMarks(held, [...shared, highlightMark, ...nodeMarks(held).slice(1).filter((mark) => !shared.some((other) => sameMark(other, mark)))]))
     run = []
     spelled.push(leaf)
   }
@@ -263,7 +266,7 @@ function spellableLine(leaves: AdfNode[], container: LineContainer, path: Conver
 }
 
 function withoutFallback(leaves: readonly AdfNode[], fallback: PlainLineFallback): AdfNode[] | undefined {
-  if (fallback.kind === 'unspellable-run') return withoutMark(leaves, fallback.run.first, fallback.run.last, fallback.run.depth)
+  if (fallback.kind === 'unspellable-run') return withoutMarks(leaves, fallback.runs)
   const first = fallback.kind === 'opening-link' ? 0 : lineStart(leaves, fallback.line)
   const mark = nodeMarks(leaves[first] ?? {})[0]
   if (mark === undefined || mark.type !== (fallback.kind === 'opening-link' ? 'link' : 'code')) return undefined
@@ -272,7 +275,7 @@ function withoutFallback(leaves: readonly AdfNode[], fallback: PlainLineFallback
   // A code span is what binds the `]` a link definition reads, and dropping it keeps the link target.
   const spans = leaves.slice(first, last + 1).some((leaf) => nodeMarks(leaf).length > 1 && nodeMarks(leaf).at(-1)?.type === 'code')
   if (mark.type === 'link' && spans) return leaves.map((leaf, index) => (index < first || index > last ? leaf : withMarks(leaf, nodeMarks(leaf).filter((held) => held.type !== 'code'))))
-  return withoutMark(leaves, first, last, 0)
+  return withoutMarks(leaves, [{ depth: 0, first, last }])
 }
 
 function lineStart(leaves: readonly AdfNode[], line: number): number {
@@ -281,6 +284,12 @@ function lineStart(leaves: readonly AdfNode[], line: number): number {
   return index
 }
 
-function withoutMark(leaves: readonly AdfNode[], first: number, last: number, depth: number): AdfNode[] {
+// The runs cover disjoint leaves, so one pass drops them all.
-  return leaves.map((leaf, index) => (index < first || index > last ? leaf : withMarks(leaf, nodeMarks(leaf).filter((_, held) => held !== depth))))
+function withoutMarks(leaves: readonly AdfNode[], runs: readonly MarkRun[]): AdfNode[] {
+  const depths = new Map<number, number>()
+  for (const run of runs) for (let index = run.first; index <= run.last; index += 1) depths.set(index, run.depth)
+  return leaves.map((leaf, index) => {
+    const depth = depths.get(index)
+    return depth === undefined ? leaf : withMarks(leaf, nodeMarks(leaf).filter((_, held) => held !== depth))
+  })
 }

src/markdown/emit/plain-reduction.test.ts

@@ -2,9 +2,8 @@ import assert from 'node:assert/strict'
 import test from 'node:test'
 
 import type { AdfAttributes, AdfDocument, AdfMark, AdfNode } from '../../adf/document.ts'
-import { adfToMarkdown } from './adf-to-markdown.ts'
+import { adfToPlainMarkdown, reduceToPlain } from './plain-reduction.ts'
 import { largestNesting } from '../../nesting.ts'
-import { reduceToPlain } from './plain-reduction.ts'
 
 const code: AdfMark = { type: 'code' }
 const em: AdfMark = { type: 'em' }
@@ -19,11 +18,8 @@ function plain(...content: AdfNode[]): string {
 }
 
 function plainDocument(input: AdfDocument): string {
-  const reduced = reduceToPlain(input)
+  const markdown = adfToPlainMarkdown(input)
-  if (!reduced.ok) return `${reduced.error.code} at /${reduced.error.path.join('/')}`
+  return markdown.ok ? markdown.value : `${markdown.error.code} at /${markdown.error.path.join('/')}`
-  const markdown = adfToMarkdown(reduced.value)
-  if (!markdown.ok) return `emit ${markdown.error.code}: ${markdown.error.message}`
-  return markdown.value
 }
 
 function text(value: string, ...marks: AdfMark[]): AdfNode {
@@ -119,17 +115,25 @@ test('spells an expand and a nested expand as a folded callout titled by the mar
   )
   assert.equal(plain(node('expand', {}, said('Line.'))), '> [!NOTE]-\n>\n> Line.\n')
   assert.equal(plain(node('expand', { title: ' *Two*\nlines ' })), '> [!NOTE]- \\*Two\\* lines\n')
+  assert.equal(plain(node('expand', { title: '\ta \t b\t ' })), '> [!NOTE]- a \t b\n')
+  assert.equal(plain(node('expand', { title: '**x** [y](z) ==w==' }, said('b'))), '> [!NOTE]- \\*\\*x\\*\\* \\[y](z) ==w==\n>\n> b\n')
 })
 
 test('spells a task list as a bullet list whose items lead with their state', () => {
   const task = (state: string, value: string): AdfNode => node('taskItem', { localId: '01a0d99b-1f58-7b95-829b-6f9860371d54', state }, text(value))
   const nested = node('taskList', {}, task('TODO', 'Review'))
   assert.equal(plain(node('taskList', {}, task('DONE', 'Write the spec'), nested, task('TODO', 'Ship it'))), '- [x] Write the spec\n  - [ ] Review\n- [ ] Ship it\n')
-  assert.equal(plain(node('taskList', {}, nested, task('DONE', ''), said('Stray'))), '- - [ ] Review\n- [x]\n- Stray\n')
+  assert.equal(plain(node('taskList', {}, nested, task('DONE', ''), said('Stray'))), '- - [ ] Review\n- \\[x]\n- Stray\n')
+  assert.equal(plain(node('taskList', {}, node('taskList', {}), task('DONE', 'a'), said('b'), node('taskList', {}, task('TODO', 'c')))), '- \\[x] a\n- b\n  - [ ] c\n')
+  assert.equal(plain(node('taskList', {}, paragraph(), task('DONE', 'a'))), '- [x] a\n')
+  assert.equal(plain(bulletList(item(said('x'))), node('taskList', {}, task('DONE', 'a'), node('taskList', {}, task('TODO', 'c')))), '- x\n- \\[x] a\n  - [ ] c\n')
   assert.equal(plain(node('taskList', {}), task('TODO', 'Loose')), 'Loose\n')
   const blockTask = node('blockTaskItem', { state: 'DONE' }, said('First.'), said('Second.'))
   const codeTask = node('blockTaskItem', { state: 'TODO' }, { content: [text('x')], type: 'codeBlock' })
   assert.equal(plain(node('taskList', {}, blockTask, codeTask)), '- [x] First.\n\n  Second.\n- [ ]\n\n  ```\n  x\n  ```\n')
+  const listTask = node('blockTaskItem', { state: 'DONE' }, bulletList(item(said('a'))))
+  assert.equal(plain(node('taskList', {}, task('TODO', ''), listTask, node('taskList', {}, task('TODO', 'b')))), '- [ ]\n- [x]\n  - a\n  - \\[ ] b\n')
+  assert.equal(plain(node('taskList', {}, task('DONE', 'a'), node('taskList', {}, task('TODO', '')))), '- [x] a\n  - [ ]\n')
 })
 
 test('spells a decision list as a plain bullet list', () => {
@@ -141,7 +145,19 @@ test('spells a highlight as a == pair around the run, whatever its colour', () =
   assert.equal(plain(paragraph(text('a '), text('hi', highlight('#fff')), text(' there', highlight('#000')), text(' b'))), 'a ==hi there== b\n')
   assert.equal(plain(paragraph(text('hi ', strong, highlight('#fff')), text('b'))), '**==hi==** b\n')
   assert.equal(plain(paragraph(text('a', strong, highlight('#fff')), text('b', highlight('#fff'), em))), '==**a**_b_==\n')
-  assert.equal(plain(paragraph(text('a', highlight('#fff'), code))), '==`a`==\n')
+  assert.equal(plain(paragraph(text('a', highlight('#fff'), code), text('b', highlight('#fff')))), '`a`==b==\n')
+  assert.equal(plain(paragraph(text('=', highlight('#fff')), text(' '), text('a==b', highlight('#fff')))), '==\\=== ==a==b==\n')
+  assert.equal(plain(paragraph(text('x'), text('y', highlight('#fff')), text(' z'))), 'xy z\n')
+})
+
+test('escapes text a renderer would take as a flavour marker, and only there', () => {
+  assert.equal(plain(said('==x== a == b a==b ===')), '\\==x\\== a == b a==b \\=\\==\n')
+  assert.equal(plain({ content: [said('[!NOTE] x'), said('[!TIP]')], type: 'blockquote' }), '> \\[!NOTE] x\n>\n> [!TIP]\n')
+  assert.equal(plain({ content: [said('[!NOTE]x')], type: 'blockquote' }, said('[!NOTE]')), '> [!NOTE]x\n\n[!NOTE]\n')
+  assert.equal(plain(bulletList(item(said('[x] a')), item(said('[ ]')))), '- \\[x] a\n- \\[ ]\n')
+  assert.equal(plain(bulletList(item(said('[x] a')), item(said('b'))), node('orderedList', { order: 1 }, item(said('[x] c')))), '- \\[x] a\n- b\n\n1. \\[x] c\n')
+  const task = (state: string, value: string): AdfNode => node('taskItem', { state }, text(value))
+  assert.equal(plain(node('taskList', {}, task('DONE', '[x] a'), task('TODO', '==b=='))), '- [x] [x] a\n- [ ] \\==b\\==\n')
 })
 
 test('unwraps the containers plain markdown has no spelling for to their body blocks in order', () => {
@@ -168,7 +184,7 @@ test('keeps the CommonMark blocks in their spelling and drops their attributes a
   assert.equal(plain(node('orderedList', { order: 1e10 }, item(said('Alpha')), item(code), item())), '- 10000000000. Alpha\n- 10000000001.\n\n  ```\n  x\n  ```\n- 10000000002.\n')
   const long = node('orderedList', { order: 1e10 }, item(said('y')))
   assert.equal(plain(bulletList(item(said('x'))), long, bulletList(item(said('z')))), '- x\n- 10000000000. y\n- z\n')
-  assert.equal(plain(node('taskList', {}, node('taskItem', { state: 'DONE' }, text('t'))), long), '- [x] t\n- 10000000000. y\n')
+  assert.equal(plain(node('taskList', {}, node('taskItem', { state: 'DONE' }, text('t'))), long), '- \\[x] t\n- 10000000000. y\n')
   assert.equal(plain(bulletList(item(said('a'), bulletList(item(said('x'))), long))), '- a\n  - x\n  - 10000000000. y\n')
   const givesWay = node('orderedList', { order: 1e10 }, item(node('rule', {})), item(bulletList(item(bulletList(item())))))
   assert.equal(plain(givesWay, bulletList(item(said('z')))), '- 10000000000.\n- 10000000001.\n  - -\n- z\n')
@@ -280,6 +296,8 @@ test('drops the mark of a run CommonMark flanking or matching cannot spell', ()
   assert.equal(plain(paragraph(text('un'), text('-real', strong), text('istic'))), 'un-realistic\n')
   assert.equal(plain(paragraph(text('a', em), text('b', strong), text('c', em))), '_a_**b**_c_\n')
   assert.equal(plain(paragraph(text('x'), text('*', em), text('y'))), 'x\\*y\n')
+  const highlight: AdfMark = { type: 'backgroundColor' }
+  assert.equal(plain(paragraph(text('a'), text('b', highlight), text(' c'), text('d', highlight), text(' '), text('e', highlight))), 'ab cd ==e==\n')
 })
 
 test('breaks a line at a newline and trims whitespace at every edge CommonMark strips', () => {
@@ -311,7 +329,8 @@ test('drops an empty paragraph and merges adjacent lists of one type', () => {
   assert.equal(plain(node('layoutSection', {}, column(bulletList(item(said('a')))), column(bulletList(item(said('b')))))), '- a\n- b\n')
 })
 
-test('returns a document the lossless emitter spells without a directive', () => {
+test('keeps the nodes the plain flavour spells and degrades only what it cannot', () => {
-  const reduced = reduceToPlain(document(node('panel', { panelType: 'info' }, said('x'))))
+  const tasks = node('taskList', {}, node('taskItem', { localId: '01a0d99b-1f58-7b95-829b-6f9860371d54', state: 'DONE' }, text('t')))
-  assert.deepEqual(reduced.ok ? reduced.value : undefined, document({ content: [said('[!NOTE]'), said('x')], type: 'blockquote' }))
+  const reduced = reduceToPlain(document(node('panel', { localId: '01a0d99b-1f56-7a50-889a-f4375f09ee05', panelType: 'info' }, said('x')), tasks))
+  assert.deepEqual(reduced.ok ? reduced.value : undefined, document(node('panel', { panelType: 'info' }, said('x')), { content: [node('taskItem', { state: 'DONE' }, text('t'))], type: 'taskList' }))
 })

src/markdown/emit/plain-reduction.ts

@@ -1,13 +1,13 @@
 import type { AdfDocument, AdfNode } from '../../adf/document.ts'
 import { adfDocumentFault, nodeAttrs, nodeContent } from '../../adf/document.ts'
-import { adfToMarkdown, commonMarkSpelling, largestListMarker, type SpellingMemo } from './adf-to-markdown.ts'
+import { commonMarkSpelling, largestListMarker, writeMarkdown, type SpellingMemo } from './adf-to-markdown.ts'
-import { alertMarker, foldedAlertMarker, taskMarker } from '../plain-conventions.ts'
 import { blockNodeModel } from '../../adf/block-nodes.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { inlineLeaves, isBlockNodeType, oneLine, reduceInline, writableHref } from './plain-inline.ts'
 import { inlineNodeModel } from '../../adf/inline-nodes.ts'
 import { languageSlot } from '../code-language.ts'
 import { largestNesting } from '../../nesting.ts'
+import { taskMarker } from '../plain-conventions.ts'
 
 // depth: the level the node reduced stands at, counted as the emitter counts it.
 type Reduction = { depth: number; memo: SpellingMemo; path: ConvertErrorPath }
@@ -20,7 +20,7 @@ type Placed = { index: number; loose: AdfNode[] } | { index: number; loose?: und
 
 const blockReducers: Readonly<Record<string, BlockReducer>> = {
   blockCard: paragraphOfNode,
-  blockquote: (node, reduction) => quoted(success([]), node, reduction),
+  blockquote: (node, reduction) => contained({ type: 'blockquote' }, node, reduction),
   bulletList: reduceList,
   caption: (node, reduction) => paragraphOf(nodeContent(node), reduction),
   codeBlock: reduceCodeBlock,
@@ -33,7 +33,7 @@ const blockReducers: Readonly<Record<string, BlockReducer>> = {
   mediaSingle: (node, reduction) => concatenated(nodeContent(node).map((child, index) => reduceStanding(child, childReduction(reduction, index)))),
   nestedExpand: reduceExpand,
   orderedList: reduceList,
-  panel: (node, reduction) => quoted(success([paragraph([text(alertMarker(nodeAttrs(node)['panelType']))])]), node, reduction),
+  panel: reducePanel,
   paragraph: (node, reduction) => paragraphOf(nodeContent(node), reduction),
   rule: () => success([{ type: 'rule' }]),
   syncBlock: paragraphOfNode,
@@ -43,7 +43,7 @@ const blockReducers: Readonly<Record<string, BlockReducer>> = {
 
 export function adfToPlainMarkdown(document: AdfDocument): Result<string> {
   const reduced = reduceToPlain(document)
-  return reduced.ok ? adfToMarkdown(reduced.value) : reduced
+  return reduced.ok ? writeMarkdown(reduced.value, 'plain') : reduced
 }
 
 export function reduceToPlain(document: AdfDocument): Result<AdfDocument> {
@@ -112,7 +112,7 @@ function plainSequence(blocks: readonly AdfNode[], reduction: Reduction): Result
     const listed = sequence[index]
     if (listed === undefined || (listed.type !== 'bulletList' && listed.type !== 'orderedList')) continue
     const block = numberedPastMarkers(listed)
-    const spelled = block === listed && commonMarkSpelling(block, reduction.path, reduction.depth, reduction.memo)?.ok === true
+    const spelled = block === listed && commonMarkSpelling(block, reduction.path, reduction.depth, { flavour: 'plain', memo: reduction.memo })?.ok === true
     if (spelled) continue
     sequence = spliced(sequence, index, block === listed ? nodeContent(block).flatMap(nodeContent) : [block])
     index = Math.max(0, index - 1) - 1
@@ -138,11 +138,12 @@ function numberedAsText(list: AdfNode): AdfNode {
   return { content: nodeContent(list).map((item, offset) => itemOf(marked(nodeContent(item), `${order + offset}.`))), type: 'bulletList' }
 }
 
+// Adjacent lists of one marker read back as one list.
 function mergedLists(blocks: readonly AdfNode[]): AdfNode[] {
   const merged: AdfNode[] = []
   for (const block of blocks) {
     let next = block
-    for (let previous = merged.at(-1); previous !== undefined && previous.type === next.type && isList(next); previous = merged.at(-1)) {
+    for (let previous = merged.at(-1); previous !== undefined && listMarker(next) !== undefined && listMarker(previous) === listMarker(next); previous = merged.at(-1)) {
       merged.pop()
       next = joinedLists(previous, next)
     }
@@ -151,17 +152,36 @@ function mergedLists(blocks: readonly AdfNode[]): AdfNode[] {
   return merged
 }
 
-function isList(block: AdfNode): boolean {
+function listMarker(block: AdfNode): string | undefined {
-  return block.type === 'bulletList' || block.type === 'orderedList'
+  if (block.type === 'orderedList') return '.'
+  return block.type === 'bulletList' || block.type === 'taskList' ? '-' : undefined
 }
 
-// Two numbered lists whose numbering breaks between them keep their numbers as text in one bullet list.
+// Two numbered lists whose numbering breaks between them keep their numbers as text in one bullet list, and a task list joining a bullet list its markers.
 function joinedLists(first: AdfNode, second: AdfNode): AdfNode {
   const breaks = first.type === 'orderedList' && nodeAttrs(second)['order'] !== Number(nodeAttrs(first)['order']) + nodeContent(first).length
-  const [head, tail] = breaks ? [numberedAsText(first), numberedAsText(second)] : [first, second]
+  const [head, tail] = breaks ? [numberedAsText(first), numberedAsText(second)] : first.type === second.type ? [first, second] : [tasksAsText(first), tasksAsText(second)]
   return { ...head, content: [...nodeContent(head), ...nodeContent(tail)] }
 }
 
+// A task keeps its marker as text; a list item stands as one, and anything else nests in the item before it.
+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()
+    items.push(itemOf(previous === undefined ? taskAsText(child) : mergedLists([...nodeContent(previous), child])))
+  }
+  return { content: items, type: 'bulletList' }
+}
+
+function taskAsText(child: AdfNode): readonly AdfNode[] {
+  const marker = taskMarker(nodeAttrs(child)['state'])
+  if (child.type === 'taskItem') return [paragraph(nodeContent(child).length === 0 ? [text(marker)] : [text(`${marker} `), ...nodeContent(child)])]
+  if (child.type === 'blockTaskItem') return marked(nodeContent(child), marker)
+  return child.type === 'listItem' ? nodeContent(child) : [child]
+}
+
 function paragraph(content: readonly AdfNode[]): AdfNode {
   return { content: [...content], type: 'paragraph' }
 }
@@ -183,15 +203,27 @@ function paragraphOfNode(node: AdfNode, reduction: Reduction): Result<AdfNode[]>
   return paragraphOf([node], reduction)
 }
 
-function quoted(head: Result<AdfNode[]>, node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
+function contained(shell: AdfNode, node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
-  const content = concatenated([head, reduceBlocks(nodeContent(node), { ...reduction, depth: reduction.depth + 1 })])
+  const content = reduceBlocks(nodeContent(node), { ...reduction, depth: reduction.depth + 1 })
-  return content.ok ? success([{ content: content.value, type: 'blockquote' }]) : content
+  return content.ok ? success([{ ...shell, content: content.value }]) : content
+}
+
+function reducePanel(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
+  const panelType = nodeAttrs(node)['panelType']
+  return contained(typeof panelType === 'string' ? { attrs: { panelType }, type: 'panel' } : { type: 'panel' }, node, reduction)
 }
 
 function reduceExpand(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
-  const title = nodeAttrs(node)['title']
+  const held = nodeAttrs(node)['title']
-  const marker = typeof title === 'string' ? `${foldedAlertMarker} ${title.replace(/^[ \t\n\r]+/, '').replaceAll('\n', ' ')}` : foldedAlertMarker
+  const title = typeof held === 'string' ? withoutTrailingBlanks(oneLine(held).replace(/^[ \t]+/, '')) : ''
-  return quoted(paragraphOf([text(marker)], { ...reduction, depth: reduction.depth + 1 }), node, reduction)
+  return contained(title === '' ? { type: node.type } : { attrs: { title }, type: node.type }, node, reduction)
+}
+
+// A backward scan: an unanchored-end regex retries from every blank in a long run.
+function withoutTrailingBlanks(text: string): string {
+  let end = text.length
+  while (end > 0 && (text.charAt(end - 1) === ' ' || text.charAt(end - 1) === '\t')) end -= 1
+  return text.slice(0, end)
 }
 
 function reduceHeading(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
@@ -230,8 +262,11 @@ function listItem(blocks: Result<AdfNode[]>): Result<AdfNode[]> {
 // A list item's first line reads as no rule and holds no line of spaces alone: the rule and the spaces give way.
 function itemOf(blocks: readonly AdfNode[]): AdfNode {
   const rules = blocks.findIndex((block) => block.type !== 'rule')
-  const content = blocks.slice(rules === -1 ? blocks.length : rules).map((block) => (block.type === 'codeBlock' ? { ...block, content: blankedLines(nodeContent(block)) } : block))
+  return { content: blankedCode(blocks.slice(rules === -1 ? blocks.length : rules)), type: 'listItem' }
-  return { content, type: 'listItem' }
+}
+
+function blankedCode(blocks: readonly AdfNode[]): AdfNode[] {
+  return blocks.map((block) => (block.type === 'codeBlock' ? { ...block, content: blankedLines(nodeContent(block)) } : block))
 }
 
 function blankedLines(code: readonly AdfNode[]): AdfNode[] {
@@ -239,26 +274,54 @@ 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.
 function reduceTaskList(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
-  const items: AdfNode[] = []
+  const kept: { blocks: AdfNode[]; child: AdfNode }[] = []
   for (const [index, child] of nodeContent(node).entries()) {
-    const blocks = taskBlocks(child, childReduction(reduction, index))
+    const at = childReduction(reduction, index)
-    if (!blocks.ok) return blocks
+    const reduced = isTask(child) ? reduceTask(child, at) : reduceStanding(child, at)
-    const previous = child.type === 'taskList' ? items.pop() : undefined
+    if (!reduced.ok) return reduced
-    items.push(itemOf(previous === undefined ? blocks.value : mergedLists([...nodeContent(previous), ...blocks.value])))
+    if (reduced.value.length > 0) kept.push({ blocks: reduced.value, child })
   }
-  return success(listOf(items, 'bulletList'))
+  const regular = isTask(kept[0]?.child) && kept.every(({ child }) => isTask(child) || child.type === 'taskList')
+  const tasks: AdfNode[] = []
+  let nested: AdfNode[] = []
+  for (const { blocks, child } of kept) {
+    if (!regular) {
+      const standsAlone = !isTask(child) && child.type !== 'taskList'
+      for (const block of standsAlone ? [{ content: blocks, type: 'listItem' }] : blocks) tasks.push(block)
+      continue
+    }
+    if (isTask(child)) {
+      nestIn(tasks, nested)
+      nested = []
+    }
+    for (const block of blocks) (isTask(child) ? tasks : nested).push(block)
+  }
+  nestIn(tasks, nested)
+  if (regular) return success([{ content: tasks, type: 'taskList' }])
+  return success(listOf(nodeContent(tasksAsText({ content: tasks, type: 'taskList' })), 'bulletList'))
 }
 
-function taskBlocks(child: AdfNode, at: Reduction): Result<AdfNode[]> {
+// The writer nests a list in the task before it, so one closing a block task item's blocks merges with it.
-  const marker = taskMarker(nodeAttrs(child)['state'])
+function nestIn(tasks: AdfNode[], nested: readonly AdfNode[]): void {
-  if (child.type === 'taskItem') {
+  const previous = tasks.at(-1)
-    const content = reduceInline(nodeContent(child), 'paragraph', at.path, at.depth)
+  if (previous?.type === 'blockTaskItem') tasks[tasks.length - 1] = { ...previous, content: mergedLists([...nodeContent(previous), ...nested]) }
-    return content.ok ? success([paragraph(content.value.length === 0 ? [text(marker)] : [text(`${marker} `), ...content.value])]) : content
+  else for (const block of mergedLists(nested)) tasks.push(block)
 }
-  if (child.type !== 'blockTaskItem') return reduceStanding(child, at)
+
-  const blocks = reduceBlocks(nodeContent(child), at)
+function isTask(node: AdfNode | undefined): boolean {
-  return blocks.ok ? success(marked(blocks.value, marker)) : blocks
+  return node?.type === 'taskItem' || node?.type === 'blockTaskItem'
+}
+
+function reduceTask(task: AdfNode, at: Reduction): Result<AdfNode[]> {
+  const attrs = { state: nodeAttrs(task)['state'] === 'DONE' ? 'DONE' : 'TODO' }
+  if (task.type === 'taskItem') {
+    const content = reduceInline(nodeContent(task), 'paragraph', at.path, at.depth)
+    return content.ok ? success([{ attrs, content: content.value, type: 'taskItem' }]) : content
+  }
+  const blocks = reduceBlocks(nodeContent(task), at)
+  return blocks.ok ? success([{ attrs, content: blankedCode(blocks.value), type: 'blockTaskItem' }]) : blocks
 }
 
 // The marker leads the first paragraph, or stands as one where the blocks open with another.

src/markdown/parse/inline-content.ts

@@ -7,9 +7,9 @@ import type { LinkDefinition } from '../commonmark/link-syntax.ts'
 import { backslashEscape, decodeTextEscapes, inlineHtmlConstruct, readBracketedAutolink, readEmailAutolink, trimTrailingSpace } from '../commonmark/grammar.ts'
 import { backtickRun, closingBacktickRun } from '../commonmark/backtick-runs.ts'
 import { commonMarkLink, linkHref } from '../mark-spellings.ts'
-import { delimiterFlags, isWordCharacter, matchEmphasis, runLength } from '../commonmark/emphasis-matching.ts'
+import { delimiterFlags, matchEmphasis, runLength } from '../commonmark/emphasis-matching.ts'
-import { highlightDelimiter } from '../plain-conventions.ts'
 import { failure, faulted, success, type ConvertErrorPath, type Result } from '../../result.ts'
+import { highlightDelimiter, highlightFlanking } from '../plain-conventions.ts'
 import { inlineNodeModel } from '../../adf/inline-nodes.ts'
 import { mergeAdjacentText, sameMarks } from '../../adf/editor-normal.ts'
 import { noSpans, readInlineDirective } from '../directive-syntax.ts'
@@ -301,22 +301,14 @@ function readDelimiterRun(scan: Scan, index: number): number {
   return index + length
 }
 
-// A delimiter is bounded outside by the code point beyond it, and flanks by the character inside it.
 function readEquals(scan: Scan, index: number): number {
   if (!scan.highlights || !scan.source.startsWith(highlightDelimiter, index)) {
     scan.pending += '='
     return index + 1
   }
-  const end = index + highlightDelimiter.length
-  const before = Array.from(scan.source.slice(Math.max(0, index - 2), index)).at(-1) ?? ''
-  const after = Array.from(scan.source.slice(end, end + 2))[0] ?? ''
   flush(scan, false)
-  scan.pieces.push({ closes: flanks(before) && !isWordCharacter(after), kind: 'highlight', opens: flanks(after) && !isWordCharacter(before) })
+  scan.pieces.push({ ...highlightFlanking(scan.source, index), kind: 'highlight' })
-  return end
+  return index + highlightDelimiter.length
-}
-
-function flanks(character: string): boolean {
-  return character !== '' && !/\s/.test(character)
 }
 
 function readAutolink(source: string, index: number): { length: number; node: AdfNode } | undefined {

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

@@ -10,11 +10,11 @@ import { commonMarkSpelling, type SpellingMemo } from '../emit/adf-to-markdown.t
 import { failure, faulted, positioned, success, type ConvertErrorPath, type ParseError, type Result, type SourcePosition } from '../../result.ts'
 import { languageSlot } from '../code-language.ts'
 import { largestNesting } from '../../nesting.ts'
+import { leadingMarker, readAlertMarker, readTaskMarker } from '../plain-conventions.ts'
 import { listBreakName, listBreakSpelling } from '../block-directive.ts'
 import { nodeAttrs, nodeContent } from '../../adf/document.ts'
 import { parseBlocks } from './blocks.ts'
 import { parseInlineContent } from './inline-content.ts'
-import { readAlertMarker, readTaskMarker } from '../plain-conventions.ts'
 import { readBlockDirectiveNode } from './directive-nodes.ts'
 import { unsupportedNodeShape } from '../directive-syntax.ts'
 
@@ -102,10 +102,8 @@ function readBlock(block: Block, reading: Reading, path: ConvertErrorPath, depth
 
 function markerLed<T extends { length: number }>(block: Block | undefined, read: (text: string) => T | undefined): { marker: T; position: SourcePosition; text: string } | undefined {
   if (block?.kind !== 'paragraph') return undefined
-  const marker = read(block.text)
+  const marker = leadingMarker(block.text, read)
-  if (marker === undefined) return undefined
+  return marker === undefined ? undefined : { marker, position: block.position, text: block.text.slice(marker.length) }
-  const text = block.text.slice(marker.length)
-  return text === '' || /^(?:[ \t\n]|\\\n)/.test(text) ? { marker, position: block.position, text } : undefined
 }
 
 function markerLine(text: string): { line: string; rest: string } {
@@ -179,7 +177,7 @@ function directiveNode(block: DirectiveBlock, reading: Reading, path: ConvertErr
   const inExpand = reading.inExpand || read.value.node.type === 'expand' || read.value.node.type === 'nestedExpand'
   const built = directiveBody(read.value, block.blocks, { ...reading, inExpand }, path, depth)
   if (!built.ok) return built
-  const readable = commonMarkSpelling(built.value, path, depth, reading.memo)
+  const readable = commonMarkSpelling(built.value, path, depth, { flavour: 'lossless', memo: reading.memo })
   if (readable === undefined) return built
   if (!readable.ok) return readable
   return failure('unsupported-node-shape', `${built.value.type} takes the CommonMark spelling, not the directive form`, path)

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

@@ -3,9 +3,9 @@ import test from 'node:test'
 
 import type { AdfAttributes, AdfDocument, AdfMark, AdfNode } from '../../adf/document.ts'
 import { adfToMarkdown } from '../emit/adf-to-markdown.ts'
+import { adfToPlainMarkdown } from '../emit/plain-reduction.ts'
 import { largestNesting } from '../../nesting.ts'
 import { markdownToAdf, plainMarkdownToAdf } from './markdown-to-adf.ts'
-import { reduceToPlain } from '../emit/plain-reduction.ts'
 import { toEditorNormal } from '../../adf/editor-normal.ts'
 
 const code: AdfMark = { type: 'code' }
@@ -26,10 +26,8 @@ function document(...content: AdfNode[]): AdfDocument {
   return { content, type: 'doc', version: 1 }
 }
 
-// ADF the reduction wrote, spelled and read back.
 function roundTripped(...content: AdfNode[]): readonly AdfNode[] | string {
-  const reduced = reduceToPlain(document(...content))
+  const markdown = adfToPlainMarkdown(document(...content))
-  const markdown = reduced.ok ? adfToMarkdown(reduced.value) : reduced
   return markdown.ok ? read(markdown.value) : markdown.error.code
 }
 
@@ -190,6 +188,16 @@ test('reads what the reduction wrote back to the node it reduced, less the attri
   assert.deepEqual(roundTripped(tasks), [plainTasks])
   const colour: AdfMark = { attrs: { color: '#c6edfb' }, type: 'backgroundColor' }
   assert.deepEqual(roundTripped(paragraph(text('a '), text('hi', colour, strong), text(' b'))), [paragraph(text('a '), text('hi', highlight, strong), text(' b'))])
+  assert.deepEqual(roundTripped(paragraph(text('=', colour), text(' '), text('a==b', colour))), [paragraph(text('=', highlight), text(' '), text('a==b', highlight))])
+  assert.deepEqual(roundTripped(paragraph(text('x'), text('y', colour))), [said('xy')])
+  assert.deepEqual(roundTripped(node('expand', { title: '**x** [y](z)' }, said('b'))), [node('expand', { title: '**x** [y](z)' }, said('b'))])
+})
+
+test('reads text the writer kept from reading as a marker back as text', () => {
+  const quote = bare('blockquote', said('[!NOTE] x'))
+  const list = bare('bulletList', bare('listItem', said('[x] a')), bare('listItem', said('[ ] b')))
+  assert.deepEqual(roundTripped(said('==x== a==b'), quote, list), [said('==x== a==b'), quote, list])
+  assert.deepEqual(roundTripped(bare('taskList', task('DONE', text('[x] ==a==')))), [bare('taskList', task('DONE', text('[x] ==a==')))])
 })
 
 test('keeps what markdownToAdf reads that no row reads, and refuses only what it refuses', () => {

src/markdown/plain-conventions.ts

@@ -1,3 +1,5 @@
+import { isWordCharacter } from './commonmark/emphasis-matching.ts'
+
 export type Flavour = 'lossless' | 'plain'
 
 type AlertMarker = { folded: boolean; length: number; panelType: string }
@@ -45,6 +47,26 @@ export function readAlertMarker(text: string): AlertMarker | undefined {
   return { folded: marker[2] !== '', length: marker[0].length, panelType: panelType ?? 'info' }
 }
 
+// A marker leads text that ends at it or goes on past whitespace or a hard break.
+export function leadingMarker<T extends { length: number }>(text: string, read: (text: string) => T | undefined): T | undefined {
+  const marker = read(text)
+  if (marker === undefined) return undefined
+  const rest = text.slice(marker.length, marker.length + 2)
+  return rest === '' || /^(?:[ \t\n]|\\\n)/.test(rest) ? marker : undefined
+}
+
+// A delimiter is bounded outside by the code point beyond it, and flanks by the character inside it.
+export function highlightFlanking(source: string, index: number): { closes: boolean; opens: boolean } {
+  const end = index + highlightDelimiter.length
+  const before = Array.from(source.slice(Math.max(0, index - 2), index)).at(-1) ?? ''
+  const after = Array.from(source.slice(end, end + 2))[0] ?? ''
+  return { closes: flanks(before) && !isWordCharacter(after), opens: flanks(after) && !isWordCharacter(before) }
+}
+
+function flanks(character: string): boolean {
+  return character !== '' && !/\s/.test(character)
+}
+
 export function taskMarker(state: unknown): string {
   return state === 'DONE' ? '[x]' : '[ ]'
 }

todo.md

@@ -2,19 +2,6 @@
 
 ## 0.2.0
 
-- **35 — Read and write plain markdown as a flavour of the markdown grammar.** Per Goal 2 and
-  `docs/decisions.md` §Plain markdown is a flavour of the grammar, README §Plain markdown's rows are
-  read by `markdownToAdf`'s parser and written by `adfToMarkdown`'s writer. The
-  exports, their refusals and those rows stay as they are.
-  - **35b — Spell the plain flavour in the writer.** Panels, expands, task lists and highlights are
-    written by the writer, which escapes text that would read back as one, so
-    `plainMarkdownToAdf(adfToPlainMarkdown(doc))` keeps a literal `==x==`, a quote opening `[!NOTE]`
-    and a list whose items all open `[x] ` as text. A highlighted `=` (today `=====`) and `a==b`
-    (today `==a==b==`, highlighting `a` alone) come back highlighted whole, or lose the highlight
-    where no spelling holds them; the plain pair's byte-for-byte property misses both, since the
-    wrong document re-spells to the same bytes. An expand titled `**x** [y](z)` comes back an
-    expand, where today its escaped marker reads back as a quote. The reduction keeps only
-    degrading what the flavour cannot spell.
 - **10f — Give task nodes read from plain markdown position ids.** Per `docs/decisions.md` §Plain
   task ids come from position, README §Plain markdown's `localId` bullet saying so. The id spelling
   is part of the chunk.
@@ -47,6 +34,10 @@
   paragraph of 2000 × `un` plus `**-r**`: each run its flanking cannot spell re-emits the whole line
   before riding the carry, quadratic in the runs (Goal 9), and the plain reduction's
   `spellableLine` drops one mark per re-emit the same way. Make both linear.
+- **42 — Trim a text leaf's trailing blanks in linear time.** `plain-inline.ts`'s `leafEdges` finds
+  the trail with an unanchored `/[ \t]*$/`, quadratic in a run of blanks inside one leaf: a
+  paragraph of `a`, 80 000 spaces, `b` takes 6.5 s in `adfToPlainMarkdown` (Goal 9). Scan backward,
+  as the expand title's trim does.
 - **34 — Read emphasis flanking by the whole character beside an astral symbol.** Check whether
   `line-escaping.ts`'s `charAt` and the parser's flanking read one UTF-16 unit beside an astral
   symbol — a lone surrogate is neither punctuation nor symbol, where CommonMark reads `😀` as