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 +88,119 @@ 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 {
-  const readable = readableBlock(node, path, depth, memo)
+export function commonMarkSpelling(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<null> | undefined {
+  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 {
-  if (node.type === 'blockquote') return tryBlockquote(node, path, depth, memo)
-  if (node.type === 'bulletList' || node.type === 'orderedList') return tryList(node, path, depth, memo)
+function spellReadableBlock(node: AdfNode, path: ConvertErrorPath, depth: number, writing: Writing): Result<EmittedBlock> | undefined {
+  const plain = writing.flavour === 'plain' ? spellPlainBlock(node, path, depth, writing) : undefined
+  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> | undefined {
+  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'), '- '))
+  return lines.includes(undefined) ? undefined : 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 +231,7 @@ 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 +240,13 @@ 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')
-    .map((line) => (line === '' ? '>' : `> ${line}`))
-    .join('\n')
-  return success(commonMarkText(text, inner.value.headroom))
+  const alert = writing.flavour === 'plain' && leadingMarker(text, readAlertMarker) !== undefined
+  return success(commonMarkText(quoted(alert ? `\\${text}` : text), inner.value.headroom))
 }
 
 function tryCodeBlock(node: AdfNode, path: ConvertErrorPath): Result<EmittedBlock> | undefined {
@@ -216,19 +289,19 @@ 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 +311,17 @@ 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 inners = walked.map((item) => joinBlocks(item.walk.blocks, 'list-item'))
+  // A bullet list whose every item opens with a task marker reads as a task list (README §Plain markdown).
+  if (writing.flavour === 'plain' && !ordered && inners.every((inner) => leadingMarker(inner, readTaskMarker) !== undefined)) inners[0] = `\\${inners[0] ?? ''}`
   const lines: string[] = []
-  for (const [offset, item] of walked.entries()) {
-    const line = tryListItemLines(item.walk.blocks, ordered ? `${start + offset}. ` : '- ')
+  for (const [offset, inner] of inners.entries()) {
+    const line = tryListItemLines(inner, ordered ? `${start + offset}. ` : '- ')
     if (line === undefined) {
       // The directive form spends a level the walk did not count.
       if (headroom < 1) return tooDeep(path)
@@ -267,8 +343,7 @@ function listStart(node: AdfNode, items: number): number | undefined {
   return start + items - 1 > largestListMarker ? undefined : start
 }
 
-function tryListItemLines(blocks: readonly PlacedBlock[], marker: string): string | undefined {
-  const inner = joinBlocks(blocks, 'list-item')
+function tryListItemLines(inner: string, marker: string): string | undefined {
   if (inner === '') return marker.trimEnd()
   const body = inner.split('\n')
   if (body.some((line) => line !== '' && isBlankLine(line))) return undefined
@@ -278,10 +353,10 @@ 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 function emitInlineLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath): Result<string> {
-  const emitted = emitLine(nodes, container, path)
+export function emitInlineLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<string> {
+  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 {
-  const emitted = emitLine(nodes, 'table-cell', path)
+export function tryPipeCell(nodes: readonly AdfNode[], path: ConvertErrorPath, flavour: Flavour): string | undefined {
+  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> {
-  const fallbacks: LineFallbacks = { carried: new Set(), openingLinkAsDirective: false }
+function emitLine(nodes: readonly AdfNode[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<EmittedLine> {
+  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 })
@@ -114,8 +117,8 @@ 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> {
-  const verdict = lineVerdict(segments, container)
+function attemptLine(segments: readonly InlineSegment[], container: LineContainer, path: ConvertErrorPath, flavour: Flavour): Result<LineAttempt> {
+  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 === 'claimed-line') return failure('unspellable-line-start', `block parsing would claim the emitted line ${JSON.stringify(verdict.text)}`, path)
@@ -123,8 +126,8 @@ function attemptLine(segments: readonly InlineSegment[], container: LineContaine
 }
 
 // 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 } {
-  const assembled = assembleInlineLine(segments, container)
+function lineVerdict(segments: readonly InlineSegment[], container: LineContainer, flavour: Flavour): PlainLineFallback | { kind: 'line'; text: string } {
+  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 lines = assembled.line.split('\n')
@@ -270,6 +273,7 @@ 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 +302,15 @@ 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'
 
@@ -16,9 +18,10 @@ 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?: undefined; escaping: 'none'; nodes: NodeRange; text: string }
-  | { emphasis?: undefined; escaping: InlineEscaping; nodes?: undefined; text: string }
+  | { emphasis: DelimiterRole; escaping: 'none'; highlight?: undefined; nodes: MarkRun; text: string }
+  | { emphasis?: undefined; escaping: 'none'; highlight: DelimiterRole; nodes: MarkRun; 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 }
 
@@ -32,8 +35,8 @@ const delimiters = ['*', '_', '`', '~']
 
 const followsLinkText = /[([]/
 
-export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
-  return escape(resolveEmphasis(segments), container)
+export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer, flavour: Flavour): AssembledLine {
+  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) {
@@ -84,28 +87,39 @@ function escape(segments: readonly InlineSegment[], container: LineContainer): A
   return { line: output, unspellableRun: unspellableRun(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)
@@ -137,7 +151,22 @@ function escapeClosedRuns(scan: string, escapings: readonly InlineEscaping[], cl
 function unspellableRun(segments: readonly InlineSegment[], output: string, placements: readonly number[]): MarkRun | undefined {
   const { nodes, runs } = emittedRuns(segments, placements, output)
   const pair = misflanked(runs) ?? unpaired(runs)
-  return pair === undefined ? undefined : nodes[pair]
+  return pair === undefined ? unreadHighlight(segments, output, placements) : nodes[pair]
+}
+
+// No `==` in text can open or close, and highlights never nest, so a pair reads back where each delimiter flanks and both share a line.
+function unreadHighlight(segments: readonly InlineSegment[], output: string, placements: readonly number[]): MarkRun | undefined {
+  let cursor = 0
+  let opener = 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)
+    if (segment.highlight === 'open') opener = start
+    if (segment.highlight === 'open' ? !flanking.opens : !flanking.closes || output.slice(opener, start).includes('\n')) return segment.nodes
+  }
+  return undefined
 }
 
 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

@@ -2,7 +2,6 @@ import type { AdfAttributes, AdfMark, AdfNode } from '../../adf/document.ts'
 import type { LineContainer } from '../line-container.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 +9,7 @@ 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']
 
@@ -140,7 +140,9 @@ 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 +165,7 @@ 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[] = []
@@ -173,10 +175,10 @@ function highlighted(leaves: readonly AdfNode[]): AdfNode[] {
     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)))
-      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)
   }

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

@@ -127,6 +127,9 @@ test('spells a task list as a bullet list whose items lead with their state', ()
   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', () => {
@@ -138,7 +141,7 @@ 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')
 })
@@ -323,5 +326,5 @@ test('drops an empty paragraph and merges adjacent lists of one type', () => {
 test('keeps the nodes the plain flavour spells and degrades only what it cannot', () => {
   const tasks = node('taskList', {}, node('taskItem', { localId: '01a0d99b-1f58-7b95-829b-6f9860371d54', state: 'DONE' }, text('t')))
   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')), node('taskList', {}, node('taskItem', { state: 'DONE' }, text('t')))))
+  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 { alertMarker, foldedAlertMarker, taskMarker } from '../plain-conventions.ts'
+import { commonMarkSpelling, largestListMarker, writeMarkdown, type SpellingMemo } from './adf-to-markdown.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 +152,30 @@ function mergedLists(blocks: readonly AdfNode[]): AdfNode[] {
   return merged
 }
 
-function isList(block: AdfNode): boolean {
-  return block.type === 'bulletList' || block.type === 'orderedList'
+function listMarker(block: AdfNode): string | undefined {
+  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)] }
 }
 
+function tasksAsText(list: AdfNode): AdfNode {
+  if (list.type !== 'taskList') return list
+  const items: AdfNode[] = []
+  for (const child of nodeContent(list)) {
+    const marker = taskMarker(nodeAttrs(child)['state'])
+    const previous = child.type === 'taskItem' || child.type === 'blockTaskItem' ? undefined : items.pop()
+    if (previous !== undefined) items.push(itemOf([...nodeContent(previous), child]))
+    else items.push(itemOf(child.type === 'taskItem' ? [paragraph(nodeContent(child).length === 0 ? [text(marker)] : [text(`${marker} `), ...nodeContent(child)])] : marked(nodeContent(child), marker)))
+  }
+  return { content: items, type: 'bulletList' }
+}
+
 function paragraph(content: readonly AdfNode[]): AdfNode {
   return { content: [...content], type: 'paragraph' }
 }
@@ -183,15 +197,20 @@ function paragraphOfNode(node: AdfNode, reduction: Reduction): Result<AdfNode[]>
   return paragraphOf([node], reduction)
 }
 
-function quoted(head: Result<AdfNode[]>, node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
-  const content = concatenated([head, reduceBlocks(nodeContent(node), { ...reduction, depth: reduction.depth + 1 })])
-  return content.ok ? success([{ content: content.value, type: 'blockquote' }]) : content
+function contained(shell: AdfNode, node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
+  const content = reduceBlocks(nodeContent(node), { ...reduction, depth: reduction.depth + 1 })
+  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 marker = typeof title === 'string' ? `${foldedAlertMarker} ${title.replace(/^[ \t\n\r]+/, '').replaceAll('\n', ' ')}` : foldedAlertMarker
-  return quoted(paragraphOf([text(marker)], { ...reduction, depth: reduction.depth + 1 }), node, reduction)
+  const held = nodeAttrs(node)['title']
+  const title = typeof held === 'string' ? oneLine(held).replace(/^[ \t]+|[ \t]+$/g, '') : ''
+  return contained(title === '' ? { type: node.type } : { attrs: { title }, type: node.type }, node, reduction)
 }
 
 function reduceHeading(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
@@ -230,8 +249,11 @@ 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, type: 'listItem' }
+  return { content: blankedCode(blocks.slice(rules === -1 ? blocks.length : rules)), 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[] {
@@ -239,7 +261,48 @@ 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 nests in the task before it.
 function reduceTaskList(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
+  const children = nodeContent(node)
+  if (!isTask(children[0]) || children.some((child) => !isTask(child) && child.type !== 'taskList')) return reduceTasksAsText(node, reduction)
+  const tasks: AdfNode[] = []
+  let nested: AdfNode[] = []
+  for (const [index, child] of children.entries()) {
+    const at = childReduction(reduction, index)
+    const reduced = isTask(child) ? reduceTask(child, at) : reduceStanding(child, at)
+    if (!reduced.ok) return reduced
+    if (isTask(child)) {
+      nestIn(tasks, nested)
+      nested = []
+    }
+    for (const block of reduced.value) (isTask(child) ? tasks : nested).push(block)
+  }
+  nestIn(tasks, nested)
+  return success([{ content: tasks, type: 'taskList' }])
+}
+
+// The writer nests a list in the task before it, so one closing a block task item's blocks merges with it.
+function nestIn(tasks: AdfNode[], nested: readonly AdfNode[]): void {
+  const previous = tasks.at(-1)
+  if (previous?.type === 'blockTaskItem') tasks[tasks.length - 1] = { ...previous, content: mergedLists([...nodeContent(previous), ...nested]) }
+  else for (const block of mergedLists(nested)) tasks.push(block)
+}
+
+function isTask(node: AdfNode | undefined): boolean {
+  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
+}
+
+function reduceTasksAsText(node: AdfNode, reduction: Reduction): Result<AdfNode[]> {
   const items: AdfNode[] = []
   for (const [index, child] of nodeContent(node).entries()) {
     const blocks = taskBlocks(child, childReduction(reduction, index))

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 { highlightDelimiter } from '../plain-conventions.ts'
+import { delimiterFlags, matchEmphasis, runLength } from '../commonmark/emphasis-matching.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) })
-  return end
-}
-
-function flanks(character: string): boolean {
-  return character !== '' && !/\s/.test(character)
+  scan.pieces.push({ ...highlightFlanking(scan.source, index), kind: 'highlight' })
+  return index + highlightDelimiter.length
 }
 
 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)
-  if (marker === undefined) return undefined
-  const text = block.text.slice(marker.length)
-  return text === '' || /^(?:[ \t\n]|\\\n)/.test(text) ? { marker, position: block.position, text } : undefined
+  const marker = leadingMarker(block.text, read)
+  return marker === undefined ? undefined : { marker, position: block.position, text: block.text.slice(marker.length) }
 }
 
 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/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)
+  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]' : '[ ]'
 }