Slack mrkdwn Format

Slack mrkdwn is Slack's proprietary lightweight markup language used in messages, blocks, and notifications. It uses *bold*, _italic_, ~strikethrough~, `code`, triple-backtick code fences, and <url|display> angle-bracket links. It is distinct from CommonMark — the syntax is simpler and incompatible.

Package: relational-text/slackNamespace: com.slack.mrkdwn.facet

Functions

import { from, to } from 'relational-text/registry'

from('slack', input: string): Document

Parse a Slack mrkdwn string into a Document.

const doc = from('slack', 'Hello *world*! Check out <https://example.com|this link>.')

Handles:

• Inline marks: *bold*, _italic_, ~strikethrough~, `code`
• Links with display text: <url|display> — display text is stored as the text content, URL in the url attribute
• Bare links: <url> — the URL is emitted as both text content and the url attribute of a link feature
• User and channel refs: <@user> / <#channel> — emitted as plain text (not structured features)
• Block types: paragraph, > blockquote lines, • / - / * bullet lists
• Code blocks: triple-backtick fences with optional language identifier

Empty lines are skipped and do not produce empty paragraph blocks. An unclosed code block at end-of-input is flushed automatically.

to('slack', doc: Document): string

Render a Document to a Slack mrkdwn string.

const mrkdwn = to('slack', doc)

Automatically applies any registered lenses targeting com.slack.mrkdwn.facet via lensGraph.autoTransform(). A Document parsed from Markdown, HTML, or another format will have its features mapped to Slack equivalents before rendering.

Block rendering behaviour:

• paragraph — rendered as a plain line followed by \n
• heading — Slack has no heading syntax; rendered as *bold text*
• list-item-text — rendered with - prefix
• code-block — rendered as a triple-backtick fence with optional language tag
• horizontal-rule — rendered as ---
• blockquote-marker, bullet-list-marker, list-item-marker — structural separator blocks that produce no output; the surrounding context (e.g. ctx.inBlockquote) controls the > prefix on paragraph lines

The trailing newline is stripped from the returned string.

ensureSlackLexicon(): void

Register the Slack lexicon and the Slack → RelationalText lens with autoApply: true. Called automatically by from('slack', ...) and to('slack', ...) on first use. Safe to call multiple times — subsequent calls are no-ops.

Feature Mapping

Inline Marks

Slack syntax	Feature name	Type ID	Expand	RT equivalent
*text*	bold	com.slack.mrkdwn.facet#bold	both	bold
_text_	italic	com.slack.mrkdwn.facet#italic	both	italic
~text~	strike	com.slack.mrkdwn.facet#strike	both	strikethrough
`text`	code	com.slack.mrkdwn.facet#code	none	code

Entities

Slack syntax	Feature name	Type ID	Attrs	RT equivalent
<url|display>	link	com.slack.mrkdwn.facet#link	url	link
<url> (bare)	link	com.slack.mrkdwn.facet#link	url	link

Block Elements

Feature name	Type ID	Attrs	RT equivalent
paragraph	com.slack.mrkdwn.facet#paragraph	—	paragraph
code-block	com.slack.mrkdwn.facet#code-block	language?	code-block
list-item-text	com.slack.mrkdwn.facet#list-item-text	—	list-item-text
blockquote-marker	com.slack.mrkdwn.facet#blockquote-marker	—	blockquote-marker
bullet-list-marker	com.slack.mrkdwn.facet#bullet-list-marker	—	bullet-list-marker
list-item-marker	com.slack.mrkdwn.facet#list-item-marker	—	list-item-marker

Lens Integration

When ensureSlackLexicon() is called it registers a Slack → RelationalText lens (com.slack.to.relationaltext.v1) with autoApply: true. The lens maps all Slack feature names directly to their RT equivalents (all mappings are identity — bold → bold, code-block → code-block, etc.). The passthrough: "keep" policy means features from other namespaces already in the document are preserved.

This enables bidirectional cross-format conversion through the lens graph:

import { from, to } from 'relational-text/registry'

// Slack → Markdown
const doc = from('slack', 'Hello *world*!')
const md = to('markdown', doc)
// 'Hello **world**!\n\n'

// Markdown → Slack
const mdDoc = from('markdown', 'Hello **world**!')
const mrkdwn = to('slack', mdDoc)
// 'Hello *world*!'

Examples

Parse a Slack message with a link

import { from } from 'relational-text/registry'

const doc = from('slack', "Check out <https://example.com|this article> — it's *great*!")
const json = doc.toJSON()
// text:   "Check out this article — it's great!"
// facets: [link over "this article" with url="https://example.com",
//          bold over "great"]

Parse a code block with language tag

import { from } from 'relational-text/registry'

const input = '```typescript\nconst x = 1\n```'
const doc = from('slack', input)
// code-block block with attrs: { language: 'typescript' }
// text content: "const x = 1\n"

Export a Markdown document to Slack

import { from, to } from 'relational-text/registry'

const doc = from('markdown', '## Introduction\n\nHello **world**!\n\n- item one\n- item two\n')
const mrkdwn = to('slack', doc)
// '*Introduction*\nHello *world*!\n- item one\n- item two'
// heading rendered as bold; bullet items with - prefix

Blockquote round-trip

import { from, to } from 'relational-text/registry'

const doc = from('slack', '> This is quoted\nAnd this is not.')
const out = to('slack', doc)
// '> This is quoted\nAnd this is not.'

Notes

• @user and #channel refs are plain text. The Slack wire format encodes user and channel references as <@U12345> or <#C12345>. The importer treats angle-bracket refs beginning with @ or # as plain text, not as structured mention features. Full Slack API integration (resolving user IDs) is outside the scope of this importer.
• No ordered list support. Slack mrkdwn does not define an ordered list syntax. The importer recognises • , - , and * as bullet list prefixes only. The exporter renders all list items with - regardless of original list type.
• Headings degrade to bold. Slack mrkdwn has no heading syntax. When exporting, heading blocks are rendered as *heading text*.
• Link url attr. The Slack link feature stores the URL in a url attribute (not href or uri). The lens maps this directly to the RT link url attr.
• Expand semantics. Bold, italic, and strikethrough marks use Peritext expand-on-both-sides semantics. Inline code does not expand — inserting text adjacent to a code span does not extend it.