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CommonMark parser and renderer in Emacs Lisp
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README.md

License BSD 2 Clause Build Status

cmark.el

CommonMark parser / HTML renderer.

This is a naive port of commonmark.js, the reference implementation of CommonMark in JavaScript.

Installation

Download latest release and execute M-x package-install-file for the .tar archive.

Usage

Instead of converting Markdown directly to HTML, as most converters do, commonmark.js parses Markdown to an AST (abstract syntax tree), and then renders this AST as HTML. This opens up the possibility of manipulating the AST between parsing and rendering. For example, one could transform emphasis into ALL CAPS.

Here's a basic usage example:

(require 'cmark)

(let ((parsed (cmark-parse "Hello *world*")) ;; parsed is a 'Node' tree
      result)
  ;; transform parsed if you like...

  (setq result (cmark-render-html parsed))) ;; result is a String

cmark-parse and cmark-render-html take optional parameters:

(cmark-parse "abc" :smart t)
(cmark-render-html parsed :sourcepos t)

cmark-parse currently supports the following:

  • smart: if non-nil, straight quotes will be made curly, -- will be changed to an en dash, --- will be changed to an em dash, and ... will be changed to ellipses.

cmark-render-html supports these options:

  • sourcepos: if non-nil, source position information for block-level elements will be rendered in the data-sourcepos attribute.
  • safe: if non-nil, raw HTML will not be passed through to HTML output (it will be replaced by comments), and potentially unsafe URLs in links and images (those beginning with javascript:, vbscript:, file:, and with a few exceptions data:) will be replaced with empty strings.
  • softbreak: specify raw string to be used for a softbreak.
  • esc: specify a function to be used to escape strings. Its first argument is the string to be escaped, the second argument is a boolean indicating whether to preserves entities in that string.

For example, to make soft breaks render as hard breaks in HTML:

(cmark-render-html parsed :softbreak "<br />")

To make them render as spaces:

(cmark-render-html parsed :softbreak " ")

The parser returns a cmark-Node. The following public properties are defined (those marked "read-only" have only a getter, not a setter):

  • type (read-only): a String, one of text, softbreak, linebreak, emph, strong, html_inline, link, image, code, document, paragraph, block_quote, item, list, heading, code_block, html_block, thematic_break.
  • firstChild (read-only): a Node or nil.
  • lastChild (read-only): a Node or nil.
  • next (read-only): a Node or nil.
  • prev (read-only): a Node or nil.
  • parent (read-only): a Node or nil.
  • sourcepos (read-only): an cons cell with the following form: ((startline . startcolumn) . (endline . endcolumn)).
  • isContainer (read-only): non-nil if the Node can contain other Nodes as children.
  • literal: the literal String content of the node or nil.
  • destination: link or image destination (String) or nil.
  • title: link or image title (String) or nil.
  • info: fenced code block info string (String) or nil.
  • level: heading level (Number).
  • listType: a String, either Bullet or Ordered.
  • listTight: non-nil if list is tight.
  • listStart: a Number, the starting number of an ordered list.
  • listDelimiter: a String, either ) or . for an ordered list.
  • onEnter, onExit: Strings, used only for custom_block or custom_inline.

Nodes have the following public methods:

  • (cmark-Node-appendChild node child): Append a Node child to the end of the Node's children.
  • (cmark-Node-prependChild node child): Prepend a Node child to the beginning of the Node's children.
  • (cmark-Node-unlink node): Remove the Node from the tree, severing its links with siblings and parents, and closing up gaps as needed.
  • (cmark-Node-insertAfter node sibling): Insert a Node sibling after the Node.
  • (cmark-Node-insertBefore node sibling): Insert a Node sibling before the Node.
  • (cmark-Node-walker node): Returns a NodeWalker that can be used to iterate through the Node tree rooted in the Node.

The cmark-NodeWalker returned by cmark-Node-walker has two methods:

  • (cmark-NodeWalker-next walker): Returns an cmark-event with properties entering (a boolean, which is non-nil when we enter a Node from a parent or sibling, and nil when we reenter it from a child). Returns nil when we have finished walking the tree.
  • (cmark-NodeWalker-resumeAt walker node entering): Resets the iterator to resume at the specified node and setting for entering. (Normally this isn't needed unless you do destructive updates to the Node tree.)

Here is an example of the use of a NodeWalker to iterate through the tree, making transformations. This simple example converts the contents of all text nodes to ALL CAPS:

(let ((walker (cmark-Node-walker parsed))
      event
      node)
  (while (setq event (cmark-NodeWalker-next walker))
    (setq node (cmark-event-node event))
    (when (and (cmark-event-entering event)
               (equal (cmark-Node-type node) "text"))
      (setf (cmark-Node-literal node) (upcase (cmark-Node-literal node))))))

This more complex example converts emphasis to ALL CAPS:

(let ((walker (cmark-Node-walker parsed))
      event
      node
      (inEmph nil))
  (while (setq event (cmark-NodeWalker-next walker))
    (setq node (cmark-event-node event))
    (cond
     ((equal (cmark-Node-type node) "emph")
      (if (cmark-event-entering event)
          (setq inEmph t)
        (setq inEmph nil)
        ;; add Emph node's children as siblings
        (while (cmark-Node-firstChild node)
          (cmark-Node-insertBefore node (cmark-Node-firstChild node)))
        ;; remove the empty Emph node
        (cmark-Node-unlink node)))
     ((and inEmph (equal (cmark-Node-type node) "text"))
      (setf (cmark-Node-literal node)
            (upcase (cmark-Node-literal node)))))))

Exercises for the reader: write a transform to

  1. De-linkify a document, transforming links to regular text.
  2. Remove all raw HTML (html_inline and html_block nodes).
  3. Run fenced code blocks marked with a language name through a syntax highlighting library, replacing them with an HtmlBlock containing the highlighted code.
  4. Print warnings to the console for images without image descriptions or titles.

A note on security

The library does not attempt to sanitize link attributes or raw HTML. If you use this library in applications that accept untrusted user input, you should either enable the safe option (see above) or run the output through an HTML sanitizer to protect against XSS attacks.

Hacking

To build the package locally, run make package. You need Cask.

To install the built package, run make install.

To run tests, run make test.

For other commands, run make help.

License

BSD-2-Clause. Some files are licensed under other conditions. See LICENSE for details. Copyright (C) 2019 taku0 and original commonmark.js authors.

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