MMark

MMark (read “em-mark”) is a strict markdown processor for writers. “Strict” means that not every input is considered a valid markdown document and parse errors are possible and even desirable, because they allow to spot markup issues without searching for them in rendered document. If a markdown document passes MMark parser, then it’ll most certainly produce HTML without quirks. This feature makes it a good choice for writers and bloggers.

MMark in its current state features:

• A parser that produces high-quality error messages and does not choke on first parse error. It is capable of reporting many parse errors where makes sense.
• An extension system allowing to create extensions that alter parsed markdown document in some way. Some of them are available in the mmark-ext package.
• A lucid-based render.

There is also a blog post announcing the project:

https://markkarpov.com/post/announcing-mmark.html

MMark and Common Mark

MMark mostly tries to follow the Common Mark specification as given here:

https://github.com/jgm/CommonMark

However, due to the fact that we do not allow inputs that do not make sense, and also try to guard against common silly mistakes (like writing ##My header and having it rendered as a paragraph starting with hashes) MMark obviously can’t follow the specification precisely. In particular, parsing of inlines differs considerably from Common Mark (see below).

Another difference between Common Mark and MMark is that the latter supports more (pun alert) common markdown extensions out-of-the-box. In particular, MMark supports:

• parsing of an optional YAML block
• strikeout using ~~this~~ syntax
• superscript using ^this^ syntax
• subscript using ~this~ syntax
• automatic assignment of ids to headers
• PHP-style footnotes, e.g. [^1] (NOT YET)
• “pipe” tables (as used on GitHub) (NOT YET)

One do not need to enable or tweak anything for these to work, they are built-in features.

Differences in inline parsing

Emphasis and strong emphasis is an especially hairy topic in the Common Mark specification. There are 17 ad-hoc rules defining interaction between * and _ -based emphasis and more than half of all Common Mark examples (that’s about 300) test just this tricky logic.

Not only it is hard to implement (for tools built around markdown too), it’s hard to understand for humans too. For example, this input:

*(*foo*)*

produces this HTML:

<p><em>(<em>foo</em>)</em></p>

(Note the nested emphasis.)

Could it produce something like this instead?

<p><em>(</em>foo<em>)</em></p>

Well, why not? Without remembering those 17 ad-hoc rules, there going to be a lot of tricky cases when a user won’t be able to tell how markdown will be parsed.

I decided to make parsing of emphasis, strong emphasis, and similar constructs like strikethrough, subscript, and superscript more symmetric and less ad-hoc. This is a work in progress and I’m not fully satisfied with the current approach as it does not allow to express some combinations of characters and markup, but in 99% of practical cases it is identical to Common Mark, and normal markdown intuitions will work OK for the users.

Let’s start by dividing all characters into three groups:

• Markup characters, including the following: *, ~, _, `, ^, [, ]. These are used for markup and whenever they appear in a document, they must form valid markup constructions. To be used as ordinary punctuation characters they must be backslash escaped.

• Space characters, including space, tab, newline, carriage return, and some Unicode space characters.

• Other characters, which include all characters not falling into the two groups described above.

Markup characters can be “converted” to other characters via backslash escaping. We’ll see how this is useful in a few moments.

We’ll call markdown characters placed between space characters and other characters left-flanking delimiter run. These markup characters sort of hang on the left hand side of a word.

Similarly we’ll call markdown characters placed between other characters and space characters right-flanking delimiter run. These hang on the right hand side of a word.

Emphasis markup (and other similar things like strikethrough, which we won’t mention explicitly anymore for brevity) can start only as left-flanking delimiter run and end only as right-flanking delimiter run.

This produces a parse error:

*Something * is not right.
Something __is __ not right.

And this too:

__foo__bar

This means that inter-word emphasis is not supported by this approach. (This is a pity, maybe I should adjust something to allow it.)

There is one more tricky thing. In some cases we want to end emphasis and have full stop or other punctuation right after it:

Here it *goes*.

You can see that the closing * is not in right-flanking position here, and so it’s a parse error. To avoid this, some punctuation characters that normally appear outside of markup were made “transparent” and thus they are regarded as white space, so the example above parses correctly and works as expected. To put a transparent character inside emphasis, backslash escaping is necessary:

We *\(can\)* have it.

Here ( and ) are transparent punctuation characters, just like ., so they must be turned into other characters to go inside the emphasis. This is a corner case and should not be common in practice.

So far the main limitation of this approach is the pains with inter-word markup, as in this example:

**We started to work on the *issue*.**

Should we escape . here? On one hand we should, to close **. But if we do, the closing * won’t be in right-flanking position anymore. God dammit.

Other differences

Block-level parsing:

• Headings, thematic breaks, code blocks should be separated from paragraphs by at least one empty line. This makes the parser a lot simpler and forces markdown sources to be in a more readable form too.
• If a line starts with hash signs it is expected to be a valid non-empty header (level 1–6 inclusive). If you want to start a paragraph with hashes, just escape the first hash with backslash and that will be enough.
• Fenced code blocks must be explicitly closed by a closing fence. They are not closed by the end of document or by start of another block.

Inline-level parsing:

• MMark does not support hard line breaks represented as double space before newline. Nevertheless, hard line breaks in the form of backslash before newline are supported (these are more explicit too).
• All URI references (in links, images, autolinks, etc.) are parsed as per RFC 3986, no special escaping is supported. In addition to that, when a URI reference in not enclosed with < and >, then closing parenthesis character ) is not considered part of URI (use <uri> syntax if you want closing parenthesis as part of a URI).
• Putting links in text of another link is not allowed, i.e. no nested links is possible.
• Putting images in description of other images is not allowed (similarly to the situation with links).

Not-yet-implemented things:

• Separate declaration of image’s source and title is not (yet?) supported.
• Blockquotes are not implemented yet.
• Lists (unordered and ordered) are not implemented yet.
• Setext headings are not implemented yet.
• Reference links are not implemented yet.
• HTML blocks are not implemented yet.
• HTML inlines are not implemented yet.
• Entity and numeric character references are not implemented yet.

Additional information about MMark-specific extensions

• YAML block must start with three hyphens --- and end with three hyphens ---. It can only be placed at the beginning of a markdown document. Trailing white space after the --- sequences is allowed.

Contribution

Issues, bugs, and questions may be reported in the GitHub issue tracker for this project.

Pull requests are also welcome and will be reviewed quickly.

License

Copyright © 2017 Mark Karpov

Distributed under BSD 3 clause license.