pulldown-cmark

Documentation

This library is a pull parser for CommonMark, written in Rust. It comes with a simple command-line tool, useful for rendering to HTML, and is also designed to be easy to use from as a library.

It is designed to be:

Fast; a bare minimum of allocation and copying
Safe; written in pure Rust with no unsafe blocks
Versatile; in particular source-maps are supported
Correct; the goal is 100% compliance with the CommonMark spec

Why a pull parser?

There are many parsers for Markdown and its variants, but to my knowledge none use pull parsing. Pull parsing has become popular for XML, especially for memory-conscious applications, because it uses dramatically less memory than constructing a document tree, but is much easier to use than push parsers. Push parsers are notoriously difficult to use, and also often error-prone because of the need for user to delicately juggle state in a series of callbacks.

In a clean design, the parsing and rendering stages are neatly separated, but this is often sacrificed in the name of performance and expedience. Many Markdown implementations mix parsing and rendering together, and even designs that try to separate them (such as the popular hoedown), make the assumption that the rendering process can be fully represented as a serialized string.

Pull parsing is in some sense the most versatile architecture. It's possible to drive a push interface, also with minimal memory, and quite straightforward to construct an AST. Another advantage is that source-map information (the mapping between parsed blocks and offsets within the source text) is readily available; you basically just call get_offset() as you consume events.

While manipulating ASTs is the most flexible way to transform documents, operating on iterators is surprisingly easy, and quite efficient. Here, for example, is the code to transform soft line breaks into hard breaks:

let parser = parser.map(|event| match event {
	Event::SoftBreak => Event::HardBreak,
	_ => event
});

Or expanding an abbreviation in text:

let parser = parser.map(|event| match event {
	Event::Text(text) => Event::Text(text.replace("abbr", "abbreviation")),
	_ => event
});

Another simple example is code to determine the max nesting level:

let mut max_nesting = 0;
let mut level = 0;
for event in parser {
	match event {
		Event::Start(_) => {
			level += 1;
			max_nesting = std::cmp::max(max_nesting, level);
		}
		Event::End(_) => level -= 1,
		_ => ()
	}
}

Using Rust idiomatically

A lot of the internal scanning code is written at a pretty low level (it pretty much scans byte patterns for the bits of syntax), but the external interface is designed to be idiomatic Rust.

Pull parsers are at heart an iterator of events (start and end tags, text, and other bits and pieces). The parser data structure implements the Rust Iterator trait directly, and Event is an enum. Thus, you can use the full power and expressivity of Rust's iterator infrastructure, including for loops and map (as in the examples above), collecting the events into a vector (for recording, playback, and manipulation), and more.

Further, the Text event (representing text) is a copy-on-write string (note: this isn't quite true yet). The vast majority of text fragments are just slices of the source document. For these, copy-on-write gives a convenient representation that requires no allocation or copying, but allocated strings are available when they're needed. Thus, when rendering text to HTML, most text is copied just once, from the source document to the HTML buffer.

Building only the pulldown-cmark library

By default, the binary is built as well. If you don't want/need it, then build like this:

> cargo build --no-default-features

Or put in your Cargo.toml file:

pulldown-cmark = { version = "0.0.11", default-features = false }

Authors

The main author is Raph Levien.

Contributions