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<main>
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<h1 id="preprocessors"><a class="header" href="#preprocessors">Preprocessors</a></h1>
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<p>A <em>preprocessor</em> is simply a bit of code which gets run immediately after the
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book is loaded and before it gets rendered, allowing you to update and mutate
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the book. Possible use cases are:</p>
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<ul>
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<li>Creating custom helpers like <code>{{#include /path/to/file.md}}</code></li>
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<li>Substituting in latex-style expressions (<code>$$ \frac{1}{3} $$</code>) with their
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mathjax equivalents</li>
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</ul>
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<p>See <a href="../format/configuration/preprocessors.html">Configuring Preprocessors</a> for more information about using preprocessors.</p>
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<h2 id="hooking-into-mdbook"><a class="header" href="#hooking-into-mdbook">Hooking Into MDBook</a></h2>
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<p>MDBook uses a fairly simple mechanism for discovering third party plugins.
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A new table is added to <code>book.toml</code> (e.g. <code>[preprocessor.foo]</code> for the <code>foo</code>
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preprocessor) and then <code>mdbook</code> will try to invoke the <code>mdbook-foo</code> program as
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part of the build process.</p>
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<p>Once the preprocessor has been defined and the build process starts, mdBook executes the command defined in the <code>preprocessor.foo.command</code> key twice.
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The first time it runs the preprocessor to determine if it supports the given renderer.
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mdBook passes two arguments to the process: the first argument is the string <code>supports</code> and the second argument is the renderer name.
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The preprocessor should exit with a status code 0 if it supports the given renderer, or return a non-zero exit code if it does not.</p>
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<p>If the preprocessor supports the renderer, then mdbook runs it a second time, passing JSON data into stdin.
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The JSON consists of an array of <code>[context, book]</code> where <code>context</code> is the serialized object <a href="https://docs.rs/mdbook/latest/mdbook/preprocess/struct.PreprocessorContext.html"><code>PreprocessorContext</code></a> and <code>book</code> is a <a href="https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html"><code>Book</code></a> object containing the content of the book.</p>
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<p>The preprocessor should return the JSON format of the <a href="https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html"><code>Book</code></a> object to stdout, with any modifications it wishes to perform.</p>
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<p>The easiest way to get started is by creating your own implementation of the
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<code>Preprocessor</code> trait (e.g. in <code>lib.rs</code>) and then creating a shell binary which
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translates inputs to the correct <code>Preprocessor</code> method. For convenience, there
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is <a href="https://github.com/rust-lang/mdBook/blob/master/examples/nop-preprocessor.rs">an example no-op preprocessor</a> in the <code>examples/</code> directory which can easily
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be adapted for other preprocessors.</p>
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<details>
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<summary>Example no-op preprocessor</summary>
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<pre><pre class="playground"><code class="language-rust edition2018">// nop-preprocessors.rs
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use crate::nop_lib::Nop;
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use clap::{Arg, ArgMatches, Command};
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use mdbook::book::Book;
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use mdbook::errors::Error;
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use mdbook::preprocess::{CmdPreprocessor, Preprocessor, PreprocessorContext};
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use semver::{Version, VersionReq};
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use std::io;
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use std::process;
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pub fn make_app() -> Command {
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Command::new("nop-preprocessor")
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.about("A mdbook preprocessor which does precisely nothing")
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.subcommand(
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Command::new("supports")
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.arg(Arg::new("renderer").required(true))
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.about("Check whether a renderer is supported by this preprocessor"),
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)
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}
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fn main() {
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let matches = make_app().get_matches();
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// Users will want to construct their own preprocessor here
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let preprocessor = Nop::new();
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if let Some(sub_args) = matches.subcommand_matches("supports") {
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handle_supports(&preprocessor, sub_args);
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} else if let Err(e) = handle_preprocessing(&preprocessor) {
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eprintln!("{}", e);
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process::exit(1);
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}
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}
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fn handle_preprocessing(pre: &dyn Preprocessor) -> Result<(), Error> {
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let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())?;
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let book_version = Version::parse(&ctx.mdbook_version)?;
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let version_req = VersionReq::parse(mdbook::MDBOOK_VERSION)?;
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if !version_req.matches(&book_version) {
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eprintln!(
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"Warning: The {} plugin was built against version {} of mdbook, \
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but we're being called from version {}",
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pre.name(),
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mdbook::MDBOOK_VERSION,
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ctx.mdbook_version
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);
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}
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let processed_book = pre.run(&ctx, book)?;
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serde_json::to_writer(io::stdout(), &processed_book)?;
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Ok(())
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}
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fn handle_supports(pre: &dyn Preprocessor, sub_args: &ArgMatches) -> ! {
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let renderer = sub_args
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.get_one::<String>("renderer")
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.expect("Required argument");
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let supported = pre.supports_renderer(renderer);
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// Signal whether the renderer is supported by exiting with 1 or 0.
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if supported {
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process::exit(0);
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} else {
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process::exit(1);
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}
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}
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/// The actual implementation of the `Nop` preprocessor. This would usually go
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/// in your main `lib.rs` file.
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mod nop_lib {
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use super::*;
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/// A no-op preprocessor.
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pub struct Nop;
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impl Nop {
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pub fn new() -> Nop {
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Nop
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}
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}
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impl Preprocessor for Nop {
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fn name(&self) -> &str {
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"nop-preprocessor"
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}
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fn run(&self, ctx: &PreprocessorContext, book: Book) -> Result<Book, Error> {
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// In testing we want to tell the preprocessor to blow up by setting a
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// particular config value
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if let Some(nop_cfg) = ctx.config.get_preprocessor(self.name()) {
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if nop_cfg.contains_key("blow-up") {
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anyhow::bail!("Boom!!1!");
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}
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}
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// we *are* a no-op preprocessor after all
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Ok(book)
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}
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fn supports_renderer(&self, renderer: &str) -> bool {
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renderer != "not-supported"
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}
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}
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#[cfg(test)]
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mod test {
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use super::*;
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#[test]
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fn nop_preprocessor_run() {
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let input_json = r##"[
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{
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"root": "/path/to/book",
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"config": {
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"book": {
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"authors": ["AUTHOR"],
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"language": "en",
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"multilingual": false,
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"src": "src",
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"title": "TITLE"
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},
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"preprocessor": {
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"nop": {}
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}
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},
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"renderer": "html",
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"mdbook_version": "0.4.21"
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},
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{
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"sections": [
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{
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"Chapter": {
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"name": "Chapter 1",
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"content": "# Chapter 1\n",
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"number": [1],
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"sub_items": [],
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"path": "chapter_1.md",
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"source_path": "chapter_1.md",
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"parent_names": []
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}
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}
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],
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"__non_exhaustive": null
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}
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]"##;
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let input_json = input_json.as_bytes();
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let (ctx, book) = mdbook::preprocess::CmdPreprocessor::parse_input(input_json).unwrap();
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let expected_book = book.clone();
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let result = Nop::new().run(&ctx, book);
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assert!(result.is_ok());
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// The nop-preprocessor should not have made any changes to the book content.
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let actual_book = result.unwrap();
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assert_eq!(actual_book, expected_book);
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}
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}
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}</code></pre></pre>
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</details>
|
|
<h2 id="hints-for-implementing-a-preprocessor"><a class="header" href="#hints-for-implementing-a-preprocessor">Hints For Implementing A Preprocessor</a></h2>
|
|
<p>By pulling in <code>mdbook</code> as a library, preprocessors can have access to the
|
|
existing infrastructure for dealing with books.</p>
|
|
<p>For example, a custom preprocessor could use the
|
|
<a href="https://docs.rs/mdbook/latest/mdbook/preprocess/trait.Preprocessor.html#method.parse_input"><code>CmdPreprocessor::parse_input()</code></a> function to deserialize the JSON written to
|
|
<code>stdin</code>. Then each chapter of the <code>Book</code> can be mutated in-place via
|
|
<a href="https://docs.rs/mdbook/latest/mdbook/book/struct.Book.html#method.for_each_mut"><code>Book::for_each_mut()</code></a>, and then written to <code>stdout</code> with the <code>serde_json</code>
|
|
crate.</p>
|
|
<p>Chapters can be accessed either directly (by recursively iterating over
|
|
chapters) or via the <code>Book::for_each_mut()</code> convenience method.</p>
|
|
<p>The <code>chapter.content</code> is just a string which happens to be markdown. While it's
|
|
entirely possible to use regular expressions or do a manual find & replace,
|
|
you'll probably want to process the input into something more computer-friendly.
|
|
The <a href="https://crates.io/crates/pulldown-cmark"><code>pulldown-cmark</code></a> crate implements a production-quality event-based
|
|
Markdown parser, with the <a href="https://crates.io/crates/pulldown-cmark-to-cmark"><code>pulldown-cmark-to-cmark</code></a> crate allowing you to
|
|
translate events back into markdown text.</p>
|
|
<p>The following code block shows how to remove all emphasis from markdown,
|
|
without accidentally breaking the document.</p>
|
|
<pre><pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>fn remove_emphasis(
|
|
num_removed_items: &mut usize,
|
|
chapter: &mut Chapter,
|
|
) -> Result<String> {
|
|
let mut buf = String::with_capacity(chapter.content.len());
|
|
|
|
let events = Parser::new(&chapter.content).filter(|e| {
|
|
let should_keep = match *e {
|
|
Event::Start(Tag::Emphasis)
|
|
| Event::Start(Tag::Strong)
|
|
| Event::End(Tag::Emphasis)
|
|
| Event::End(Tag::Strong) => false,
|
|
_ => true,
|
|
};
|
|
if !should_keep {
|
|
*num_removed_items += 1;
|
|
}
|
|
should_keep
|
|
});
|
|
|
|
cmark(events, &mut buf, None).map(|_| buf).map_err(|err| {
|
|
Error::from(format!("Markdown serialization failed: {}", err))
|
|
})
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<p>For everything else, have a look <a href="https://github.com/rust-lang/mdBook/blob/master/examples/nop-preprocessor.rs">at the complete example</a>.</p>
|
|
<h2 id="implementing-a-preprocessor-with-a-different-language"><a class="header" href="#implementing-a-preprocessor-with-a-different-language">Implementing a preprocessor with a different language</a></h2>
|
|
<p>The fact that mdBook utilizes stdin and stdout to communicate with the preprocessors makes it easy to implement them in a language other than Rust.
|
|
The following code shows how to implement a simple preprocessor in Python, which will modify the content of the first chapter.
|
|
The example below follows the configuration shown above with <code>preprocessor.foo.command</code> actually pointing to a Python script.</p>
|
|
<pre><code class="language-python">import json
|
|
import sys
|
|
|
|
|
|
if __name__ == '__main__':
|
|
if len(sys.argv) > 1: # we check if we received any argument
|
|
if sys.argv[1] == "supports":
|
|
# then we are good to return an exit status code of 0, since the other argument will just be the renderer's name
|
|
sys.exit(0)
|
|
|
|
# load both the context and the book representations from stdin
|
|
context, book = json.load(sys.stdin)
|
|
# and now, we can just modify the content of the first chapter
|
|
book['sections'][0]['Chapter']['content'] = '# Hello'
|
|
# we are done with the book's modification, we can just print it to stdout,
|
|
print(json.dumps(book))
|
|
</code></pre>
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