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% Pandoc filters % John MacFarlane

Summary

Pandoc provides an interface for users to write programs (known as filters) which act on pandoc’s AST.

Pandoc consists of a set of readers and writers. When converting a document from one format to another, text is parsed by a reader into pandoc’s intermediate representation of the document---an "abstract syntax tree" or AST---which is then converted by the writer into the target format. The pandoc AST format is defined in the module Text.Pandoc.Definition in the pandoc-types package .

A "filter" is a program that modifies the AST, between the reader and the writer:

INPUT --reader--> AST --filter--> AST --writer--> OUTPUT

Filters are "pipes" that read from standard input and write to standard output. They consume and produce a JSON representation of the pandoc AST. (In recent versions, this representation includes a pandoc-api-version field which refers to a version of pandoc-types.) Filters may be written in any programming language. To use a filter, you need only specify it on the command line using --filter, e.g.

pandoc -s input.txt --filter pandoc-citeproc -o output.htl

For a gentle introduction into writing your own filters, continue this guide. There’s also a list of third party filters on the wiki.

A simple example

Suppose you wanted to replace all level 2+ headers in a markdown document with regular paragraphs, with text in italics. How would you go about doing this?

A first thought would be to use regular expressions. Something like this:

perl -pe 's/^##+ (.*)$/\*\1\*/' source.txt

This should work most of the time. But don't forget that ATX style headers can end with a sequence of #s that is not part of the header text:

## My header ##

And what if your document contains a line starting with ## in an HTML comment or delimited code block?

<!--
## This is just a comment
-->

~~~~
### A third level header in standard markdown
~~~~

We don't want to touch these lines. Moreover, what about setext style second-level headers?

A header
--------

We need to handle those too. Finally, can we be sure that adding asterisks to each side of our string will put it in italics? What if the string already contains asterisks around it? Then we'll end up with bold text, which is not what we want. And what if it contains a regular unescaped asterisk?

How would you modify your regular expression to handle these cases? It would be hairy, to say the least. What we need is a real parser.

Well, pandoc has a real markdown parser, the library function readMarkdown. This transforms markdown text to an abstract syntax tree (AST) that represents the document structure. Why not manipulate the AST directly in a short Haskell script, then convert the result back to markdown using writeMarkdown?

First, let's see what this AST looks like. We can use pandoc's native output format:

% cat test.txt
## my header

text with *italics*
% pandoc -s -t native test.txt
Pandoc (Meta {unMeta = fromList []})
[Header 3 ("my-header",[],[]) [Str "My",Space,Str "header"]
, Para [Str "text",Space,Str "with",Space,Emph [Str "italics"]] ]

A Pandoc document consists of a Meta block (containing metadata like title, authors, and date) and a list of Block elements. In this case, we have two Blocks, a Header and a Para. Each has as its content a list of Inline elements. For more details on the pandoc AST, see the haddock documentation for Text.Pandoc.Definition.

Here's a short Haskell script that reads markdown, changes level 2+ headers to regular paragraphs, and writes the result as markdown. If you save it as behead.hs, you can run it using runhaskell behead.hs. It will act like a unix pipe, reading from stdin and writing to stdout. Or, if you want, you can compile it, using ghc --make behead, then run the resulting executable behead.

-- behead.hs
import Text.Pandoc
import Text.Pandoc.Walk (walk)

behead :: Block -> Block
behead (Header n _ xs) | n >= 2 = Para [Emph xs]
behead x = x

readDoc :: String -> Pandoc
readDoc s = readMarkdown def s
-- or, for pandoc 1.14 and greater, use:
-- readDoc s = case readMarkdown def s of
--                  Right doc -> doc
--                  Left err  -> error (show err)

writeDoc :: Pandoc -> String
writeDoc doc = writeMarkdown def doc

main :: IO ()
main = interact (writeDoc . walk behead . readDoc)

The magic here is the walk function, which converts our behead function (a function from Block to Block) to a transformation on whole Pandoc documents. (See the haddock documentation for Text.Pandoc.Walk.)

Queries: listing URLs

We can use this same technique to do much more complex transformations and queries. Here's how we could extract all the URLs linked to in a markdown document (again, not an easy task with regular expressions):

-- extracturls.hs
import Text.Pandoc

extractURL :: Inline -> [String]
extractURL (Link _ _ (u,_)) = [u]
extractURL (Image _ _ (u,_)) = [u]
extractURL _ = []

extractURLs :: Pandoc -> [String]
extractURLs = query extractURL

readDoc :: String -> Pandoc
readDoc = readMarkdown def
-- or, for pandoc 1.14, use:
-- readDoc s = case readMarkdown def s of
--                Right doc -> doc
--                Left err  -> error (show err)

main :: IO ()
main = interact (unlines . extractURLs . readDoc)

query is the query counterpart of walk: it lifts a function that operates on Inline elements to one that operates on the whole Pandoc AST. The results returned by applying extractURL to each Inline element are concatenated in the result.

JSON filters

behead.hs is a very special-purpose program. It reads a specific input format (markdown) and writes a specific output format (HTML), with a specific set of options (here, the defaults). But the basic operation it performs is one that would be useful in many document transformations. It would be nice to isolate the part of the program that transforms the pandoc AST, leaving the rest to pandoc itself. What we want is a filter that just operates on the AST---or rather, on a JSON representation of the AST that pandoc can produce and consume:

                         source format
                              ↓
                           (pandoc)
                              ↓
                      JSON-formatted AST
                              ↓
                           (filter)
                              ↓
                      JSON-formatted AST
                              ↓
                           (pandoc)
                              ↓
                        target format

The module Text.Pandoc.JSON (from pandoc-types) contains a function toJSONFilter that makes it easy to write such filters. Here is a filter version of behead.hs:

#!/usr/bin/env runhaskell
-- behead2.hs
import Text.Pandoc.JSON

main :: IO ()
main = toJSONFilter behead
  where behead (Header n _ xs) | n >= 2 = Para [Emph xs]
        behead x = x

It can be used this way:

pandoc -f SOURCEFORMAT -t json | runhaskell behead2.hs | \
  pandoc -f json -t TARGETFORMAT

But it is easier to use the --filter option with pandoc:

pandoc -f SOURCEFORMAT -t TARGETFORMAT --filter ./behead2.hs

Note that this approach requires that behead2.hs be executable, so we must

chmod +x behead2.hs

Alternatively, we could compile the filter:

ghc --make behead2.hs
pandoc -f SOURCEFORMAT -t TARGETFORMAT --filter ./behead

Note that if the filter is placed in the system PATH, then the initial ./ is not needed. Note also that the command line can include multiple instances of --filter: the filters will be applied in sequence.

LaTeX for WordPress

Another easy example. WordPress blogs require a special format for LaTeX math. Instead of $e=mc^2$, you need: $LaTeX e=mc^2$. How can we convert a markdown document accordingly?

Again, it's difficult to do the job reliably with regexes. A $ might be a regular currency indicator, or it might occur in a comment or code block or inline code span. We just want to find the $s that begin LaTeX math. If only we had a parser...

We do. Pandoc already extracts LaTeX math, so:

#!/usr/bin/env runhaskell
-- wordpressify.hs
import Text.Pandoc.JSON

main = toJSONFilter wordpressify
  where wordpressify (Math x y) = Math x ("LaTeX " ++ y)
        wordpressify x = x

Mission accomplished. (I've omitted type signatures here, just to show it can be done.)

But I don't want to learn Haskell!

While it's easiest to write pandoc filters in Haskell, it is fairly easy to write them in python using the pandocfilters package. The package is in PyPI and can be installed using pip install pandocfilters or easy_install pandocfilters.

Here's our "beheading" filter in python:

#!/usr/bin/env python

"""
Pandoc filter to convert all level 2+ headers to paragraphs with
emphasized text.
"""

from pandocfilters import toJSONFilter, Emph, Para

def behead(key, value, format, meta):
  if key == 'Header' and value[0] >= 2:
    return Para([Emph(value[2])])

if __name__ == "__main__":
  toJSONFilter(behead)

toJSONFilter(behead) walks the AST and applies the behead action to each element. If behead returns nothing, the node is unchanged; if it returns an object, the node is replaced; if it returns a list, the new list is spliced in.

Note that, although these parameters are not used in this example, format provides access to the target format, and meta provides access to the document's metadata.

There are many examples of python filters in the pandocfilters repository.

For a more Pythonic alternative to pandocfilters, see the panflute library. Don't like Python? There are also ports of pandocfilters in PHP, perl, javascript/node.js, Groovy, and Ruby.

Starting with pandoc 2.0, pandoc includes built-in support for writing filters in lua. The lua interpreter is built in to pandoc, so a lua filter does not require any additional software to run. See the documentation on lua filters.

Include files

So none of our transforms have involved IO. How about a script that reads a markdown document, finds all the inline code blocks with attribute include, and replaces their contents with the contents of the file given?

#!/usr/bin/env runhaskell
-- includes.hs
import Text.Pandoc.JSON

doInclude :: Block -> IO Block
doInclude cb@(CodeBlock (id, classes, namevals) contents) =
  case lookup "include" namevals of
       Just f     -> return . (CodeBlock (id, classes, namevals)) =<< readFile f
       Nothing    -> return cb
doInclude x = return x

main :: IO ()
main = toJSONFilter doInclude

Try this on the following:

Here's the pandoc README:

~~~~ {include="README"}
this will be replaced by contents of README
~~~~

Removing links

What if we want to remove every link from a document, retaining the link's text?

#!/usr/bin/env runhaskell
-- delink.hs
import Text.Pandoc.JSON

main = toJSONFilter delink

delink :: Inline -> [Inline]
delink (Link _ txt _) = txt
delink x              = [x]

Note that delink can't be a function of type Inline -> Inline, because the thing we want to replace the link with is not a single Inline element, but a list of them. So we make delink a function from an Inline element to a list of Inline elements. toJSONFilter can still lift this function to a transformation of type Pandoc -> Pandoc.

A filter for ruby text

Finally, here's a nice real-world example, developed on the pandoc-discuss list. Qubyte wrote:

I'm interested in using pandoc to turn my markdown notes on Japanese into nicely set HTML and (Xe)LaTeX. With HTML5, ruby (typically used to phonetically read chinese characters by placing text above or to the side) is standard, and support from browsers is emerging (Webkit based browsers appear to fully support it). For those browsers that don't support it yet (notably Firefox) the feature falls back in a nice way by placing the phonetic reading inside brackets to the side of each Chinese character, which is suitable for other output formats too. As for (Xe)LaTeX, ruby is not an issue.

At the moment, I use inline HTML to achieve the result when the conversion is to HTML, but it's ugly and uses a lot of keystrokes, for example

<ruby>ご<rt></rt>飯<rp>（</rp><rt>はん</rt><rp>）</rp></ruby>

sets ご飯 "gohan" with "han" spelt phonetically above the second character, or to the right of it in brackets if the browser does not support ruby. I'd like to have something more like

r[はん](飯)

or any keystroke saving convention would be welcome.

We came up with the following script, which uses the convention that a markdown link with a URL beginning with a hyphen is interpreted as ruby:

[はん](-飯)

-- handleruby.hs
import Text.Pandoc.JSON
import System.Environment (getArgs)

handleRuby :: Maybe Format -> Inline -> Inline
handleRuby (Just format) (Link _ [Str ruby] ('-':kanji,_))
  | format == Format "html"  = RawInline format
    $ "<ruby>" ++ kanji ++ "<rp>(</rp><rt>" ++ ruby ++ "</rt><rp>)</rp></ruby>"
  | format == Format "latex" = RawInline format
    $ "\\ruby{" ++ kanji ++ "}{" ++ ruby ++ "}"
  | otherwise = Str ruby
handleRuby _ x = x

main :: IO ()
main = toJSONFilter handleRuby

Note that, when a script is called using --filter, pandoc passes it the target format as the first argument. When a function's first argument is of type Maybe Format, toJSONFilter will automatically assign it Just the target format or Nothing.

We compile our script:

ghc --make handleRuby

Then run it:

% pandoc -F ./handleRuby -t html
[はん](-飯)
^D
<p><ruby>飯<rp>(</rp><rt>はん</rt><rp>)</rp></ruby></p>
% pandoc -F ./handleRuby -t latex
[はん](-飯)
^D
\ruby{飯}{はん}

Exercises

  1. Put all the regular text in a markdown document in ALL CAPS (without touching text in URLs or link titles).

  2. Remove all horizontal rules from a document.

  3. Renumber all enumerated lists with roman numerals.

  4. Replace each delimited code block with class dot with an image generated by running dot -Tpng (from graphviz) on the contents of the code block.

  5. Find all code blocks with class python and run them using the python interpreter, printing the results to the console.