Tissevert/pandoc

Fork 0

John MacFarlane f28f34244a Update title and authors on lua-filters.md.

2017-11-20 16:51:45 -08:00

29 KiB

Raw Blame History

title

author

date

Pandoc Lua Filters

Albert Krewinkel

John MacFarlane

November 20, 2017

Introduction

Pandoc has long supported filters, which allow the pandoc abstract syntax tree (AST) to be manipulated between the parsing and the writing phase. Traditional pandoc filters accept a JSON representation of the pandoc AST and produce an altered JSON representation of the AST. They may be written in any programming language, and invoked from pandoc using the --filter option.

Although traditional filters are very flexible, they have a couple of disadvantages. First, there is some overhead in writing JSON to stdout and reading it from stdin (twice, once on each side of the filter). Second, whether a filter will work will depend on details of the user's environment. A filter may require an interpreter for a certain programming language to be available, as well as a library for manipulating the pandoc AST in JSON form. One cannot simply provide a filter that can be used by anyone who has a certain version of the pandoc executable.

Starting with pandoc 2.0, we have made it possible to write filters in lua without any external dependencies at all. A lua interpreter and a lua library for creating pandoc filters is built into the pandoc executable. Pandoc data types are marshalled to lua directly, avoiding the overhead of writing JSON to stdout and reading it from stdin.

Here is an example of a lua filter that converts strong emphasis to small caps:

return {
  {
    Strong = function (elem)
      return pandoc.SmallCaps(elem.c)
    end,
  }
}

or equivalently,

function Strong(elem)
  return pandoc.SmallCaps(elem.c)
end

This says: walk the AST, and when you find a Strong element, replace it with a SmallCaps element with the same content.

To run it, save it in a file, say smallcaps.lua, and invoke pandoc with --lua-filter=smallcaps.lua.

Here's a quick performance comparison, using a version of the pandoc manual, MANUAL.txt, and versions of the same filter written in compiled Haskell (smallcaps) and interpreted Python (smallcaps.py):

Command Time

pandoc MANUAL.txt 1.01s pandoc MANUAL.txt --filter ./smallcaps 1.36s pandoc MANUAL.txt --filter ./smallcaps.py 1.40s pandoc MANUAL.txt --lua-filter ./smallcaps.lua 1.03s

As you can see, the lua filter avoids the substantial overhead associated with marshalling to and from JSON over a pipe.

Lua filter structure

Lua filters are tables with element names as keys and values consisting of functions acting on those elements.

Filters are expected to be put into separate files and are passed via the --lua-filter command-line argument. For example, if a filter is defined in a file current-date.lua, then it would be applied like this:

pandoc --lua-filter=current-date.lua -f markdown MANUAL.txt

The --lua-filter can be supplied multiple times, causing the filters to be applied sequentially in the order they were given. If other, non-Lua filters are given as well (via --filter), then those are executed after all Lua filters have been applied.

Pandoc expects each lua file to return a list of filters. The filters in that list are called sequentially, each on the result of the previous filter. If there is no value returned by the filter script, then pandoc will try to generate a single filter by collecting all top-level functions whose names correspond to those of pandoc elements (e.g., Str, Para, Meta, or Pandoc). (That is why the two examples above are equivalent.)

For each filter, the document is traversed and each element subjected to the filter. Elements for which the filter contains an entry (i.e. a function of the same name) are passed to lua element filtering function. In other words, filter entries will be called for each corresponding element in the document, getting the respective element as input.

The return of a filter function must one of the following:

nil: this means that the object should remain unchanged.
a pandoc object: this must be of the same type as the input and will replace the original object.
a list of pandoc objects: these will replace the original object; the list is merged with the neighbors of the orignal objects (spliced into the list the original object belongs to); returning an empty list deletes the object.

The function's output must result in an element of the same type as the input. This means a filter function acting on an inline element must return either nil, an inline, or a list of inlines, and a function filtering a block element must return one of nil, a block, or a list of block elements. Pandoc will throw an error if this condition is violated.

If there is no function matching the element's node type, then the filtering system will look for a more general fallback function. Two fallback functions are supported, Inline and Block. Each matches elements of the respective type.

Elements without matching functions are left untouched.

See module documentation for a list of pandoc elements.

The global FORMAT is set to the format of the pandoc writer being used (html5, latex, etc.), so the behavior of a filter can be made conditional on the eventual output format.

Pandoc Module

The pandoc lua module is loaded into the filter's lua environment and provides a set of functions and constants to make creation and manipulation of elements easier. The global variable pandoc is bound to the module and should generally not be overwritten for this reason.

Two major functionalities are provided by the module: element creator functions and access to some of pandoc's main functionalities.

Element creation

Element creator functions like Str, Para, and Pandoc are designed to allow easy creation of new elements that are simple to use and can be read back from the lua environment. Internally, pandoc uses these functions to create the lua objects which are passed to element filter functions. This means that elements created via this module will behave exactly as those elements accessible through the filter function parameter.

Exposed pandoc functionality

Some pandoc functions have been made available in lua:

walk_block and walk_inline allow filters to be applied inside specific block or inline elements.
read allows filters to parse strings into pandoc documents
pipe runs an external command with input from and output to strings
sha1 generates a SHA1 hash
The mediabag module allows access to the "mediabag," which stores binary content such as images that may be included in the final document.

Examples

Macro substitution.

The following filter converts the string {{helloworld}} into emphasized text "Hello, World".

return {
  {
    Str = function (elem)
      if elem.text == "{{helloworld}}" then
        return pandoc.Emph {pandoc.Str "Hello, World"}
      else
        return elem
      end
    end,
  }
}

Default metadata file

This filter causes metadata defined in an external file (metadata-file.yaml) to be used as default values in a document's metadata:

-- read metadata file into string
local metafile = io.open('metadata-file.yaml', 'r')
local content = metafile:read("*a")
metafile:close()
-- get metadata
local default_meta = pandoc.read(content, "markdown").meta

return {
  {
    Meta = function(meta)
      -- use default metadata field if it hasn't been defined yet.
      for k, v in pairs(default_meta) do
        if meta[k] == nil then
          meta[k] = v
        end
      end
      return meta
    end,
  }

Setting the date in the metadata

This filter sets the date in the document's metadata to the current date:

function Meta(m)
  m.date = os.date("%B %e, %Y")
  return m
end

Extracting information about links

This filter prints a table of all the URLs linked to in the document, together with the number of links to that URL.

links = {}

function Link (el)
  if links[el.target] then
    links[el.target] = links[el.target] + 1
  else
    links[el.target] = 1
  end
  return el
end

function Doc (blocks, meta)
  function strCell(str)
    return {pandoc.Plain{pandoc.Str(str)}}
  end
  local caption = {pandoc.Str "Link", pandoc.Space(), pandoc.Str "count"}
  local aligns = {pandoc.AlignDefault, pandoc.AlignLeft}
  local widths = {0.8, 0.2}
  local headers = {strCell "Target", strCell "Count"}
  local rows = {}
  for link, count in pairs(links) do
    rows[#rows + 1] = {strCell(link), strCell(count)}
  end
  return pandoc.Doc(
    {pandoc.Table(caption, aligns, widths, headers, rows)},
    meta
  )
end

Replacing placeholders with their metadata value

Lua filter functions are run in the order

Inlines → Blocks → Meta → Pandoc.

Passing information from a higher level (e.g., metadata) to a lower level (e.g., inlines) is still possible by using two filters living in the same file:

local vars = {}

function get_vars (meta)
  for k, v in pairs(meta) do
    if v.t == 'MetaInlines' then
      vars["$" .. k .. "$"] = v
    end
  end
end

function replace (el)
  if vars[el.text] then
    return pandoc.Span(vars[el.text])
  else
    return el
  end
end

return {{Meta = get_vars}, {Str = replace}}

If the contents of file occupations.md is

---
name: Samuel Q. Smith
occupation: Professor of Phrenology
---

Name

: \$name\$

Occupation

: \$occupation\$

then running pandoc --lua-filter=meta-vars.lua occupations.md will output:

<dl>
<dt>Name</dt>
<dd><p><span>Samuel Q. Smith</span></p>
</dd>
<dt>Occupation</dt>
<dd><p><span>Professor of Phrenology</span></p>
</dd>
</dl>

Modifying pandoc's `MANUAL.txt` for man pages

This is the filter we use when converting MANUAL.txt to man pages. It converts level-1 headers to uppercase (using walk_block to transform inline elements inside headers), removes footnotes, and replaces links with regular text.

-- we use preloaded text to get a UTF-8 aware 'upper' function
local text = require('text')

function Header(el)
    if el.level == 1 then
      return pandoc.walk_block(el, {
        Str = function(el)
            return pandoc.Str(text.upper(el.text))
        end })
    end
end

function Link(el)
    return el.content
end

function Note(el)
    return {}
end

Creating a handout from a paper

This filter extracts all the numbered examples, section headers, block quotes, and figures from a document, in addition to any divs with class handout. (Note that only blocks at the "outer level" are included; this ignores blocks inside nested constructs, like list items.)

-- creates a handout from an article, using its headings,
-- blockquotes, numbered examples, figures, and any
-- Divs with class "handout"

function Pandoc(doc)
    local hblocks = {}
    for i,el in pairs(doc.blocks) do
        if (el.t == "Div" and el.classes[1] == "handout") or
           (el.t == "BlockQuote") or
           (el.t == "OrderedList" and el.style == "Example") or
           (el.t == "Para" and #el.c == 1 and el.c[1].t == "Image") or
           (el.t == "Header") then
           table.insert(hblocks, el)
        end
    end
    return pandoc.Pandoc(hblocks, doc.meta)
end

Counting words in a document

This filter counts the words in the body of a document (omitting metadata like titles and abstracts), including words in code. It should be more accurate than wc -w run directly on a Markdown document, since the latter will count markup characters, like the # in front of an ATX header, or tags in HTML documents, as words. To run it, pandoc --lua-filter wordcount.lua myfile.md.

-- counts words in a document

words = 0

wordcount = {
  Str = function(el)
    -- we don't count a word if it's entirely punctuation:
    if el.text:match("%P") then
        words = words + 1
    end
  end,

  Code = function(el)
    _,n = el.text:gsub("%S+","")
    words = words + n
  end,

  CodeBlock = function(el)
    _,n = el.text:gsub("%S+","")
    words = words + n
  end
}

function Pandoc(el)
    -- skip metadata, just count body:
    pandoc.walk_block(pandoc.Div(el.blocks), wordcount)
    print(words .. " words in body")
    os.exit(0)
end

Converting ABC code to music notation

This filter replaces code blocks with class abc with images created by running their contents through abcm2ps and ImageMagick's convert. (For more on ABC notation, see http://abcnotation.com.)

Images are added to the mediabag. For output to binary formats, pandoc will use images in the mediabag. For textual formats, use --extract-media to specify a directory where the files in the mediabag will be written, or (for HTML only) use --self-contained.

-- Pandoc filter to process code blocks with class "abc" containing
-- ABC notation into images.
--
-- * Assumes that abcm2ps and ImageMagick's convert are in the path.
-- * For textual output formats, use --extract-media=abc-images
-- * For HTML formats, you may alternatively use --self-contained

local filetypes = { html = {"png", "image/png"}
                  , latex = {"pdf", "application/pdf"}
                  }
local filetype = filetypes[FORMAT][1] or "png"
local mimetype = filetypes[FORMAT][2] or "image/png"

local function abc2eps(abc, filetype)
    local eps = pandoc.pipe("abcm2ps", {"-q", "-O", "-", "-"}, abc)
    local final = pandoc.pipe("convert", {"-", filetype .. ":-"}, eps)
    return final
end

function CodeBlock(block)
    if block.classes[1] == "abc" then
        local img = abc2eps(block.text, filetype)
        local fname = pandoc.sha1(img) .. "." .. filetype
        pandoc.mediabag.insert(fname, mimetype, img)
        return pandoc.Para{ pandoc.Image({pandoc.Str("abc tune")}, fname) }
    end
end

Module text

UTF-8 aware text manipulation functions, implemented in Haskell. These are available to any lua filter. However, the module must be explicitly loaded:

-- uppercase all regular text in a document:
text = require 'text'
function Str (s)
  s.text = text.upper(s.text)
  return s
end

[lower (s)]{#text-lower}: Returns a copy of a UTF-8 string, converted to lowercase.
[upper (s)]{#text-upper}: Returns a copy of a UTF-8 string, converted to uppercase.
[reverse (s)]{#text-reverse}: Returns a copy of a UTF-8 string, with characters reversed.
[len (s)]{#text-len}: Returns the length of a UTF-8 string.
[sub (s)]{#text-sub}: Returns a substring of a UTF-8 string, using lua's string indexing rules.

Module pandoc

Lua functions for pandoc scripts.

Pandoc Document

[Pandoc (blocks[, meta])]{#Pandoc}

A complete pandoc document

Parameters:

blocks:: document content
meta:: document meta data

MetaValue

[MetaBlocks (blocks)]{#MetaBlocks}

Meta blocks

Parameters:

blocks:: blocks

[MetaInlines (inlines)]{#MetaInlines}

Meta inlines

Parameters:

inlines:: inlines

[MetaList (meta_values)]{#MetaList}

Meta list

Parameters:

meta_values:: list of meta values

[MetaMap (key_value_map)]{#MetaMap}

Meta map

Parameters:

key_value_map:: a string-indexed map of meta values

[MetaString (str)]{#MetaString}

Creates string to be used in meta data.

Parameters:

str:: string value

[MetaBool (bool)]{#MetaBool}

Creates boolean to be used in meta data.

Parameters:

bool:: boolean value

Blocks

[Block]{#Block}

Block elements

[BlockQuote (content)]{#BlockQuote}

Creates a block quote element

Parameters:

content:: block content

Returns: block quote element

[BulletList (content)]{#BulletList}

Creates a bullet (i.e.

Parameters:

content:: list of items

Returns: block quote element

[CodeBlock (text[, attr])]{#CodeBlock}

Creates a code block element

Parameters:

text:: code string
attr:: element attributes

Returns: code block element

[DefinitionList (content)]{#DefinitionList}

Creates a definition list, containing terms and their explanation.

Parameters:

content:: list of items

Returns: block quote element

[Div (content[, attr])]{#Div}

Creates a div element

Parameters:

content:: block content
attr:: element attributes

Returns: code block element

[Header (level, content[, attr])]{#Header}

Creates a block quote element.

Parameters:

level:: header level
content:: inline content
attr:: element attributes

Returns: header element

[HorizontalRule ()]{#HorizontalRule}

Creates a horizontal rule.

Returns: horizontal rule

[LineBlock (content)]{#LineBlock}

Creates a line block element.

Parameters:

content:: inline content

Returns: block quote element

[Null ()]{#Null}

Creates a null element.

Returns: null element

[OrderedList (items[, listAttributes])]{#OrderedList}

Creates an ordered list.

Parameters:

items:: list items
listAttributes:: list parameters

Returns:

[Para (content)]{#Para}

Creates a para element.

Parameters:

content:: inline content

Returns: block quote element

[Plain (content)]{#Plain}

Creates a plain element.

Parameters:

content:: inline content

Returns: block quote element

[RawBlock (format, text)]{#RawBlock}

Creates a raw content block of the specified format.

Parameters:

format:: format of content
text:: string content

Returns: block quote element

[Table (caption, aligns, widths, headers, rows)]{#Table}

Creates a table element.

Parameters:

caption:: table caption
aligns:: alignments
widths:: column widths
headers:: header row
rows:: table rows

Returns: block quote element

Inline

[Inline]{#Inline}

Inline element class

[Cite (content, citations)]{#Cite}

Creates a Cite inline element

Parameters:

content:: List of inlines
citations:: List of citations

Returns: citations element

[Code (text[, attr])]{#Code}

Creates a Code inline element

Parameters:

text:: brief image description
attr:: additional attributes

Returns: code element

[Emph (content)]{#Emph}

Creates an inline element representing emphasised text.

Parameters:

content:: inline content

Returns: emphasis element

[Image (caption, src[, title[, attr]])]{#Image}

Creates a Image inline element

Parameters:

caption:: text used to describe the image
src:: path to the image file
title:: brief image description
attr:: additional attributes

Returns: image element

[LineBreak ()]{#LineBreak}

Create a LineBreak inline element

Returns: linebreak element

[Link (content, target[, title[, attr]])]{#Link}

Creates a link inline element, usually a hyperlink.

Parameters:

content:: text for this link
target:: the link target
title:: brief link description
attr:: additional attributes

Returns: image element

[Math (mathtype, text)]{#Math}

Creates a Math element, either inline or displayed.

Parameters:

mathtype:: rendering specifier
text:: Math content

Returns: Math element

[DisplayMath (text)]{#DisplayMath}

Creates a DisplayMath element (DEPRECATED).

Parameters:

text:: Math content

Returns: Math element

[InlineMath (text)]{#InlineMath}

Creates an InlineMath inline element (DEPRECATED).

Parameters:

text:: Math content

Returns: Math element

[Note (content)]{#Note}

Creates a Note inline element

Parameters:

content:: footnote block content

[Quoted (quotetype, content)]{#Quoted}

Creates a Quoted inline element given the quote type and quoted content.

Parameters:

quotetype:: type of quotes to be used
content:: inline content

Returns: quoted element

[SingleQuoted (content)]{#SingleQuoted}

Creates a single-quoted inline element (DEPRECATED).

Parameters:

content:: inline content

Returns: quoted element

Helpers

[Attr ([identifier[, classes[, attributes]]])]{#Attr}

Create a new set of attributes (Attr).

Parameters:

identifier:: element identifier
classes:: element classes
attributes:: table containing string keys and values

Returns: element attributes

[Citation (id, mode[, prefix[, suffix[, note_num[, hash]]]])]{#Citation}

Creates a single citation.

Parameters:

id:: citation identifier (like a bibtex key)
mode:: citation mode
prefix:: citation prefix
suffix:: citation suffix
note_num:: note number
hash:: hash number

Constants

[AuthorInText]{#AuthorInText}

Author name is mentioned in the text.

See also: Table

[AlignRight]{#AlignRight}

Table cells right-aligned.

See also: Table

[AlignCenter]{#AlignCenter}

Table cell content is centered.

See also: Table

[AlignDefault]{#AlignDefault}

Table cells are alignment is unaltered.

See also: Table

[DefaultDelim]{#DefaultDelim}

Default list number delimiters are used.

See also: OrderedList

[Period]{#Period}

List numbers are delimited by a period.

See also: OrderedList

[OneParen]{#OneParen}

List numbers are delimited by a single parenthesis.

See also: OrderedList

[TwoParens]{#TwoParens}

List numbers are delimited by a double parentheses.

See also: OrderedList

[DefaultStyle]{#DefaultStyle}

List are numbered in the default style

See also: OrderedList

[Example]{#Example}

List items are numbered as examples.

See also: OrderedList

[Decimal]{#Decimal}

List are numbered using decimal integers.

See also: OrderedList

[LowerRoman]{#LowerRoman}

List are numbered using lower-case roman numerals.

See also: OrderedList

[UpperRoman]{#UpperRoman}

List are numbered using upper-case roman numerals

See also: OrderedList

[LowerAlpha]{#LowerAlpha}

List are numbered using lower-case alphabetic characters.

See also: OrderedList

[UpperAlpha]{#UpperAlpha}

List are numbered using upper-case alphabetic characters.

See also: OrderedList

Helper Functions

[walk_block (element, filter)]{#walk_block}

Apply a filter inside a block element, walking its contents.

Parameters:

element:: the block element
filter:: a lua filter (table of functions) to be applied within the block element

Returns: the transformed block element

[walk_inline (element, filter)]{#walk_inline}

Apply a filter inside an inline element, walking its contents.

Parameters:

element:: the inline element
filter:: a lua filter (table of functions) to be applied within the inline element

Returns: the transformed inline element

[read (markup[, format])]{#read}

Parse the given string into a Pandoc document.

Parameters:

markup:: the markup to be parsed
format:: format specification, defaults to "markdown".

Returns: pandoc document

Usage:

local org_markup = "/emphasis/"  -- Input to be read
local document = pandoc.read(org_markup, "org")
-- Get the first block of the document
local block = document.blocks[1]
-- The inline element in that block is an `Emph`
assert(block.content[1].t == "Emph")

[global_filter ()]{#global_filter}

Use functions defined in the global namespace to create a pandoc filter.

Returns: A list of filter functions

Usage:

-- within a file defining a pandoc filter:
function Str(text)
  return pandoc.Str(utf8.upper(text))
end

return {pandoc.global_filter()}
-- the above is equivallent to
-- return {{Str = Str}}

[sha1 (contents)]{#mediabag-sha1}

Returns the SHA1 has of the contents.

Returns:

SHA1 hash of the contents.

Usage:

local fp = pandoc.mediabag.sha1("foobar")

[pipe (command, args, input)]{#mediabag-sha1}

Runs command with arguments, passing it some input, and returns the output.

Returns:

Output of command.

Raises:

A table containing the keys command, error_code, and output is thrown if the command exits with a non-zero error code.

Usage:

local output = pandoc.pipe("sed", {"-e","s/a/b/"}, "abc")

Submodule mediabag

The submodule mediabag allows accessing pandoc's media storage. The "media bag" is used when pandoc is called with the --extract-media or --standalone/-s option.

[insert (filepath, mime_type, contents)]{#mediabag-insert}

Adds a new entry to pandoc's media bag.

Parameters:

filepath:: filename and path relative to the output folder.
mime_type:: the file's MIME type
contents:: the binary contents of the file.

Usage:

local fp = "media/hello.txt"
local mt = "text/plain"
local contents = "Hello, World!"
pandoc.mediabag(fp, mt, contents)

[list ()]{#mediabag-list}

Get a summary of the current media bag contents.

Returns: A list of elements summarizing each entry in the media bag. The summary item contains the keys path, type, and length, giving the filepath, MIME type, and length of contents in bytes, respectively.

Usage:

-- calculate the size of the media bag.
local mb_items = pandoc.mediabag.list()
local sum = 0
for i = 1, #mb_items:
    sum = sum + mb_items[i].length
end
print(sum)

[lookup (filepath)]{#mediabag-lookup}

Lookup a media item in the media bag, returning mime type and contents.

Parameters:

filepath:: name of the file to look up.

Returns:

the entries MIME type, or nil if the file was not found.
contents of the file, or nil if the file was not found.

Usage:

local filename = "media/diagram.png"
local mt, contents = pandoc.mediabag.lookup(filename)

[fetch (source, base_url)]{#mediabag-fetch}

Fetches the given source from a URL or local file. Returns two values: the contents of the file and the mime type (or an empty string).

Returns:

the entries MIME type, or nil if the file was not found.
contents of the file, or nil if the file was not found.

Usage:

local diagram_url = "https://pandoc.org/diagram.jpg"
local contents = pandoc.mediabag.fetch(diagram_url, ".")

29 KiB Raw Blame History

Introduction

Lua filter structure

Pandoc Module

Element creation

Exposed pandoc functionality

Examples

Macro substitution.

Default metadata file

Setting the date in the metadata

Extracting information about links

Replacing placeholders with their metadata value

Modifying pandoc's MANUAL.txt for man pages

Creating a handout from a paper

Counting words in a document

Converting ABC code to music notation

Module text

Module pandoc

Pandoc Document

MetaValue

Blocks

Inline

Helpers

Constants

Helper Functions

Submodule mediabag

29 KiB

Raw Blame History

Modifying pandoc's `MANUAL.txt` for man pages