author

date

title

Albert Krewinkel

John MacFarlane

December 6, 2017

Pandoc Lua Filters

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 option may be supplied multiple times. Pandoc applies all filters (including JSON filters specified via --filter and lua filters specified via --lua-filter) in the order they appear on the command line.

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 original 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.

Global variables

Pandoc passes additional data to Lua filters by setting global variables.

FORMAT : 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_READER_OPTIONS : Table of the options which were provided to the parser.

PANDOC_VERSION : Contains the pandoc version as a numerically indexed table, most significant number first. E.g., for pandoc 2.1.1, the value of the variable is a table {2, 1, 1}. Use table.concat(PANDOC_VERSION, '.') to produce a version string. This variable is also set in custom writers.

PANDOC_API_VERSION : Contains the version of the pandoc-types API against which pandoc was compiled. It is given as a numerically indexed table, most significant number first. E.g., if pandoc was compiled against pandoc-types 1.17.3, then the value of the variable will be a table {1, 17, 3}. Use table.concat(PANDOC_API_VERSION, '.') to produce a version string from this table. This variable is also set in custom writers.

PANDOC_SCRIPT_FILE : The name used to involve the filter. This value can be used to find files relative to the script file. This variable is also set in custom writers.

PANDOC_STATE : The state shared by all readers and writers. It is used by pandoc to collect and pass information. The value of this variable is of type CommonState and is read-only.

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;
the pandoc.mediabag module allows access to the "mediabag," which stores binary content such as images that may be included in the final document;
the pandoc.utils module contains various utility functions.

Lua interpreter initialization

The way the Lua interpreter is set-up can be controlled by placing a file init.lua in pandoc's data directory. The default init file loads the pandoc and pandoc.mediabag modules:

pandoc = require 'pandoc'
pandoc.mediabag = require 'pandoc.mediabag'

A common use-case would be to add code to load additional modules or to alter default modules. E.g., the following snippet adds all unicode-aware functions defined in the text module to the default string module, prefixed with the string uc_.

for name, fn in pairs(require 'text') do
  string['uc_' .. name] = fn
end

This makes it possible to apply these functions on strings using colon syntax (mystring:uc_upper()).

Examples

The following filters are presented as examples. A repository of useful lua filters (which may also serve as good examples) is available at https://github.com/pandoc/lua-filters.

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["$" .. k .. "$"] = {table.unpack(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

Building images with tikz

This filter converts raw LaTeX tikz environments into images. It works with both PDF and HTML output. The tikz code is compiled to an image using pdflatex, and the image is converted (if necessary) from pdf to png format using ImageMagick's convert, so both of these must be in the system path. Converted images are cached in the working directory and given filenames based on a hash of the source, so that they need not be regenerated each time the document is built. (A more sophisticated version of this might put these in a special cache directory.)

local function tikz2image(src, filetype, outfile)
    local tmp = os.tmpname()
    local tmpdir = string.match(tmp, "^(.*[\\/])") or "."
    local f = io.open(tmp .. ".tex", 'w')
    f:write("\\documentclass{standalone}\n\\usepackage{tikz}\n\\begin{document}\n")
    f:write(src)
    f:write("\n\\end{document}\n")
    f:close()
    os.execute("pdflatex -output-directory " .. tmpdir  .. " " .. tmp)
    if filetype == 'pdf' then
        os.rename(tmp .. ".pdf", outfile)
    else
        os.execute("convert " .. tmp .. ".pdf " .. outfile)
    end
    os.remove(tmp .. ".tex")
    os.remove(tmp .. ".pdf")
    os.remove(tmp .. ".log")
    os.remove(tmp .. ".aux")
end

extension_for = {
    html = 'png',
    html4 = 'png',
    html5 = 'png',
    latex = 'pdf',
    beamer = 'pdf' }

local function file_exists(name)
    local f = io.open(name, 'r')
    if f ~= nil then
        io.close(f)
        return true
    else
        return false
    end
end

function RawBlock(el)
    local filetype = extension_for[FORMAT] or "png"
    local fname = pandoc.sha1(el.text) .. "." .. filetype
    if not file_exists(fname) then
        tikz2image(el.text, filetype, fname)
    end
    return pandoc.Para({pandoc.Image({}, fname)})
end

Example of use:

pandoc --lua-filter tikz.lua -s -o cycle.html <<EOF
Here is a diagram of the cycle:

\begin{tikzpicture}

\def \n {5}
\def \radius {3cm}
\def \margin {8} % margin in angles, depends on the radius

\foreach \s in {1,...,\n}
{
  \node[draw, circle] at ({360/\n * (\s - 1)}:\radius) {$\s$};
  \draw[->, >=latex] ({360/\n * (\s - 1)+\margin}:\radius)
    arc ({360/\n * (\s - 1)+\margin}:{360/\n * (\s)-\margin}:\radius);
}
\end{tikzpicture}
EOF

Lua type reference

This section describes the types of objects available to Lua filters. See the pandoc module for functions to create these objects.

Pandoc {#type-ref-pandoc}

Pandoc document

blocks : document content (List of Blocks)

meta : document meta information ([Meta] object)

Meta {#type-ref-meta}

Meta information on a document; string-indexed collection of MetaValues.

MetaValue {#type-ref-MetaValue}

Document meta information items.

MetaBlocks {#type-ref-MetaBlocks}

A list of blocks usable as meta value (List of Blocks)

Fields:

tag, t : the literal MetaBlocks (string)

MetaBool {#type-ref-MetaBool}

Plain Lua boolean value (boolean)

MetaInlines {#type-ref-MetaInlines}

List of inlines used in metadata (List of Inlines)

Fields:

tag, t : the literal MetaInlines (string)

MetaList {#type-ref-iMetaList}

A list of other MetaValues. (List)

Fields:

tag, t : the literal MetaList (string)

MetaMap {#type-ref-MetaMap}

A string-indexed map of meta-values. (table)

Fields:

tag, t : the literal MetaMap (string)

Note: The fields will be shadowed if the map contains a field with the same name as those listed.

MetaString {#type-ref-MetaString}

Plain Lua string value (string)

Block {#type-ref-Block}

BlockQuote {#type-ref-BlockQuote}

A block quote element

content: : block content (List of Blocks)

tag, t : the literal BlockQuote (string)

BulletList {#type-ref-BulletList}

A bullet list

content : list of items (List of Blocks)

tag, t : the literal BulletList (string)

CodeBlock {#type-ref-CodeBlock}

Block of code.

text : code string (string)

attr : element attributes (Attr)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal CodeBlock (string)

DefinitionList {#type-ref-DefinitionList}

Definition list, containing terms and their explanation.

content : list of items

tag, t : the literal DefinitionList (string)

Div {#type-ref-Div}

Generic block container with attributes

content : block content (List of Blocks)

attr : element attributes (Attr)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Div (string)

Header {#type-ref-Header}

Creates a header element.

level : header level (integer)

content : inline content (List of Inlines)

attr : element attributes (Attr)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Header (string)

HorizontalRule {#type-ref-HorizontalRule}

A horizontal rule.

tag, t : the literal HorizontalRule (string)

LineBlock {#type-ref-LineBlock}

A line block, i.e. a list of lines, each separated from the next by a newline.

content : inline content

tag, t : the literal LineBlock (string)

Null {#type-ref-Null}

A null element; this element never produces any output in the target format.

tag, t : the literal Null (string)

OrderedList {#type-ref-OrderedList}

An ordered list.

Parameters:

items : list items (List of Blocks)

listAttributes : list parameters ([ListAttributes])

start : alias for listAttributes.start (integer)

style : alias for listAttributes.style (string)

delimiter : alias for listAttributes.delimiter (string)

tag, t : the literal OrderedList (string)

Para {#type-ref-Para}

A paragraph

content : inline content (List of Inlines)

tag, t : the literal Para (string)

Plain {#type-ref-Plain}

Plain text, not a paragraph

content : inline content (List of Inlines)

tag, t : the literal Plain (string)

RawBlock {#type-ref-RawBlock}

Raw content of a specified format.

format : format of content (string)

text : raw content (string)

tag, t : the literal RawBlock (string)

Table {#type-ref-Table}

A table.

caption : table caption (List of Inlines)

aligns : column alignments (List of Alignments)

widths : column widths (number)

headers : header row (List of table cells)

rows : table rows (List of Lists of table cells)

tag, t : the literal Table (string)

A [table cell]{#table-cell} is a list of blocks.

Alignment{#Alignment} is a string value indicating the horizontal alignment of a table column. AlignLeft, AlignRight, and AlignCenter leads cell content tob be left-aligned, right-aligned, and centered, respectively. The default alignment is AlignDefault (often equivalent to centered).

Inline {#type-ref-Inline}

Cite {#type-ref-Cite}

Citation

content : (List of Inlines)

citations : citation entries (List of citations)

tag, t : the literal Cite (string)

Code {#type-ref-Code}

Inline code

text : code string (string)

attr : attributes (Attr)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Code (string)

Emph {#type-ref-Emph}

Emphasized text

content : inline content (List of Inlines)

tag, t : the literal Emph (string)

Image {#type-ref-Image}

Image: alt text (list of inlines), target

attr : attributes (Attr)

caption : text used to describe the image (List of Inlines)

src : path to the image file (string)

title : brief image description

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Image (string)

LineBreak {#type-ref-LineBreak}

Hard line break

tag, t : the literal LineBreak (string)

Link {#type-ref-Link}

Hyperlink: alt text (list of inlines), target

attr : attributes (Attr)

content : text for this link (List of Inlines)

target : the link target (string)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Link (string)

Math {#type-ref-Math}

TeX math (literal)

mathype : specifier determining whether the math content should be shown inline (InlineMath) or on a separate line (DisplayMath) (string)

text : math content (string)

tag, t : the literal Math (string)

Note {#type-ref-Note}

Footnote or endnote

content : (List of Blocks)

tag, t : the literal Note (string)

Quoted {#type-ref-Quoted}

Quoted text

quotetype : type of quotes to be used; one of SingleQuote or DoubleQuote (string)

content : quoted text (List of Inlines)

tag, t : the literal Quoted (string)

RawInline {#type-ref-RawInline}

Raw inline

format : the format of the content (string)

text : raw content (string)

tag, t : the literal RawInline (string)

SmallCaps {#type-ref-SmallCaps}

Small caps text

content : (List of Inlines)

tag, t : the literal SmallCaps (string)

SoftBreak {#type-ref-SoftBreak}

Soft line break

tag, t : the literal SoftBreak (string)

Space {#type-ref-Space}

Inter-word space

tag, t : the literal Space (string)

Span {#type-ref-Span}

Generic inline container with attributes

attr : attributes (Attr)

content : wrapped content (List of Inlines)

identifier : alias for attr.identifier (string)

classes : alias for attr.classes (List of strings)

attributes : alias for attr.attributes (Attributes)

tag, t : the literal Span (string)

Str {#type-ref-Str}

Text

text : content (string)

tag, t : the literal Str (string)

Strikeout {#type-ref-Strikeout}

Strikeout text

content : inline content (List of Inlines)

tag, t : the literal Strikeout (string)

Strong {#type-ref-Strong}

Strongly emphasized text

content : inline content (List of Inlines)

tag, t : the literal Strong (string)

Subscript {#type-ref-Subscript}

Subscripted text

content : inline content (List of Inlines)

tag, t : the literal Subscript (string)

Superscript {#type-ref-Superscript}

Superscripted text

content : inline content (List of Inlines)

tag, t : the literal Superscript (string)

Element components

Attr {#type-ref-Attr}

A set of element attributes

identifier : element identifier (string)

classes : element classes (List of strings)

attributes : collection of key/value pairs (Attributes)

Attributes {#type-ref-Attributes}

List of key/value pairs. Values can be accessed by using keys as indices to the list table.

Citation {#type-ref-Citation}

Single citation entry

id : citation identifier, e.g., a bibtex key (string)

mode : citation mode, one of AuthorInText, SuppressAuthor, or NormalCitation (string)

prefix : citation prefix (List of Inlines)

suffix : citation suffix (List of Inlines)

note_num : note number (integer)

hash : hash (integer)

ListAttributes {#type-ref-ListAttributes}

List attributes

start : number of the first list item (integer)

style : style used for list numbers; possible values are DefaultStyle, Example, Decimal, LowerRoman, UpperRoman, LowerAlpha, and UpperAlpha (string)

delimiter : delimiter of list numbers; one of DefaultDelim, Period, OneParen, and TwoParens (string)

Hierarchical Element {#type-ref-Element}

Hierarchical elements can be either Sec (sections) or Blk (blocks). Blk elements are treated like Blocks.

Sec {#type-ref-Sec}

Section elements used to provide hierarchical information on document contents.

Objects of this type are read-only.

level : header level (integer)

numbering : section numbering (list of integers)

attr : header attributes (Attr)

label : header content (list of Inlines)

contents : list of contents in this section (list of hierarchical elements)

tag, t : constant Sec (string)

ReaderOptions {#type-ref-ReaderOptions}

Pandoc reader options

abbreviations : set of known abbreviations (set of strings)

columns : number of columns in terminal (integer)

default_image_extension : default extension for images (string)

extensions : string representation of the syntax extensions bit field (string)

indented_code_classes : default classes for indented code blocks (list of strings)

standalone : whether the input was a standalone document with header (boolean)

strip_comments : HTML comments are stripped instead of parsed as raw HTML (boolean)

tab_stop : width (i.e. equivalent number of spaces) of tab stops (integer)

track_changes : track changes setting for docx; one of AcceptChanges, RejectChanges, and AllChanges (string)

CommonState {#type-ref-CommonState}

The state used by pandoc to collect information and make it available to readers and writers.

input_files : List of input files from command line (List of strings)

output_file : Output file from command line (string or nil)

log : A list of log messages in reverse order (List of LogMessages)

request_headers : Headers to add for HTTP requests; table with header names as keys and header contents as value (table)

resource_path : Path to search for resources like included images (List of strings)

source_url : Absolute URL or directory of first source file (string or nil)

user_data_dir : Directory to search for data files (string or nil)

trace : Whether tracing messages are issued (boolean)

verbosity : Verbosity level; one of INFO, WARNING, ERROR (string)

LogMessage {#type-ref-LogMessage}

A pandoc log message. Object have no fields, but can be converted to a string via tostring.

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 {#type-ref-}

[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: bullet list 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: definition list element

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

: Creates a div element

Parameters:

`content`:
:   block content

`attr`:
:   element attributes

Returns: div element

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

: Creates a header 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: line block 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: ordered list element

[Para (content)]{#Para}

: Creates a para element.

Parameters:

`content`:
:   inline content

Returns: paragraph element

[Plain (content)]{#Plain}

: Creates a plain element.

Parameters:

`content`:
:   inline content

Returns: plain element

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

: Creates a raw content block of the specified format.

Parameters:

`format`:
:   format of content

`text`:
:   string content

Returns: raw block 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: table 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`:
:   code string

`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

See also: [Quoted](#Quoted)

[DoubleQuoted (content)]{#DoubleQuoted}

: Creates a single-quoted inline element (DEPRECATED).

Parameters:

`content`:
:   inline content

Returns: quoted element

See also: [Quoted](#Quoted)

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

: Creates a RawInline inline element

Parameters:

`format`:
:   format of the contents

`text`:
:   string content

Returns: raw inline element

[SmallCaps (content)]{#SmallCaps}

: Creates text rendered in small caps

Parameters:

`content`:
:   inline content

Returns: smallcaps element

[SoftBreak ()]{#SoftBreak}

: Creates a SoftBreak inline element.

Returns: softbreak element

[Space ()]{#Space}

: Create a Space inline element

Returns: space element

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

: Creates a Span inline element

Parameters:

`content`:
:   inline content

`attr`:
:   additional attributes

Returns: span element

[Str (text)]{#Str}

: Creates a Str inline element

Parameters:

`text`:
:   content

Returns: string element

[Strikeout (content)]{#Strikeout}

: Creates text which is striked out.

Parameters:

`content`:
:   inline content

Returns: strikeout element

[Strong (content)]{#Strong}

: Creates a Strong element, whose text is usually displayed in a bold font.

Parameters:

`content`:
:   inline content

Returns: strong element

[Subscript (content)]{#Subscript}

: Creates a Subscript inline element

Parameters:

`content`:
:   inline content

Returns: subscript element

[Superscript (content)]{#Superscript}

: Creates a Superscript inline element

Parameters:

`content`:
:   inline content

Returns: strong element

Element components

[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

ListAttributes ([start[, style[, delimiter]]])

: Creates a set of list attributes

Parameters:

`start`:
:   number of the first list item (default: 1)

`style`:
:   style used for list numbering (default: `DefaultStyle`)

`delimiter`:
:   delimiter of list numbers (default: `DefaultDelim`)

Returns: list attributes table

Constants

[AuthorInText]{#AuthorInText}

: Author name is mentioned in the text.

See also: [Citation](#Citation)

[SuppressAuthor]{#SuppressAuthor}

: Author name is suppressed.

See also: [Citation](#Citation)

[NormalCitation]{#NormalCitation}

: Default citation style is used.

See also: [Citation](#Citation)

[AlignLeft]{#AlignLeft}

: Table cells aligned left.

See also: [Table](#Table)

[AlignRight]{#AlignRight}

: Table cells right-aligned.

See also: [Table](#Table)

[AlignCenter]{#AlignCenter}

: Table cell content is centered.

See also: [Table](#Table)

[AlignDefault]{#AlignDefault}

: Table cells are alignment is unaltered.

See also: [Table](#Table)

[DefaultDelim]{#DefaultDelim}

: Default list number delimiters are used.

See also: [OrderedList](#OrderedList)

[Period]{#Period}

: List numbers are delimited by a period.

See also: [OrderedList](#OrderedList)

[OneParen]{#OneParen}

: List numbers are delimited by a single parenthesis.

See also: [OrderedList](#OrderedList)

[TwoParens]{#TwoParens}

: List numbers are delimited by a double parentheses.

See also: [OrderedList](#OrderedList)

[DefaultStyle]{#DefaultStyle}

: List are numbered in the default style

See also: [OrderedList](#OrderedList)

[Example]{#Example}

: List items are numbered as examples.

See also: [OrderedList](#OrderedList)

[Decimal]{#Decimal}

: List are numbered using decimal integers.

See also: [OrderedList](#OrderedList)

[LowerRoman]{#LowerRoman}

: List are numbered using lower-case roman numerals.

See also: [OrderedList](#OrderedList)

[UpperRoman]{#UpperRoman}

: List are numbered using upper-case roman numerals

See also: [OrderedList](#OrderedList)

[LowerAlpha]{#LowerAlpha}

: List are numbered using lower-case alphabetic characters.

See also: [OrderedList](#OrderedList)

[UpperAlpha]{#UpperAlpha}

: List are numbered using upper-case alphabetic characters.

See also: [OrderedList](#OrderedList)

Helper functions

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

: 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")

[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")

Module pandoc.utils

This module exposes internal pandoc functions and utility functions.

[blocks_to_inlines (blocks[, sep])]{#utils-blocks_to_inlines}

: Squash a list of blocks into a list of inlines.

Parameters:

`blocks`:
:   List of blocks to be flattened.

`sep`:
:   List of inlines inserted as separator between two
    consecutive blocks; defaults to `{ pandoc.Space(),
    pandoc.Str'¶', pandoc.Space()}`.

Returns:

-   ({[Inline][#Inline]}) List of inlines

Usage:

    local blocks = {
      pandoc.Para{ pandoc.Str 'Paragraph1' },
      pandoc.Para{ pandoc.Emph 'Paragraph2' }
    }
    local inlines = pandoc.utils.blocks_to_inlines(blocks)
    -- inlines = {
    --   pandoc.Str 'Paragraph1',
    --   pandoc.Space(), pandoc.Str'¶', pandoc.Space(),
    --   pandoc.Emph{ pandoc.Str 'Paragraph2' }
    -- }

[hierarchicalize (blocks)]{#utils-hierarchicalize}

: Convert list of blocks into an hierarchical list. An hierarchical elements is either a normal block (but no Header), or a Sec element. The latter has the following fields:

-   level: level in the document hierarchy;
-   numbering: list of integers of length `level`,
    specifying the absolute position of the section in the
    document;
-   attr: section attributes (see [Attr](#Attr));
-   contents: nested list of hierarchical elements.

Returns:

-   List of hierarchical elements

Usage:

    local blocks = {
      pandoc.Header(2, pandoc.Str 'first'),
      pandoc.Header(2, pandoc.Str 'second'),
    }
    local elements = pandoc.utils.hierarchicalize(blocks)
    print(table.concat(elements[1].numbering, '.')) -- 0.1
    print(table.concat(elements[2].numbering, '.')) -- 0.2

[run_json_filter (doc, filter[, args])]{#utils-run_json_filter}

: Filter the given doc by passing it through the a JSON filter.

Parameters:

`doc`:
:   the Pandoc document to filter

`filter`:
:   filter to run

`args`:
:   list of arguments passed to the filter. Defaults to
    `{FORMAT}`.

Returns:

-   ([Pandoc](#Pandoc)) Filtered document

Usage:

    -- Assumes `some_blocks` contains blocks for which a
    -- separate literature section is required.
    local sub_doc = pandoc.Pandoc(some_blocks, metadata)
    sub_doc_with_bib = pandoc.utils.run_json_filter(
      sub_doc,
      'pandoc-citeproc'
    )
    some_blocks = sub_doc.blocks -- some blocks with bib

[normalize_date (date_string)]{#utils-normalize_date}

: Parse a date and convert (if possible) to "YYYY-MM-DD" format. We limit years to the range 1601-9999 (ISO 8601 accepts greater than or equal to 1583, but MS Word only accepts dates starting 1601).

Returns:

-   A date string, or nil when the conversion failed.

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

: Returns the SHA1 has of the contents.

Returns:

-   SHA1 hash of the contents.

Usage:

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

[stringify (element)]{#utils-stringify}

: Converts the given element (Pandoc, Meta, Block, or Inline) into a string with all formatting removed.

Returns:

-   A plain string representation of the given element.

Usage:

    local inline = pandoc.Emph{pandoc.Str 'Moin'}
    -- outputs "Moin"
    print(pandoc.utils.stringify(inline))

[to_roman_numeral (integer)]{#utils-to_roman_numeral}

: Converts an integer < 4000 to uppercase roman numeral.

Returns:

-   A roman numeral string.

Usage:

    local to_roman_numeral = pandoc.utils.to_roman_numeral
    local pandoc_birth_year = to_roman_numeral(2006)
    -- pandoc_birth_year == 'MMVI'

Module pandoc.mediabag

The pandoc.mediabag module 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, ".")

Module pandoc.List

Pandoc's List type and helper methods

Metamethods

[pandoc.List:__concat (list)]{#pandoc.List:__concat}

: Concatenates two lists.

Parameters:

`list`:
:   second list concatenated to the first

Returns: a new list containing all elements from list1 and
list2

Methods

[pandoc.List:clone ()]{#pandoc.List:clone}

: Returns a (shallow) copy of the list.

[pandoc.List:includes (needle, init)]{#pandoc.List:includes}

: Checks if the list has an item equal to the given needle.

Parameters:

`needle`:
:   item to search for

`init`:
:   index at which the search is started

Returns: true if a list item is equal to the needle, false
otherwise

[pandoc.List:find (needle, init)]{#pandoc.List:find}

: Returns the value and index of the first occurrence of the given item.

Parameters:

`needle`:
:   item to search for

`init`:
:   index at which the search is started

Returns: first item equal to the needle, or nil if no such
item exists.

[pandoc.List:find_if (pred, init)]{#pandoc.List:find_if}

: Returns the value and index of the first element for which the predicate holds true.

Parameters:

`pred`:
:   the predicate function

`init`:
:   index at which the search is started

Returns: first item for which \`test\` succeeds, or nil if
no such item exists.

[pandoc.List:extend (list)]{#pandoc.List:extend}

: Adds the given list to the end of this list.

Parameters:

`list`:
:   list to appended

[pandoc.List:map (fn)]{#pandoc.List:map}

: Returns a copy of the current list by applying the given function to all elements.

Parameters:

`fn`:
:   function which is applied to all list items.

[pandoc.List:filter (pred)]{#pandoc.List:filter}

: Returns a new list containing all items satisfying a given condition.

Parameters:

`pred`:
:   condition items must satisfy.

Returns: a new list containing all items for which \`test\`
was true.

Files

lua-filters.md

Latest commit

History

lua-filters.md

File metadata and controls

Introduction

Lua filter structure

Global variables

Pandoc Module

Element creation

Exposed pandoc functionality

Lua interpreter initialization

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

Building images with tikz

Lua type reference

Pandoc {#type-ref-pandoc}

Meta {#type-ref-meta}

MetaValue {#type-ref-MetaValue}

MetaBlocks {#type-ref-MetaBlocks}

MetaBool {#type-ref-MetaBool}

MetaInlines {#type-ref-MetaInlines}

MetaList {#type-ref-iMetaList}

MetaMap {#type-ref-MetaMap}

MetaString {#type-ref-MetaString}

Block {#type-ref-Block}

BlockQuote {#type-ref-BlockQuote}

BulletList {#type-ref-BulletList}

CodeBlock {#type-ref-CodeBlock}

DefinitionList {#type-ref-DefinitionList}

Div {#type-ref-Div}

Header {#type-ref-Header}

HorizontalRule {#type-ref-HorizontalRule}

LineBlock {#type-ref-LineBlock}

Null {#type-ref-Null}

OrderedList {#type-ref-OrderedList}

Para {#type-ref-Para}

Plain {#type-ref-Plain}

RawBlock {#type-ref-RawBlock}

Table {#type-ref-Table}

Inline {#type-ref-Inline}

Cite {#type-ref-Cite}

Code {#type-ref-Code}

Emph {#type-ref-Emph}

Image {#type-ref-Image}

LineBreak {#type-ref-LineBreak}

Link {#type-ref-Link}

Math {#type-ref-Math}

Note {#type-ref-Note}

Quoted {#type-ref-Quoted}

RawInline {#type-ref-RawInline}

SmallCaps {#type-ref-SmallCaps}

SoftBreak {#type-ref-SoftBreak}

Space {#type-ref-Space}

Span {#type-ref-Span}

Str {#type-ref-Str}

Strikeout {#type-ref-Strikeout}

Strong {#type-ref-Strong}

Subscript {#type-ref-Subscript}

Superscript {#type-ref-Superscript}

Element components

Attr {#type-ref-Attr}

Attributes {#type-ref-Attributes}

Citation {#type-ref-Citation}

ListAttributes {#type-ref-ListAttributes}

Hierarchical Element {#type-ref-Element}

Sec {#type-ref-Sec}

ReaderOptions {#type-ref-ReaderOptions}

CommonState {#type-ref-CommonState}

LogMessage {#type-ref-LogMessage}

Module text

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