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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

Initialization of pandoc's Lua interpreter can be controlled by placing a file init.lua in pandoc's data directory. A common use-case would be to load additional modules, or even to alter default modules.

The following snippet is an example of code that might be useful when added to init.lua. The 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

Pandoc document

Object equality is determined via pandoc.utils.equals.

blocks: document content (List of Blocks)
meta: document meta information ([Meta] object)

MetaValue

Document meta information items.

Object equality is determined via pandoc.utils.equals.

MetaBlocks

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

Fields:

tag, t: the literal MetaBlocks (string)

MetaBool

Plain Lua boolean value (boolean)

MetaInlines

List of inlines used in metadata (List of Inlines)

Fields:

tag, t: the literal MetaInlines (string)

MetaList

A list of other MetaValues. (List)

Fields:

tag, t: the literal MetaList (string)

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

Plain Lua string value (string)

Block

Object equality is determined via pandoc.utils.equals.

BlockQuote

A block quote element

content:: block content (List of Blocks)
tag, t: the literal BlockQuote (string)

BulletList

A bullet list

content: list of items (List of Blocks)
tag, t: the literal BulletList (string)

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

Definition list, containing terms and their explanation.

content: list of items
tag, t: the literal DefinitionList (string)

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

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

A horizontal rule.

tag, t: the literal HorizontalRule (string)

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

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

tag, t: the literal Null (string)

OrderedList

An ordered list.

content: 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

A paragraph

content: inline content (List of Inlines)
tag, t: the literal Para (string)

Plain

Plain text, not a paragraph

content: inline content (List of Inlines)
tag, t: the literal Plain (string)

RawBlock

Raw content of a specified format.

format: format of content (string)
text: raw content (string)
tag, t: the literal RawBlock (string)

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

Object equality is determined via pandoc.utils.equals.

Cite

Citation

content: (List of Inlines)
citations: citation entries (List of citations)
tag, t: the literal Cite (string)

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

Emphasized text

content: inline content (List of Inlines)
tag, t: the literal Emph (string)

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

Hard line break

tag, t: the literal LineBreak (string)

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

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

Footnote or endnote

content: (List of Blocks)
tag, t: the literal Note (string)

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

Raw inline

format: the format of the content (string)
text: raw content (string)
tag, t: the literal RawInline (string)

SmallCaps

Small caps text

content: (List of Inlines)
tag, t: the literal SmallCaps (string)

SoftBreak

Soft line break

tag, t: the literal SoftBreak (string)

Space

Inter-word space

tag, t: the literal Space (string)

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

Text

text: content (string)
tag, t: the literal Str (string)

Strikeout

Strikeout text

content: inline content (List of Inlines)
tag, t: the literal Strikeout (string)

Strong

Strongly emphasized text

content: inline content (List of Inlines)
tag, t: the literal Strong (string)

Subscript

Subscripted text

content: inline content (List of Inlines)
tag, t: the literal Subscript (string)

Superscript

Superscripted text

content: inline content (List of Inlines)
tag, t: the literal Superscript (string)

Element components

Attr

A set of element attributes

Object equality is determined via pandoc.utils.equals.

identifier: element identifier (string)
classes: element classes (List of strings)
attributes: collection of key/value pairs (Attributes)

Attributes

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

Citation

Single citation entry

Object equality is determined via pandoc.utils.equals.

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

List attributes

Object equality is determined via pandoc.utils.equals.

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

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

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

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

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

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. The module is made available as part of the pandoc module via pandoc.text. The text module can also be loaded explicitly:

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

lower

lower (s)

Returns a copy of a UTF-8 string, converted to lowercase.

upper

upper (s)

Returns a copy of a UTF-8 string, converted to uppercase.

reverse

reverse (s)

Returns a copy of a UTF-8 string, with characters reversed.

len

len (s)

Returns the length of a UTF-8 string.

sub

sub (s)

Returns a substring of a UTF-8 string, using Lua's string indexing rules.

Module pandoc

Lua functions for pandoc scripts.

Pandoc Document

Pandoc

Pandoc (blocks[, meta])

A complete pandoc document

Parameters:

blocks:: list of [Blocks]
meta:: [Meta] value (see below)

Metadata

MetaValue

MetaBlocks

MetaBlocks (blocks)

Block-level metadata content.

Parameters:

blocks:: list of [Blocks]

MetaInlines

MetaInlines (inlines)

Inline-level metadata content.

Parameters:

inlines:: list of [Inlines]

MetaList

MetaList (meta_values)

List of metadata items.

Parameters:

meta_values:: list of MetaValues

MetaMap

MetaMap (key_value_map)

Field/value map of metadata items.

Parameters:

key_value_map:: a table with string keys and MetaValue values

MetaString

MetaString (str)

String metadata content.

Parameters:

str:: string value

MetaBool

MetaBool (bool)

Boolean metadata content.

Parameters:

bool:: boolean value

Blocks

BlockQuote

BlockQuote (content)

Creates a BlockQuote element

Parameters:

content:: list of [Blocks]

Returns: BlockQuote element

BulletList

BulletList (content)

Creates a BulletList element

Parameters:

content:: list of items (where each item is a list of [Blocks])

Returns: BulletList element

CodeBlock

CodeBlock (text[, attr])

Creates a CodeBlock element

Parameters:

text:: string (the code)
attr:: Attr{#Attr} (code attributes)

Returns: CodeBlock element

DefinitionList

DefinitionList (content)

Creates a DefinitionList element

Parameters:

content:: list of items (where each item is a two element list, where the first element is a list of [Inlines], the term, and the second is a list of lists of [Blocks], the definitions)

Returns: DefinitionList element

Div

Div (content[, attr])

Creates a Div element

Parameters:

content:: list of [Blocks]
attr:: Attr{#Attr} (Div attributes)

Returns: Div element

Header

Header (level, content[, attr])

Creates a Header element.

Parameters:

level:: Header level (integer)
content:: list of [Inlines] (header title)
attr:: Attr{#Attr} (header attributes)

Returns: Header element

HorizontalRule

HorizontalRule ()

Creates a HorizontalRule element.

Returns: HorizontalRule element

LineBlock

LineBlock (content)

Creates a LineBlock element.

Parameters:

content:: list of lines (where each line is a list of [Inlines])

Returns: LineBlock element

Null

Null ()

Creates a Null block element.

Returns: Null element

OrderedList

OrderedList (items[, listAttributes])

Creates an OrderedList element.

Parameters:

items:: list of items (where each item is a list of [Blocks])
listAttributes:: [ListAttributes]{#ListAttributes}

Returns: OrderedList element

Para

Para (content)

Creates a Para element.

Parameters:

content:: list of [Inlines]

Returns: Para element

Plain

Plain (content)

Creates a Plain element.

Parameters:

content:: list of [Inlines]

Returns: Plain element

RawBlock

RawBlock (format, text)

Creates a RawBlock of the specified format.

Parameters:

format:: string (format of content, e.g. 'latex')
text:: string content

Returns: RawBlock element

Table

Table (caption, aligns, widths, headers, rows)

Creates a Table element.

Parameters:

caption:: table caption (list of [Inlines])
aligns:: alignments (a list of pandoc.AlignDefault, pandoc.AlignLeft, pandoc.AlignRight, pandoc.AlignCenter, one for each column)
widths:: column widths (a list of floats, one for each column, denoting the fraction of the textwidth needed for the column, 0.5 = half width; OR an empty list for a simple table where cells need not wrap)
headers:: header row (a list of cells, each cell a list of [Blocks])
rows:: table rows (a list of rows, each row a list of cells, each cell a list of [Blocks])

Returns: Table element

Inlines

Cite

Cite (content, citations)

Creates a Cite element

Parameters:

content:: list of [Inlines]
citations:: list of [Citation]s

Returns: citations element

Code

Code (text[, attr])

Creates a Code inline element

Parameters:

text:: string (the code)
attr:: Attr{#Attr} (code attributes)

Returns: Code element

Emph

Emph (content)

Creates an Emph inline element

Parameters:

content:: list of [Inlines]

Returns: Emph element

Image

Image (alt, src[, title[, attr]])

Creates a Image inline element

Parameters:

alt:: list of [Inlines]: alt text (or, for implicit figures, caption)
src:: string: path to the image file
title:: string: title attribute
attr:: Attr{#Attr}: additional image attributes

Returns: Image element

LineBreak

LineBreak ()

Create a LineBreak inline element

Returns: linebreak element

Link

Link (content, target[, title[, attr]])

Creates a Link inline element

Parameters:

content:: list of [Inlines]: the linked text
target:: string: the link target
title:: string: the title attribute
attr:: Attr{#Attr}: additional link attributes

Returns: image element

Math

Math (mathtype, text)

Creates a Math inline element, either inline or displayed.

Parameters:

mathtype:: either pandoc.InlineMath or pandoc.DisplayMath
text:: string: raw tex math

Returns: Math element

DisplayMath

DisplayMath (text)

Creates a DisplayMath element (DEPRECATED, use Math).

Parameters:

text:: string: raw tex math

Returns: Math element

InlineMath

InlineMath (text)

Creates an InlineMath inline element (DEPRECATED, use [Math]{#Math}).

Parameters:

text:: string: raw tex math

Returns: Math element

Note

Note (content)

Creates a Note inline element

Parameters:

content:: list of [Blocks] (content of footnote)

Quoted

Quoted (quotetype, content)

Creates a Quoted inline element

Parameters:

quotetype:: either pandoc.DoubleQuote or pandoc.SingleQuote
content:: list of [Inlines]

Returns: Quoted element

SingleQuoted

SingleQuoted (content)

Creates a single-quoted inline element (DEPRECATED, use [Quoted]{#Quoted}).

Parameters:

content:: list of [Inlines]

Returns: Quoted element

DoubleQuoted

DoubleQuoted (content)

Creates a double-quoted inline element (DEPRECATED, use [Quoted]{#Quoted}).

Parameters:

content:: list of [Inlines]

Returns: Quoted element

RawInline

RawInline (format, text)

Creates a RawInline inline element

Parameters:

format:: string (format of the contents)
text:: string (content)

Returns: RawInline element

Smallcaps

SmallCaps (content)

Creates text rendered in small caps

Parameters:

content:: list of [Inlines]

Returns: SmallCaps element

SoftBreak

SoftBreak ()

Creates a SoftBreak inline element.

Returns: SoftBreak element

Space

Space ()

Create a Space inline element

Returns: Space element

Span

Span (content[, attr])

Creates a Span inline element

Parameters:

content:: list of [Inlines]
attr:: Attr{#Attr}: span attributes

Returns: Span element

Str

Str (text)

Creates a Str inline element

Parameters:

text:: string

Returns: String element

Strikeout

Strikeout (content)

Creates a Strikeout inline element

Parameters:

content:: list of [Inlines]

Returns: Strikeout element

Strong

Strong (content)

Creates a Strong inline element.

Parameters:

content:: list of [Inlines]

Returns: Strong element

Subscript

Subscript (content)

Creates a Subscript inline element

Parameters:

content:: list of [Inlines]

Returns: Subscript element

Superscript

Superscript (content)

Creates a Superscript inline element

Parameters:

content:: list of [Inlines]

Returns: Superscript element

Element components

Attr

Attr ([identifier[, classes[, attributes]]])

Create a new set of attributes (Attr).

Parameters:

identifier:: string: element identifier
classes:: list of strings: classes
attributes:: table containing string keys and values

Returns: Attr

Citation

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

Creates a single Citation.

Parameters:

id:: string citation identifier (like a bibtex key)
mode:: pandoc.AuthorInText, pandoc.SuppressAuthor, or pandoc.NormalCitation
prefix:: list of [Inlines] for citation prefix
suffix:: list of [Inlines] for citation suffix
note_num:: int: note number
hash:: int: hash number

ListAttributes

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

Creates a set of list attributes

Parameters:

start:: int: number of the first list item (default: 1)
style:: pandoc.DefaultStyle (default), pandoc.Decimal, pandoc.LowerRoman, pandoc.UpperRoman, pandoc.LowerAlpha, or pandoc.UpperAlpha
delimiter:: pandoc.DefaultDelim (default), pandoc.Period, pandoc.OneParen, pandoc.TwoParens

Returns: list attributes table

Helper functions

pipe

pipe (command, args, input)

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

walk_block (element, filter)

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

walk_inline (element, filter)

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

read (markup[, format])

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.

The module is loaded as part of the pandoc module and available as pandoc.utils. In versions up-to and including pandoc 2.6, this module had to be loaded explicitly. Example:

local utils = require 'pandoc.utils'

Use this for backwards compatibility.

blocks_to_inlines

blocks_to_inlines (blocks[, sep])

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:

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' }
-- }

equals

equals (element1, element2)

Test equality of AST elements. Elements in Lua are considered equal if and only if the objects obtained by unmarshaling are equal.

Parameters:

element1, element2:: Objects to be compared. Acceptable input types are Pandoc, Meta, MetaValue, Block, Inline, Attr, ListAttributes, and Citation.

Returns:

Whether the two objects represent the same element (boolean)

hierarchicalize

hierarchicalize (blocks)

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);
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

run_json_filter (doc, filter[, args])

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) 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

normalize_date (date_string)

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

sha1 (contents)

Returns the SHA1 has of the contents.

Returns:

SHA1 hash of the contents.

Usage:

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

stringify

stringify (element)

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

to_roman_numeral (integer)

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.

The module is loaded as part of module pandoc and can either be accessed via the pandoc.mediabag field, or explicitly required, e.g.:

local mb = require 'pandoc.mediabag'

insert

insert (filepath, mime_type, contents)

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

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

lookup (filepath)

Lookup a media item in the media bag, and return its MIME type and contents.

Parameters:

filepath:: name of the file to look up.

Returns:

the entry's 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

fetch (source, base_url)

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.

This module is loaded and available as pandoc.List. Older versions up-to and including pandoc 2.6 require the module to be loaded explicitly. Example:

local List = require 'pandoc.List'

The above remains the recommended method to use this module; it provides the List type under an idiomatic name and is fully backwards compatible.

Metamethods

concat

pandoc.List:__concat (list)

Concatenates two lists.

Parameters:

list:: second list concatenated to the first

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

Methods

clone

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

Returns a (shallow) copy of the list.

includes

pandoc.List:includes (needle, init)

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

find

pandoc.List:find (needle, init)

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.

find_if

pandoc.List:find_if (pred, init)

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.

extend

pandoc.List:extend (list)

Adds the given list to the end of this list.

Parameters:

list:: list to appended

map

pandoc.List:map (fn)

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.

filter

pandoc.List:filter (pred)

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.

52 KiB Raw Blame History

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

Meta

MetaValue

MetaBlocks

MetaBool

MetaInlines

MetaList

MetaMap

MetaString

Block

BlockQuote

BulletList

CodeBlock

DefinitionList

Div

Header

HorizontalRule

LineBlock

Null

OrderedList

Para

Plain

RawBlock

Table

Inline

Cite

Code

Emph

Image

LineBreak

Link

Math

Note

Quoted

RawInline

SmallCaps

SoftBreak

Space

Span

Str

Strikeout

Strong

Subscript

Superscript

Element components

Attr

Attributes

Citation

ListAttributes

Hierarchical Element

Sec

ReaderOptions

CommonState

LogMessage

Module text

lower

upper

reverse

len

sub

52 KiB

Raw Blame History

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