Lua Filters

Pandoc expects lua files 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 filter by collecting all top-level functions whose names correspond to those of pandoc elements (e.g., Str, Para, Meta, or Pandoc).

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

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

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

Lua Filter Structure

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

Filter Application

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 element function's output must be an element of the same type as the input. This means a filter function acting on an inline element must return an inline, and a block element must remain a block element after filter application. Pandoc will throw an error if this condition is violated.

Elements without matching functions are left untouched.

See module documentation for a list of pandoc elements.

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 filters will require access to certain functions provided by pandoc. This is currently limited to the read function which allows to parse strings into pandoc documents from within the lua filter.

Examples

Macro substitution.

The following filter converts strings containing {{helloworld}} with emphasized text.

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

Default metadata file

Using the metadata from an external file as default values.

-- 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

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

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