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.

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](pandoc-module.html) 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 `Doc` 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.

``` lua
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.

``` lua
-- 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

```lua
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.

```lua
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
```