Pandoc Lua filters: how to specify attributes for Span element

Question

I have a Markdown document containing raw LaTeX commands. I am trying to use a Lua filter with Pandoc (2.0.1.1) to convert the LaTeX commands into something more portable. In particular, commands that specify the language of text should be converted into spans with a lang attribute. The problem is that I don't know how to pass the attributes to the pandoc.Span constructor. This is my attempt at a filter ( filter.lua ):

function RawInline(elem)
  if elem.format == "tex" then
    text = string.match(elem.text, "\\textspanish{(.+)}")
    if text then
      contents = {pandoc.Str(text)}
      attrs = pandoc.Attr("",{},{lang = "es-SP"})
      return pandoc.Span(contents, attrs)
    end
  else
    return elem
  end
end

Sample usage:

echo '\textspanish{hola}' | pandoc -f markdown -t native --lua-filter=filter.lua

The output is [Para [Span ("",[],[]) [Str "hola"]]] , with no attributes on the span.

If I pass a name and/or class to pandoc.Attr , these come through, eg, attrs = pandoc.Attr("name",{"class"},{lang = "es-SP"}) produces [Para [Span ("name",["class"],[]) [Str "hola"]]] . But attributes I pass to the constructor never appear in the output. What is the right way to pass attributes to pandoc.Attr ?

Answer 1

You found one of the rough edges in the lua filter implementation; this should be ironed out and be made more user friendly.

The current implementation uses two-element tables to hold key-value pairs:

attrs = pandoc.Attr("", {}, {{"lang", "es-SP"}})

This is not a great way to represent pairs. The reason for the current implementation is two-fold:

1. It mirrors how way pairs (and attributes in general) are encoded in pandoc's JSON output.
2. These pairs have a fixed order.

The last part is important when one wants to guarantee that the order of attributes won't be changed when passing through a filter. There is no rule in lua which determines the order of keys in a table: the lua table {one = 1, two = 2} could be read back into pandoc as the attribute list {one="1" two="2} or as {two="2" one="1"} . Now, the order of attributes shouldn't matter for most applications, but we cannot be sure. Hence the less-than-intuitive representation.