A Pandoc filter for exporting HTML comments

export_comments.lua

-- Default: Don't export comments
local comments = nil

function Meta (meta)
  -- If there is a "comments" field in the YAML frontmatter, extract this
  if meta.comments then
    comments = pandoc.utils.stringify(meta.comments)
  end
  return meta
end

function RawBlock (raw)
  -- Only export if the input format is HTML (== HTML comment)
  -- and if we have a comments value
  if not comments then return end
  if raw.format ~= 'html' then return end

  -- Let's see if we actually have a comment
  local comment = raw.text:match '%<%!%-%-%s*(.*)%s*%-%-%>'
  if not comment then return end

  -- Export the comment always with the correct class, and as an element depending
  -- on the comments value. If the comments value is not supported, this is the same
  -- as not exporting the comments.
  if comments == "paragraph" then return pandoc.Div(comment, {class='comment'})
  elseif comments == "bold" then return pandoc.Strong(comment, {class='comment'})
  elseif comments == "italic" then return pandoc.Emph(comment, {class='comment'})
  elseif comments == "footnote" then return pandoc.Note(comment, {class='comment'})
  elseif comments == "blockquote" then return pandoc.Quoted(comment, {class='comment'})
  elseif comments == "small-caps" then return pandoc.SmallCaps(comment, {class='comment'})
  elseif comments == "strikeout" then return pandoc.Strikeout(comment, {class='comment'})
  elseif comments == "underline" then return pandoc.Underline(comment, {class='comment'})
  end
end

-- This line is required so that the meta values are read in first and THEN the
-- RawBlocks are traversed
return {
  { Meta = Meta },
  { RawBlock = RawBlock }
}