Lua scripting: set dir attribute to auto by default

[avidseeker]

I'm trying to write a filter for #10014 that adds the dir="auto" attribute to every element. That is: it outputs <p dir="auto">text</p> instead of just <p>text</p>. Here's what I tried:

$ pandoc -f markdown -t html -L all_dir_auto.lua

Where the script is

function Para(elem)
    elem.attributes.dir = 'auto'
    return elem
end

But I get attempt to index a nil value (field 'attributes') error.

[massifrg]

Para have no attributes in Pandoc AST.

Try this:

function Para(elem)
  local inlines = pandoc.List(elem.content)
  inlines:insert(1, pandoc.RawInline('html', '<p dir="auto">'))
  inlines:insert(pandoc.RawInline('html', '</p>'))
  return pandoc.Plain(inlines)
end

(you may also write local inlines = elem.content instead of creating a new List)

[avidseeker]

Thank you. That was what I needed. I wrote an additional filter for headers too. Hopefully this is robust enough

function Header(elem)
  local inlines = pandoc.List(elem.content)
  inlines:insert(1, pandoc.RawInline('html', '<h' .. elem.level .. ' dir="auto">'))
  inlines:insert(pandoc.RawInline('html', '</h' .. elem.level .. '>'))
  return pandoc.Plain(inlines)
end
function Para(elem)
  local inlines = pandoc.List(elem.content)
  inlines:insert(1, pandoc.RawInline('html', '<p dir="auto">'))
  inlines:insert(pandoc.RawInline('html', '</p>'))
  return pandoc.Plain(inlines)
end

[massifrg]

I would not use the same trick with Header, because it has an Attr data structure, so you can use:

  elem.attributes.dir = 'auto'

as you did first.

You can set that attribute on every Block or Inline that has an Attr.

For blocks, you can use the Plain trick, for inlines, e.g. Emph or Strong, just return the inlines list, without boxing it inside a Plain.

You can also look in the Pandoc Lua Filters page.
Here's the links for Header and Para, where you can see the fields that are defined (e.g. attributes is present in Header but not in Para).

A note on attributes' conversion to HTML: usually they are prefixed with data-, but dir is a known HTML attribute, so Pandoc keeps it without the prefix.

[avidseeker]

I just found this to be unnecessary as there is a css attribute for this:

* {
  unicode-bidi: plaintext;
  text-align: start; /* Align text automatically left/right*/
}

from: https://stackoverflow.com/a/72463737