I think you need a two-pass filter, one to pick up the metadata and the other to use it (this is because Meta is called nearly last).

This works (I've re-ordered your functions and declared a local variable, but this probably wasn't necessary).

% cat default-code-class.lua 

local default_code_classes = {}

function get_default_code_class(meta)

  if meta['default-code-class'] then

    default_code_classes = {pandoc.utils.stringify(meta['default-code-class'])}

  end

end

function add_default_code_class(el)

  if #(el.classes) == 0 then

    el.classes = default_code_classes

    return el

  end

end

return {{Meta=get_default_code_class}, {CodeBlock=add_default_code_class}}

% pandoc -L default-code-class.lua code.md -M default-code-class="c numberLines"

<pre class="c numberLines"><code></code></pre>

On Fri, 20 Aug 2021 at 16:29, Clément Aubert <aubert.clement-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

Hi,

This question relates to

https://github.com/jgm/pandoc/issues/2104#issuecomment-595983954

There is a lua filter containing

``` lua
function add_default_code_class(el)
  if #(el.classes) == 0 then
    el.classes = default_code_classes
    return el
  end
end

function get_default_code_class(meta)
  if meta['default-code-class'] then
    default_code_classes = {pandoc.utils.stringify(meta['default-code-class'])}
  end
end
```

and I'm trying to tweak it so that two classes can be passed from the command-line, eg. using

--lua-filter default-code-class.lua -M default-code-class=c, numberLines

but can't find a way.

Thanks!

Clément.

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