Thanks a lot for your solution.
With your help and the logging module, programming Lua filters for Pandoc 
is starting to be fun and I came up with this solution by myself:

function Meta (m)
  if FORMAT:match 'docx' then
    authors = pandoc.List:new()
    for i, author in ipairs(m.author) do
      if m.author[i] then
        authors = authors .. author .. {pandoc.Str(', ')}
      else
        authors = authors .. author
      end
    end
    m.author = pandoc.MetaInlines(authors)
  end
  return m
end


BPJ schrieb am Donnerstag, 15. Juni 2023 um 18:35:18 UTC+2:

> It is better to not hard code the number of authors:
>
> ``````lua
> function Meta (meta)
>   if 'docx' == FORMAT then
>     if 'List' ~= pandoc.utils.type(meta.author) then
>       -- hopefully Inlines
>       return nil
>     elseif 1 == #meta.author then -- only one
>       meta.author = meta.author[1]
>     else -- more than one
>       -- Define once, use many times
>       local sep = pandoc.Inlines(", ") -- DWIM
>       -- Initialize to an empty list and use `:extend` to populate
>       local authors = pandoc.Inlines({ })
>       -- Loop through any number of authors
>       for i, author in ipairs(meta.author) do
>         if 1 == i then -- no comma for the first
>           authors:extend(author)
>         else -- append ", A. U. Thor"
>           authors:extend(sep .. author)
>         end
>       end
>       -- Put them back
>       meta.author = authors
>       -- We have modified, so return it
>       return meta
>     end
>   end
>   return nil -- nothing modified
> end
> ``````
>
> Den tors 15 juni 2023 17:18Stephan Boltzmann <stephan2...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> 
> skrev:
>
>> Thanks a lot for your help, which enabled me to write the following short 
>> filter, which puts the name of all authors on the same line in 'docx' 
>> output, while preserving the nice default layout in 'latex' output.
>> Debugging filters working on the yaml metadata provided a particular 
>> challenge to me:
>>
>> function Meta (m)
>>   if FORMAT:match 'docx' then
>>         authors = m.author[1] .. {pandoc.Str(', ')} .. m.author[2]
>>         m.author = pandoc.MetaInlines(authors)
>>   end
>>   return m
>> end
>>
>> It only handles two and not more authors, but in my case, that's fine.
>> William Lupton schrieb am Mittwoch, 14. Juni 2023 um 18:55:04 UTC+2:
>>
>>> Your metadata has this structure (hmm... perhaps some of the Inlines are 
>>> really MetaInlines, in which case that's a problem with my logging module):
>>>
>>> (#) meta Meta {
>>>   author: List[2] {
>>>     [1] Inlines[3] {
>>>       [1] Str "Author"
>>>       [2] Space
>>>       [3] Str "A"
>>>     }
>>>     [2] Inlines[3] {
>>>       [1] Str "Author"
>>>       [2] Space
>>>       [3] Str "B"
>>>     }
>>>   }
>>>   title: Inlines[1] {
>>>     [1] Str "Test"
>>>   }
>>> }
>>>
>>> ... so I think you want this filter (MetaInlines isn't an attribute 
>>> name, and you have to count the space):
>>>
>>> function Meta (m)
>>>     print(pandoc.utils.stringify(m.author[1][3]))
>>> end
>>>
>>> On Wed, 14 Jun 2023 at 17:47, Stephan Boltzmann <stephan2...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> 
>>> wrote:
>>>
>>>> Hello everybody out there using Pandoc,
>>>>
>>>> For the following short document,
>>>> ---
>>>> title: Test
>>>> author:
>>>>   - Author A
>>>>   - Author B
>>>> ---
>>>> # Introduction
>>>>
>>>> I would expect the Lua filter
>>>>
>>>> function Meta (m)
>>>>     print(pandoc.utils.stringify(m.author[1].MetaInlines[2]))
>>>> end
>>>>
>>>> to print out "A", but it prints nothing at all.
>>>>
>>>> What is the conceptual misunderstanding I have here?
>>>>
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>>>> .
>>>>
>>> -- 
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>> To unsubscribe from this group and stop receiving emails from it, send an 
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>>
> To view this discussion on the web visit 
>> https://groups.google.com/d/msgid/pandoc-discuss/64f0f40b-5c49-47fa-97f5-f824735f6cebn%40googlegroups.com 
>> <https://groups.google.com/d/msgid/pandoc-discuss/64f0f40b-5c49-47fa-97f5-f824735f6cebn%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>>
>

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