Converting filter to custom writer

[BPJ <melroch-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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I have a not-so-old "pseudo-writer" implemented as a filter which mostly
injects  tons of raw markup into the document tree so that the output of
the plain writer looks like my target format, returning a table of filters
at the end like this

``````lua
return {
  {
    Meta = get_config
  },
  {
    SmallCaps = SmallCaps,
    Str = Str
  },
  note_filter,
  main_filter
}
``````

where `get_config` just extracts some info from the metadata and populates
a table declared at the top of the script which the functions in the other
filters refer to.

Am I right that I in principle could convert this into a new style custom
writer by removing the Meta filter, replacing `return {<filters>}` above
with `local filters = {<filters>}` and adding the following at the bottom
of the script?

``````lua
function Writer (doc, opts)
  get_config(doc.meta)
  for _,filter in ipairs(filters) do
    doc = doc:walk(filter)
  end
  doc.blocks = blocks
  return pandoc.write(doc, 'plain', opts)
``````

(This got me thinking: what if `walk` accepted one more argument which will
be passed as an extra argument to the functions in the filters, which could
be used to pass config and/or maintain state? It somehow feels more
intuitive than defining a table outside the metadata handler which all
handlers close around.)

(BTW the main reason that the `note_filter` is separate is that it needs to
be topdown because it counts and collects the notes, while `main_filter`
needs to be bottomup because it sometimes relies on the content of elements
already having been processed. I was very happy when I realized that this
would work!)

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