Confused about filters, new style and classic style writers

[A A <amine.aboufirass-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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Hi all,

There’s something which I don’t quite understand in with respect to the
following:

   - lua writers
      - classic style
      - new style
   - lua filters

As far as I can tell, lua filters and classic style writers are pretty much
identical, basically defining Lua functions corresponding to node names,
and adding the code inside these functions. The only notable difference I
can think of is that classic style writers return raw output (i.e. strings)
from these functions, whereas filters typically return some other pandoc
object, or node in the AST, effectively modifying it during runtime.

Enter new style writers. The *only* example I have been able to find of
these writers is the one in the main documentation, and it is super
confusing:

function Writer (doc, opts)
  local filter = {
    CodeBlock = function (cb)
      -- only modify if code block has no attributes
      if cb.attr == pandoc.Attr() then
        local delimited = '```\n' .. cb.text .. '\n```'
        return pandoc.RawBlock('markdown', delimited)
      end
    end
  }
  return pandoc.write(doc:walk(filter), 'gfm', opts)
end

Here we have a global function called Writer which takes in doc, a
pandoc.Pandoc and then proceeds to defining a *filter* which it then
applies to this document using the walk method. It then uses the gfm writer
to actually generate the raw output. So basically, a writer within a writer.

I’m not sure if this is just a bad example, or whether I’m missing
something. It would have been more useful to rewrite the example provided
for classic style writers in the new style , in my opinion.

Are there any more down-to-earth examples for making writers in the new
style? Why are we defining filters inside these new style writers? Couldn’t
we just make a simple filter instead?

Regards,

Amine

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