Lua: calling :new function to create Pandoc Lua objects fails in v2.16

[jjallaire]

The following calls all yield an error in Pandoc v2.16 whereas they worked w/o problem in Pandoc 2.14.2:

Pandoc = function(el)
  local plain = pandoc.Plain:new({})
  local str = pandoc.Str:new('foo')
  local link = pandoc.Link:new({}, "#href")
end

[tarleb]

Oh, that's interesting: the :new method was more of a side-effect of the way we handled types and their constructors. It is documented only for pandoc.List elements, but used to work with almost all AST-element constructors.

Adding the :new method back would require a bit of refactoring, e.g. by moving all current element constructor functions into the pandoc.types module so we could replace pandoc.Plain, pandoc.Str, etc. with callable tables.

For the time being, you could put the following code into your DATADIR/init.lua:

for k, v in pairs(pandoc) do
  if type(v) == 'function' then
    local mt = {
      __call = function(_, ...)
        return v(...)
      end,
      __index = {
        new = function(_, ...)
          return v(...)
        end
      }
    }
    pandoc[k] = setmetatable({}, mt)
  end
end

Is the use of :new historic or also a stylistic preference?

[jjallaire]

It's historic, so it's no problem to change (but just wonder if other filters out in the wild will have the same problem -- probably just something to keep an eye on)

[jgm]

@tarleb is your plan to do this refactoring so that 'new' would work here, or to treat this as an undocumented quirk of earlier versions? It would be good to do a point release soon because of the other Lua-related fixes, but if you were going to make this change I'd wait.

[tarleb]

I hadn't decided yet, but knowing that the filters were not too difficult to adjust, I'm leaning towards keeping the current behavior.

[cagix]

i'd vote for keeping new for all elements (+ documenting this). this makes it nice and symmetrical :)

[tarleb]

It does have a certain appeal. I'll see how I can achieve it in a non-hacky way.

@jgm please don't wait for this for the next point release. It might take a while.

[tarleb]

I gave this some more thought: If we bring back :new, then I'd like it to take a table argument instead of multiple arguments:

local img = pandoc.Image:new{
  src = 'logo.png',
  caption = 'A really nice logo'
}

Making :new just an alias for the normal constructor would seem like a wasted opportunity.

[jgm]

That does seem like a nice feature!