[luatex] What are user-defined whatsit nodes?

[Stephan Hennig]

Am 21.11.2014 um 19:10 schrieb luigi scarso:
> On Fri, Nov 21, 2014 at 6:31 PM, Stephan Hennig <sh-list at posteo.net> wrote:
> 
>>
>> To put it differently, is user-defined whatsits inhibiting ligatures a
>> bug or intentional?
>>
>> intentional
> anything non-glyph or non-disc will inhibit ligatures  building

OK, thanks for the clarification!

These (side-)effects of user-defined whatsits make me wonder what
use-cases have been in mind when introducing this type of node?
Attributes seem so much more attractive for squeezing additional
information into a node list, because they should be handled
transparently (I think).

But never mind, I'm currently looking into ways to make TeX smarter with
regards to ligatures.  I've recently found the selnolig package
utilizing user-defined whatsit nodes for inhibiting selected ligatures.
 Before asking further questions regarding ligatures I just wanted to
get some clarification about the whatsit approach.

The selnolig's whatsit approach works, but it's again not free of
side-effects.  It gets in the way with the hyphenation algorithm in that
the whatsit marks a word boundary causing problems with minimum
hyphenation length calculation.  Note how setting \righthyphenmin=5 in
the attached example prevents t-t hyphenation in the word 'butterflies.'
(Let's ignore the fact that the fl ligature is indeed valid in this
example.)  The problem is more serious in German with its many compound
words.  Babel shortcuts like "|, which insert real glue if I recall
correctly, suffer from the same problem.

Any ideas how to prevent selected ligatures without causing side-effects?

Best regards,
Stephan Hennig

% -*- coding: utf-8 -*-
\directlua{
% Declare constants.
  local GLYPH = node.id('glyph')
  local WHATSIT = node.id('whatsit')
  local USER_DEFINED = node.subtype('user_defined')
  local CHAR_f = string.byte('f')
  local CHAR_l = string.byte('l')
  local Ncopy = node.copy
  local Nnew = node.new
  local Ninsert_before = node.insert_before
  local Ntraverse = node.traverse
% Create user-defined whatsit.
  local what = Nnew(WHATSIT, USER_DEFINED)
  what.user_id = 20141117
  what.type = 100
  what.value = 0
% Register callback.
  callback.register('hyphenate',
    function (head, tail)
%     Iterate over node list.
      for n in Ntraverse(head) do
        if n.id == GLYPH and n.char == CHAR_l then
          local p = n.prev
          if p.id == GLYPH and p.char == CHAR_f then
            Ninsert_before(head, n, Ncopy(what))
          end
        end
      end
      lang.hyphenate(head, tail)
    end
  )
}
\righthyphenmin=3
\showhyphens{butterflies}
\righthyphenmin=5
\showhyphens{butterflies}
\bye