In ConTeXt mkiv, how do I register a callback? callback.register() does nothing (and IIRC most callbacks are frozen anyway) and somewhere I found tasks.appendaction, but this gives me an error that tasks is not defined (nil).

I'd like to add something to the post_linebreak_filter callback.

Edit: thanks to Khaled's answer and help from the Mailing list, I've got a solution now. The key is to create my own namespace:


and add my function:

moduledata.mystuff.show_hyph = show_hyph

and to register the function to the "finalizers" callbacks, which are the post_linebreak_filter equivalent in ConTeXt.


See my answer for a complete example

2 Answers 2


It should be nodes.tasks.appendaction(), I don't know the official way to use it, but after trial and error (and grepping through the source), I came up with:

bidi.handle_bidi = nodes.installattributehandler {
    name      = "bidi",
    namespace = bidi,
    processor = bidi.ctxprocess,
nodes.tasks.appendaction("processors", "characters",  "bidi.handle_bidi")


nodes.tasks.enableaction("processors", "bidi.handle_bidi")
nodes.tasks.disableaction("processors", "bidi.handle_bidi")

(copied from my bidi module, and this is equivalent to pre_linepreak_filter)

Better ask Hans for a definite answer.

  • I have asked on the NTG ConTeXt list, waiting for an answer.
    – topskip
    Feb 25, 2011 at 11:30
  • I've got a reply from Hans, I'll post it as a separate answer.
    – topskip
    Feb 25, 2011 at 14:57

I have taken the example from the LuaTeX wiki and transformed it into ConTeXt code with the help of Khaled and Hans:

show_hyph = function(head)
  while head do
    if head.id == 0 or head.id == 1 then -- % hlist, vlist
      show_hyph(head.list)               -- % head.head in LuaTeX > 0.65
    elseif head.id == 7 then             -- % disc
      local n = node.new("whatsit","pdf_literal")
      n.mode = 0
      n.data = "q 0.3 w 0 2 m 0 7 l S Q"
      n.next = head.next
      n.prev = head
      head.next = n
      head = n
  head = head.next
  return true

moduledata.mystuff.show_hyph = show_hyph


Seit zwei Jahren...

The "finalizer" is the ConTeXt equivalent of the post_linebreak_filter callback.

  • @Charles: Sorry for entering this discussion late. (I can't quite follow since parts are already deleted.) Just wanted to say: I usually strongly discourage people to include answers into the question. Just a pointer is OK, but I think we should keep answers to answers. What do you think? (@Patrick: In my opinion you've already got to much of an answer in the question now.) Mar 2, 2011 at 14:39
  • @Hendrik: Putting a summary at the end of questions is fairly well established on SO when there are several useful answers. E.g., stackoverflow.com/questions/9033/hidden-features-of-c Maybe this should be a meta qn? Mar 2, 2011 at 15:03
  • @Charles: Thanks for the feedback. Well, for a question with three hundred and two answers, a summary is somewhat necessary, but here we have two answers. But still it might be worth asking for other opinions on meta. Mar 2, 2011 at 15:10

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