[luatex] The node returned by node.traverse().

[luigi scarso]

On Wed, Feb 15, 2012 at 2:44 PM, Paul Isambert <zappathustra at free.fr> wrote:
> luigi scarso <luigi.scarso at gmail.com> a écrit:
>>
>> On Wed, Feb 15, 2012 at 9:02 AM, Paul Isambert <zappathustra at free.fr> wrote:
>> > Hello there,
>> >
>> > I have a problem with the identity, so to speak, of nodes returned by
>> > node.traverse(); I don't know if I'm missing something (perhaps related
>> > to Lua rather than LuaTeX) or what. The following code should illustrate
>> > the problem (I do not bother with breaks since there is only one node in
>> > the list and one entry in the table):
>> >
>> > %%%%%%%%%%%
>> > \setbox0=\hbox{a}
>> >
>> > \directlua{
>> >  local head = tex.box[0].head
>> >  local t = { [head] = true }
>> >  for n in node.traverse(head) do
>> >    texio.write_nl(tostring(t[n]))
>> >    texio.write_nl(tostring(n == head))
>> >    for a in pairs(t) do
>> >      if a == n then
>> >        texio.write_nl("They match!")
>> >      end
>> >    end
>> >  end
>> > }
>> The key of t is head,  and head is a userdata.
>>
>>  n==head is managed by __eq(n,head)  handler so can happen that
>> __eq(n,head) is true based on some considerations of n and head.
>>
>> On the other side rawequal(n,head) gives false
>> so n and head have not the same identity (can be a different address,
>> but we don't have a
>> way to print addresses in Lua).
>> With
>> local head_copy = head
>> rawequal(head_copy,head)
>> gives true , i.e head_copy and head have the same identity.
>> (and of course head_copy == head  is true )
>>
>> As far I know, keys are based on  identities, so t[n] and t[head] differs.
>
> Thank you Luigi; I'd thought about metatable magic but I hadn't considered
> rawequal to make sure.
>
> Now I should recast the question: why are "head" an "n" different? I
> suppose the answer of course is: because node.traverse() doesn't really
> return the node, but then what does it return and why?
>
> Best,
> Paul
>
another pointer to the same object, but for  the key its address is
used, not the content
so we avoid collisions:
\starttext
\setbox0=\hbox{a}
\startluacode
  local head = tex.box[0].head
  local head1 = tex.box[0].head
  local t1={[head]="I'm head!",[head1]="I'm head1!"}
  print("rawequal(head,head1):", rawequal(head,head1))
  print("t1[head]="..t1[head],"t1[head1]="..t1[head1])
  print("head==head1?", head==head1)
  head1=head
  print("rawequal(head,head1):", rawequal(head,head1))
  print("t1[head]="..t1[head],"t1[head1]="..t1[head1])
  print("head==head1?", head==head1)


 local t = { [head] = true }
  for n in node.traverse(head) do
    texio.write_nl(tostring(t[n]))
    texio.write_nl(tostring(n == head))
    for a in pairs(t) do
      if a == n then
        texio.write_nl("They match!")
      end
    end
  end
\stopluacode
\stoptext

gives

rawequal(head,head1):	false
t1[head]=I'm head!	t1[head1]=I'm head1!
head==head1?	true
rawequal(head,head1):	true
t1[head]=I'm head!	t1[head1]=I'm head!
head==head1?	true

nil
true
They match! )



-- 
luigi