HTML::Element traverse - discussion of HTML::Element's traverse method
# $element->traverse is unnecessary and obscure. # Don't use it in new code.
HTML::Element provides a method traverse that traverses the tree
and calls user-specified callbacks for each node, in pre- or
post-order. However, use of the method is quite superfluous: if you
want to recursively visit every node in the tree, it's almost always
simpler to write a subroutine does just that, than it is to bundle up
the pre- and/or post-order code in callbacks for the traverse
method.
Suppose you want to traverse at/under a node $tree and give elements an 'id' attribute unless they already have one.
You can use the traverse method:
{
my $counter = 'x0000';
$start_node->traverse(
[ # Callbacks;
# pre-order callback:
sub {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
return HTML::Element::OK; # keep traversing
},
# post-order callback:
undef
],
1, # don't call the callbacks for text nodes
);
}
or you can just be simple and clear (and not have to understand the
calling format for traverse) by writing a sub that traverses the
tree by just calling itself:
{
my $counter = 'x0000';
sub give_id {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
foreach my $c ($x->content_list) {
give_id($c) if ref $c; # ignore text nodes
}
};
give_id($start_node);
}
See, isn't that nice and clear?
But, if you really need to know:
The traverse() method is a general object-method for traversing a
tree or subtree and calling user-specified callbacks. It accepts the
following syntaxes:
These all mean to traverse the element and all of its children. That is, this method starts at node $h, "pre-order visits" $h, traverses its children, and then will "post-order visit" $h. "Visiting" means that the callback routine is called, with these arguments:
$_[0] : the node (element or text segment),
$_[1] : a startflag, and
$_[2] : the depth
If the $ignore_text parameter is given and true, then the pre-order call will not be happen for text content.
The startflag is 1 when we enter a node (i.e., in pre-order calls) and 0 when we leave the node (in post-order calls).
Note, however, that post-order calls don't happen for nodes that are text segments or are elements that are prototypically empty (like "br", "hr", etc.).
If we visit text nodes (i.e., unless $ignore_text is given and true), then when text nodes are visited, we will also pass two extra arguments to the callback:
$_[3] : the element that's the parent
of this text node
$_[4] : the index of this text node
in its parent's content list
Note that you can specify that the pre-order routine can be a different routine from the post-order one:
$h->traverse( [\&pre_callback,\&post_callback], ...);
You can also specify that no post-order calls are to be made, by providing a false value as the post-order routine:
$h->traverse([ \&pre_callback,0 ], ...);
And similarly for suppressing pre-order callbacks:
$h->traverse([ 0,\&post_callback ], ...);
Note that these two syntaxes specify the same operation:
$h->traverse([\&foo,\&foo], ...);
$h->traverse( \&foo , ...);
The return values from calls to your pre- or post-order routines are significant, and are used to control recursion into the tree.
These are the values you can return, listed in descending order of my estimation of their usefulness:
...to keep on traversing.
Note that HTML::Element::OK et
al are constants. So if you're running under use strict
(as I hope you are), and you say:
return HTML::Element::PRUEN
the compiler will flag this as an error (an unallowable
bareword, specifically), whereas if you spell PRUNE correctly,
the compiler will not complain.
return;,
it is returning undef -- at least in scalar context, and
traverse always calls your callbacks in scalar context.)
Almost every task to do with extracting information from a tree can be expressed in terms of traverse operations (usually in only one pass, and usually paying attention to only pre-order, or to only post-order), or operations based on traversing. (In fact, many of the other methods in this class are basically calls to traverse() with particular arguments.)
The source code for HTML::Element and HTML::TreeBuilder contain several examples of the use of the "traverse" method to gather information about the content of trees and subtrees.
(Note: you should not change the structure of a tree while you are traversing it.)
[End of documentation for the traverse() method]
Now, if you've been reading Structure and Interpretation of Computer Programs too much, maybe you even want a recursive lambda. Go ahead:
{
my $counter = 'x0000';
my $give_id;
$give_id = sub {
my $x = $_[0];
$x->attr('id', $counter++) unless defined $x->attr('id');
foreach my $c ($x->content_list) {
$give_id->($c) if ref $c; # ignore text nodes
}
};
$give_id->($start_node);
undef $give_id;
}
It's a bit nutty, and it's still more concise than a call to the
traverse method!
It is left as an exercise to the reader to figure out how to do the
same thing without using a $give_id symbol at all.
It is also left as an exercise to the reader to figure out why I
undefine $give_id, above; and why I could achieved the same effect
with any of:
$give_id = 'I like pie!';
# or...
$give_id = [];
# or even;
$give_id = sub { print "Mmmm pie!\n" };
But not:
$give_id = sub { print "I'm $give_id and I like pie!\n" };
# nor...
$give_id = \$give_id;
# nor...
$give_id = { 'pie' => \$give_id, 'mode' => 'a la' };
Note that you may at times see an iterative implementation of pre-order traversal, like so:
{
my @to_do = ($tree); # start-node
while(@to_do) {
my $this = shift @to_do;
# "Visit" the node:
$this->attr('id', $counter++)
unless defined $this->attr('id');
unshift @to_do, grep ref $_, $this->content_list;
# Put children on the stack -- they'll be visited next
}
}
This can under certain circumstances be more efficient than just a
normal recursive routine, but at the cost of being rather obscure. It
gains efficiency by avoiding the overhead of function-calling, but
since there are several method dispatches however you do it (to
attr and content_list), the overhead for a simple function call
is insignificant.
The traverse method does have the fairly neat features of
the ABORT, PRUNE_UP and PRUNE_SOFTLY signals. None of these
can be implemented totally straightforwardly with recursive
routines, but it is quite possible. ABORT-like behavior can be
implemented either with using non-local returning with eval/die:
my $died_on; # if you need to know where...
sub thing {
... visits $_[0]...
... maybe set $died_on to $_[0] and die "ABORT_TRAV" ...
... else call thing($child) for each child...
...any post-order visiting $_[0]...
}
eval { thing($node) };
if($@) {
if($@ =~ m<^ABORT_TRAV>) {
...it died (aborted) on $died_on...
} else {
die $@; # some REAL error happened
}
}
or you can just do it with flags:
my($abort_flag, $died_on);
sub thing {
... visits $_[0]...
... maybe set $abort_flag = 1; $died_on = $_[0]; return;
foreach my $c ($_[0]->content_list) {
thing($c);
return if $abort_flag;
}
...any post-order visiting $_[0]...
return;
}
$abort_flag = $died_on = undef; thing($node); ...if defined $abort_flag, it died on $died_on
Copyright 2000,2001 Sean M. Burke
Sean M. Burke, <sburke@cpan.org>