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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:
traverse(\&callback)
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.
...to block traversing under the current element's content.
(This is ignored if received from a post-order callback,
since by then the recursion has already happened.)
If this is returned by a pre-order callback, no
post-order callback for the current node will happen.
(Recall that if your callback exits with just return;,
it is returning undef -- at least in scalar context, and
traverse always calls your callbacks in scalar context.)
...to abort the whole traversal immediately. This is often useful when you're looking for just the first node in the tree that meets some criterion of yours.
...to abort continued traversal into this node and its parent node. No post-order callback for the current or parent node will happen.
Like PRUNE, except that the post-order call for the current node is not blocked.
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>