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HTML::Element::traversUsermContributed Perl DocumeHTML::Element::traverse(3pm)



NAME
       HTML::Element::traverse - discussion of HTML::Element's traverse method

SYNOPSIS
         # $element->traverse is unnecessary and obscure.
         #   Don't use it in new code.

DESCRIPTION
       "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.

EXAMPLES
       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
       The "traverse()" method is a general object-method for traversing a
       tree or subtree and calling user-specified callbacks.  It accepts the
       following syntaxes:

       $h->traverse(\&callback)
       or $h->traverse(\&callback, $ignore_text)
       or $h->traverse( [\&pre_callback,\&post_callback] , $ignore_text)

       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 pro-
       viding 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:

       HTML::Element::OK, 1, or any other true value
           ...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.

       undef, 0, '0', '', or HTML::Element::PRUNE
           ...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.)

       HTML::Element::ABORT
           ...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.

       HTML::Element::PRUNE_UP
           ...to abort continued traversal into this node and its parent node.
           No post-order callback for the current or parent node will happen.

       HTML::Element::PRUNE_SOFTLY
           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 argu-
       ments.)

       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]

       Traversing with Recursive Anonymous Routines

       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 "tra-
       verse" 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 unde-
       fine $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' };

       Doing Recursive Things Iteratively

       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 nor-
       mal 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.

       Pruning and Whatnot

       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

SEE ALSO
       HTML::Element

COPYRIGHT
       Copyright 2000,2001 Sean M. Burke

AUTHOR
       Sean M. Burke, <sburkeATcpan.org>



perl v5.8.8                       2006-11-15      HTML::Element::traverse(3pm)