HTML::Element::traverse - phpMan

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

VERSION
    This document describes version 5.07 of HTML::Element::traverse,
    released August 31, 2017 as part of HTML-Tree.

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
    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:

    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 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]

  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
    "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' };

  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
    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.

  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

AUTHOR
    Current maintainers:

    *   Christopher J. Madsen "<perl AT cjmweb.net>"

    *   Jeff Fearn "<jfearn AT cpan.org>"

    Original HTML-Tree author:

    *   Gisle Aas

    Former maintainers:

    *   Sean M. Burke

    *   Andy Lester

    *   Pete Krawczyk "<petek AT cpan.org>"

    You can follow or contribute to HTML-Tree's development at
    <https://github.com/kentfredric/HTML-Tree>.

COPYRIGHT
    Copyright 2000,2001 Sean M. Burke