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

- NAME
- SYNOPSIS
- DESCRIPTION
- EXAMPLES
- THE TRAVERSE METHOD

- Traversing with Recursive Anonymous Routines
- Doing Recursive Things Iteratively
- Pruning and Whatnot

- SEE ALSO
- COPYRIGHT
- AUTHOR

# 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 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

the HTML::Element manpage

# COPYRIGHT

Copyright 2000,2001 Sean M. Burke

# AUTHOR

Sean M. Burke, <sburke@cpan.org>

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