Provided by: libdata-walk-perl_1.00-3_all 
NAME
Data::Walk - Traverse Perl data structures
SYNOPSIS
use Data::Walk;
walk \&wanted, @items_to_walk;
use Data::Walk;
walkdepth \&wanted, @items_to_walk;
use Data::Walk;
walk { wanted => \&process, follow => 1 }, $self;
DESCRIPTION
The above synopsis bears an amazing similarity to File::Find(3pm) and this is not
coincidental.
Data::Walk(3pm) is for data what File::Find(3pm) is for files. You can use it for rolling
your own serialization class, for displaying Perl data structures, for deep copying or
comparing, for recursive deletion of data, or ...
If you are impatient and already familiar with File::Find(3pm), you can skip the following
documentation and proceed with "DIFFERENCES TO FILE::FIND".
FUNCTIONS
The module exports two functions by default:
walk
walk \&wanted, @items;
walk \%options, @items;
As the name suggests, the function traverses the items in the order they are given.
For every object visited, it calls the &wanted subroutine. See "THE WANTED FUNCTION"
for details.
walkdepth
walkdepth \&wanted, @items;
walkdepth \%options, @items;
Works exactly like "walk()" but it first descends deeper into the structure, before
visiting the nodes on the current level. If you want to delete visited nodes, then
"walkdepth()" is probably your friend.
OPTIONS
The first argument to "walk()" and "walkdepth()" is either a code reference to your
&wanted function, or a hash reference describing the operations to be performed for each
visited node.
Here are the possible keys for the hash.
wanted
The value should be a code reference. This code reference is described in "THE WANTED
FUNCTION" below.
bydepth
Visits nodes on the current level of recursion only after descending into subnotes.
The entry point "walkdepth()" is a shortcut for specifying "{ bydepth => 1 }".
preprocess
The value should be a code reference. This code reference is used to preprocess the
current node $Data::Walk::container. Your preprocessing function is called before the
loop that calls the "wanted()" function. It is called with a list of member nodes and
is expected to return such a list. The list will contain all sub-nodes, regardless of
the value of the option follow! The list is normally a shallow copy of the data
contained in the original structure. You can therefore safely delete items in it,
without affecting the original data. You can use the option copy, if you want to
change that behavior.
The behavior is identical for regular arrays and hashes, so you probably want to
coerce the list passed as an argument into a hash then. The variable
$Data::Walk::type will contain the string "HASH" if the currently inspected node is a
hash.
You can use the preprocessing function to sort the items contained or to filter out
unwanted items. The order is also preserved for hashes!
preprocess_hash
The value should be a code reference. The code is executed right after an eventual
preprocess_hash handler, but only if the current container is a hash. It is skipped
for regular arrays.
You will usually prefer a preprocess_hash handler over a preprocess handler if you
only want to sort hash keys.
postprocess
The value should be a code reference. It is invoked just before leaving the currently
visited node. It is called in void context with no arguments. The variable
$Data::Walk::container points to the currently visited node.
follow
Causes cyclic references to be followed. Normally, the traversal will not descend
into nodes that have already been visited. If you set the option follow to a truth
value, you can change this behavior. Unless you take additional measures, this will
always imply an infinite loop!
Please note that the &wanted function is also called for nodes that have already been
visited! The effect of follow is to suppress descending into subnodes.
copy
Normally, the &preprocess function is called with a shallow copy of the data. If you
set the option copy to a false value, the &preprocess function is called with one
single argument, a reference to the original data structure. In that case, you also
have to return a suitable reference.
Using this option will result in a slight performance win, and can make it sometimes
easier to manipulate the original data.
What is a shallow copy? Think of a list containing references to hashes:
my @list = ({ foo => 'bar' }, { foo => 'baz' });
my @shallow = @list;
After this, @shallow will contain a new list, but the items stored in it are exactly
identical to the ones stored in the original. In other words, @shallow occupies new
memory, whereas both lists contain references to the same memory for the list members.
All other options are silently ignored.
THE WANTED FUNCTION
The &wanted function does whatever verifications you want on each item in the data
structure. Note that despite its name, the &wanted function is a generic callback and
does not tell Data::Walk(3pm) if an item is "wanted" or not. In fact, its return value is
ignored.
The wanted function takes no arguments but rather does its work through a collection of
variables:
$_ The currently visited node. Think "file" in terms of File::Find(3pm)!
$Data::Walk::container
The node containing the currently visited node, either a reference to a hash or an
array. Think "directory" in terms of File::Find(3pm)!
$Data::Walk::type
The base type of the object that $Data::Walk::container references. This is either
"ARRAY" or "HASH".
$Data::Walk::seen
For references, this will hold the number of times the currently visited node has
been visited before. The value is consequently set to 0 not 1 on the first visit.
For non-references, the value is undefined.
$Data::Walk::address
For references, this will hold the memory address it points to. It can be used as a
unique identifier for the current node. For non- references, the value is undefined.
$Data::Walk::depth
The depth of the current recursion.
These variables should not be modified.
DIFFERENCES TO FILE::FIND
The API of Data::Walk(3pm) tries to mimic the API of File::Find(3pm) to a certain extent.
If you are already familiar with File::Find(3pm) you will find it very easy to use
Data::Walk(3pm). Even the documentation for Data::Walk(3pm) is in parts similar or
identcal to that of File::Find(3pm).
Analogies
The equivalent of directories in File::Find(3pm) are the container data types in
Data::Walk(3pm). Container data types are arrays (aka lists) and associative arrays (aka
hashes). Files are equivalent to scalars. Wherever File::Find(3pm) passes lists of
strings to functions, Data::Walk(3pm) passes lists of variables.
Function Names
Instead of "find()" and "finddepth()", Data::Walk(3pm) uses "walk()" and "walkdepth()",
like the smart reader has already guessed after reading the "SYNOPSIS".
Variables
The variable $Data::Walk::container is vaguely equivalent to $File::Find::dir. All other
variables are specific to the corresponding module.
Wanted Function
Like its archetype from File::Find(3pm), the wanted function of Data::Walk(3pm) is called
with $_ set to the currently inspected item.
Options
The option follow has the effect that Data::Walk(3pm) also descends into nodes it has
already visited. Unless you take extra measures, this will lead to an infinite loop!
A number of options are not applicable to data traversion and are ignored by
Data::Walk(3pm). Examples are follow_fast, follow_skip, no_chdir, untaint,
untaint_pattern, and untaint_skip. To give truth the honor, all unrecognized options are
skipped.
You may argue, that the options untaint and friends would be useful, too, allowing you to
recursively untaint data structures. But, hey, that is what Data::Walk(3pm) is all about.
It makes it very easy for you to write that yourself.
EXAMPLES
Following are some recipies for common tasks.
Recursive Untainting
sub untaint {
s/(.*)/$1/s unless ref $_;
};
walk \&untaint, $data;
See perlsec(1), if you don't understand why the untaint() function untaints your data
here.
Recurse To Maximum Depth
If you want to stop the recursion at a certain level, do it as follows:
my $max_depth = 20;
sub not_too_deep {
if ($Data::Walk::depth > $max_depth) {
return ();
} else {
return @_;
}
}
sub do_something1 {
# Your code goes here.
}
walk { wanted => \&do_something, preprocess => \¬_too_deep };
BUGS
If you think you have spotted a bug, you can share it with others in the bug tracking
system at http://rt.cpan.org/NoAuth/Bugs.html?Dist=Data-Walk.
COPYING
Copyright (C) 2005-2006, Guido Flohr <guido@imperia.net>, all rights reserved.
This program is free software; you can redistribute it and/or modify it under the terms of
the GNU Library General Public License as published by the Free Software Foundation;
either version 2, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU Library General Public License for more details.
You should have received a copy of the GNU Library General Public License along with this
program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.
SEE ALSO
Data::Dumper(3pm), Storable(3pm), File::Find(3pm), perl(1)