NAME

Object::Iterate - iterators for objects that know the next element

SYNOPSIS

        use Object::Iterate qw(iterate igrep imap);

        iterate {...} $object;

        my @filtered    = igrep {...} $object;

        my @transformed = imap {...} $object;

DESCRIPTION

This module provides control structures to iterate through the elements of an object that cannot be represented as list of items all at once. Objects can represent a virtual collection that is beyond the reaches of foreach, map, and grep because they cannot turn themselves into a list.

If the object can return the next object, it can use this module. Iterate assumes that the object responds to __next__ with the next element, and to __more__ with TRUE or FALSE if more elements remain to be processed. The __init__ method is called before the first iteration if it exists, and silently skipped otherwise. The control structure continues until the __more__ method returns FALSE (which does not mean that it visited all of the elements but that the object has decided to stop iterating). At the end of all iterations (when __more__ returns false), Object::Iterate calls __final__ if it exists, and skips it otherwise.

Each control structure sets $_ to the current element, just like foreach, map, and grep.

Mutable method names

You do not really have to use the __next__, __more__, __init__, or __final__ names. They are just the defaults which Iterate stores in the package variables $Next, $More, $Init, and $Final respectively. This module does not export these variables, so you need to use the full package specification to change them (i.e. $Object::Iterate::$Next). If your object does not have the specified methods, the functions will die. You may want to wrap them in eval blocks.

Since this module uses package variables to storethese methods names, the method names apply to every use of the functions no matter the object. You might want to local()-ise the variables for different objects.

Before any control structure does its job, it checks the object to see if it can respond to these two methods, whatever you decide to call them, so your object must know that it can respond to these methods. AUTOLOADed methods cannot work since the module cannot know if they exist.

iterate BLOCK, OBJECT

Applies BLOCK to each item returned by ODJECT->__next__.

        iterate { print "$_\n" } $object;

This is the same thing as using a while loop, but iterate() stays out of your way.

        while( $object->__more__ )
                {
                local $_ = $object->__next__;

                ...BLOCK...
                }

igrep BLOCK, OBJECT

Applies BLOCK to each item returned by ODJECT->__next__, and returns all of the elements for which the BLOCK returns TRUE.

        my $output = igrep { print "$_\n" } $object;

This is a grep for something that cannot be represented as a list at one time.

        while( $object->__more__ )
                {
                local $_ = $object->__next__;

                push @output, $_ if ...BLOCK...;
                }

imap BLOCK, OBJECT

Applies BLOCK to each item returned by ODJECT->__next__, and returns the combined lists that BLOCK returns for each of the elements.

        my $output = imap { print "$_\n" } $object;

This is a map for something that cannot be represented as a list at one time.

        while( $object->$More )
                {
                local $_ = $object->__next__;

                push @output, ...BLOCK...;
                }

ERROR MESSAGES

iterate object has no __more__() method at script line N

You need to provide the method to let Object::Iterate determine if more elements are available. You don't have to call it __more__ if you change the value of $Object::Iterate::More.

iterate object has no __next__() method at script line N

You need to provide the method to let Object::Iterate fetch the next element. You don't have to call it __next__ if you change the value of $Object::Iterate::Next.

SOURCE AVAILABILITY

This source is part of a SourceForge project which always has the latest sources in CVS, as well as all of the previous releases.

        http://sourceforge.net/projects/brian-d-foy/

If, for some reason, I disappear from the world, one of the other members of the project can shepherd this module appropriately.

TO DO

* let the methods discover the method names per object.

CREDITS

Thanks to Slaven Rezic for adding __init__ support

AUTHOR

brian d foy, <bdfoy@cpan.org>.

COPYRIGHT and LICENSE

Copyright 2002 by brian d foy.

This software is available under the same terms as perl.