Provided by: libparams-coerce-perl_0.14-1_all 
NAME
Params::Coerce - Allows your classes to do coercion of parameters
SYNOPSIS
# Coerce a object of class Foo to a Bar
my $bar = Params::Coerce::coerce('Bar', $Foo)
# Create a coercion param function
use Params::Coerce '_Bar' => 'Bar';
my $bar = _Bar($Foo);
# Usage when Bar has a 'from' method
my $bar = Bar->from($Foo);
Real world example using HTML::Location.
# My class needs a URI
package Web::Spider;
use URI;
use Params::Coerce 'coerce';
sub new {
my $class = shift;
# Where do we start spidering
my $start = coerce('URI', shift) or die "Wasn't passed a URI";
bless { root => $start }, $class;
}
#############################################
# Now we can do the following
# Pass a URI as normal
my $URI = URI->new('http://ali.as/');
my $Spider1 = Web::Spider->new( $URI );
# We can also pass anything that can be coerced into being a URI
my $Website = HTML::Location->new( '/home/adam/public_html', 'http://ali.as' );
my $Spider2 = Web::Spider->new( $Website );
DESCRIPTION
A big part of good API design is that we should be able to be flexible in the ways that we
take parameters.
Params::Coerce attempts to encourage this, by making it easier to take a variety of
different arguments, while adding negligable additional complexity to your code.
What is Coercion
"Coercion" in computing terms generally referse to "implicit type conversion". This is
where data and object are converted from one type to another behind the scenes, and you
just just magically get what you need.
The overload pragma, and its string overloading is the form of coercion you are most
likely to have encountered in Perl programming. In this case, your object is automatically
(within perl itself) coerced into a string.
"Params::Coerce" is intended for higher-order coercion between various types of different
objects, for use mainly in subroutine and (mostly) method parameters, particularly on
external APIs.
__as_Another_Class Methods
At the heart of "Params::Coerce" is the ability to transform objects from one thing to
another. This can be done by a variety of different mechanisms.
The prefered mechanism for this is by creating a specially named method in a class that
indicates it can be coerced into another type of object.
As an example, HTML::Location provides an object method that returns an equivalent URI
object.
# In the package HTML::Location
# Coerce to a URI
sub __as_URI {
my $self = shift;
return URI->new( $self->uri );
}
__from_Another_Class Methods
From version 0.04 of "Params::Coerce", you may now also provide __from_Another_Class
methods as well. In the above example, rather then having to define a method in
HTML::Location, you may instead define one in URI. The following code has an identical
effect.
# In the package URI
# Coerce from a HTML::Location
sub __from_HTML_Location {
my $Location = shift;
return URI->new( $Location->uri );
}
"Params::Coerce" will only look for the __from method, if it does not find a __as method.
Loading Classes
One thing to note with the "__as_Another_Class" methods is that you are not required to
load the class you are converting to in the class you are converting from.
In the above example, HTML::Location does not have to load the URI class. The need to load
the classes for every object we might some day need to be coerced to would result in
highly excessive resource usage.
Instead, "Params::Coerce" guarentees that the class you are converting to "will" be loaded
before it calls the __as_Another_Class method. Of course, in most situations you will have
already loaded it for another purpose in either the From or To classes and this won't be
an issue.
If you make use of some class other than the class you are being coerced to in the
__as_Another_Class method, you will need to make sure that is loaded in your code, but it
is suggested that you do it at run-time with a "require" if you are not using it already
elsewhere.
Coercing a Parameter
The most explicit way of accessing the coercion functionality is with the
Params::Coerce::coerce function. It takes as its first argument the name of the class you
wish to coerce to, followed by the parameter to which you wish to apply the coercion.
package My::Class;
use URI ();
use Params::Coerce '_URI' => 'URI';
sub new {
my $class = shift;
# Take a URI argument
my $URI = Params::Coerce::coerce('URI', shift) or return;
...
}
For people doing procedural programming, you may also import this function.
# Import the coerce function
use Params::Coerce 'coerce';
Please note thatThe "coerce|Params::Coerce" function is the only function that can be
imported, and that the two argument pragma (or the passing of two or more arguments to
->import) means something different entirely.
Importing Parameter Coercion Methods
The second way of using Params::Coerce, and the more common one for Object-Oriented
programming, is to create method specifically for taking parameters in a coercing manner.
package My::Class;
use URI ();
use Params::Coerce '_URI' => 'URI';
sub new {
my $class = shift;
# Take a URI as parameter
my $URI1 = $class->_URI(shift) or return;
my $URI2 = _URI(shift) or return;
...
}
The "from" Constructor
From version 0.11 of "Params::Coerce", an additional mechanism is available with the
importable "from" constructor.
package My::Class;
use Params::Coerce 'from';
package Other::Class;
sub method {
my $self = shift;
my $My = My::Class->from(shift) or die "Bad param";
...
}
This is mainly a convenience. The above is equivalent to
package My::Class;
use Params::Coerce 'from' => 'Params::Coerce';
In future versions, this "->from" syntax may also tweak the resolution order of the
coercion.
Chained Coercion
While it is intended that Params::Coerce will eventually support coercion using multiple
steps, like "<Foo::Bar-"__as_HTML_Location->__as_URI>>, it is not currently capable of
this. At this time only a single coercion step is supported.
FUNCTIONS
coerce $class, $param
The "coerce" function takes a class name and a single parameter and attempts to coerce the
parameter into the intended class, or one of its subclasses.
Please note that it is the responsibility of the consuming class to ensure that the class
you wish to coerce to is loaded. "coerce" will check this and die is it is not loaded.
Returns an instance of the class you specify, or one of its subclasses. Returns "undef"
if the parameter cannot be coerced into the class you wish.
TO DO
- Write more unit tests
- Implement chained coercion
- Provide a way to coerce to string, int, etc that is compatible with overload and other
types of things.
SUPPORT
Bugs should always be submitted via the CPAN bug tracker
<http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Param-Coerce>
For other issues, contact the maintainer
AUTHORS
Adam Kennedy <adamk@cpan.org>
COPYRIGHT
Copyright 2004 - 2006 Adam Kennedy.
This program is free software; you can redistribute it and/or modify it under the same
terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.