bionic (3) Params::Coerce.3pm.gz
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.