NAME
Moose::Manual::Unsweetened - Moose idioms in plain old Perl 5 without the sugar
DESCRIPTION
If you're trying to figure out just what the heck Moose does, and how it saves you time, you might find it helpful to see what Moose is really doing for you. This document shows you the translation from Moose sugar back to plain old Perl 5.
CLASSES AND ATTRIBUTES
First, we define two very small classes the Moose way.
package
Person;
use
DateTime;
use
Moose;
has
name
=> (
is
=>
'rw'
,
isa
=>
'Str'
,
required
=> 1,
);
# Moose doesn't know about non-Moose-based classes.
class_type
'DateTime'
;
my
$en_parser
= DateTime::Format::Natural->new(
lang
=>
'en'
,
time_zone
=>
'UTC'
,
);
coerce
'DateTime'
=> from
'Str'
=> via {
$en_parser
->parse_datetime(
$_
) };
has
birth_date
=> (
is
=>
'rw'
,
isa
=>
'DateTime'
,
coerce
=> 1,
handles
=> {
birth_year
=>
'year'
},
);
subtype
'ShirtSize'
=> as
'Str'
=> where { /^(?:s|m|l|xl|xxl)$/i }
=> message {
"$_ is not a valid shirt size (s, m, l, xl, xxl)"
};
has
shirt_size
=> (
is
=>
'rw'
,
isa
=>
'ShirtSize'
,
default
=>
'l'
,
);
This is a fairly simple class with three attributes. We also define a type to validate t-shirt sizes because we don't want to end up with something like "blue" for the shirt size!
package
User;
use
Email::Valid;
use
Moose;
subtype
'Email'
=> as
'Str'
=> where { Email::Valid->address(
$_
) }
=> message {
"$_ is not a valid email address"
};
has
email_address
=> (
is
=>
'rw'
,
isa
=>
'Email'
,
required
=> 1,
);
This class subclasses Person to add a single attribute, email address.
Now we will show what these classes would look like in plain old Perl 5. For the sake of argument, we won't use any base classes or any helpers like Class::Accessor
.
package
Person;
use
strict;
use
warnings;
use
DateTime;
sub
new {
my
$class
=
shift
;
my
%p
=
ref
$_
[0] ? %{
$_
[0] } :
@_
;
exists
$p
{name}
or confess
'name is a required attribute'
;
$class
->_validate_name(
$p
{name} );
exists
$p
{birth_date}
or confess
'birth_date is a required attribute'
;
$p
{birth_date} =
$class
->_coerce_birth_date(
$p
{birth_date} );
$class
->_validate_birth_date(
$p
{birth_date} );
$p
{shirt_size} =
'l'
unless
exists
$p
{shirt_size}:
$class
->_validate_shirt_size(
$p
{shirt_size} );
return
bless
\
%p
,
$class
;
}
sub
_validate_name {
shift
;
my
$name
=
shift
;
local
$Carp::CarpLevel
=
$Carp::CarpLevel
+ 1;
defined
$name
or confess
'name must be a string'
;
}
{
my
$en_parser
= DateTime::Format::Natural->new(
lang
=>
'en'
,
time_zone
=>
'UTC'
,
);
sub
_coerce_birth_date {
shift
;
my
$date
=
shift
;
return
$date
unless
defined
$date
&& !
ref
$date
;
my
$dt
=
$en_parser
->parse_datetime(
$date
);
return
$dt
?
$dt
:
undef
;
}
}
sub
_validate_birth_date {
shift
;
my
$birth_date
=
shift
;
local
$Carp::CarpLevel
=
$Carp::CarpLevel
+ 1;
$birth_date
->isa(
'DateTime'
)
or confess
'birth_date must be a DateTime object'
;
}
sub
_validate_shirt_size {
shift
;
my
$shirt_size
=
shift
;
local
$Carp::CarpLevel
=
$Carp::CarpLevel
+ 1;
defined
$shirt_size
or confess
'shirt_size cannot be undef'
;
$shirt_size
=~ /^(?:s|m|l|xl|xxl)$/
or confess
"$shirt_size is not a valid shirt size (s, m, l, xl, xxl)"
;
}
sub
name {
my
$self
=
shift
;
if
(
@_
) {
$self
->_validate_name(
$_
[0] );
$self
->{name} =
$_
[0];
}
return
$self
->{name};
}
sub
birth_date {
my
$self
=
shift
;
if
(
@_
) {
my
$date
=
$self
->_coerce_birth_date(
$_
[0] );
$self
->_validate_birth_date(
$date
);
$self
->{birth_date} =
$date
;
}
return
$self
->{birth_date};
}
sub
birth_year {
my
$self
=
shift
;
return
$self
->birth_date->year;
}
sub
shirt_size {
my
$self
=
shift
;
if
(
@_
) {
$self
->_validate_shirt_size(
$_
[0] );
$self
->{shirt_size} =
$_
[0];
}
return
$self
->{shirt_size};
}
Wow, that was a mouthful! One thing to note is just how much space the data validation code consumes. As a result, it's pretty common for Perl 5 programmers to just not bother. Unfortunately, not validating arguments leads to surprises down the line ("why is birth_date an email address?").
Also, did you spot the (intentional) bug?
It's in the _validate_birth_date()
method. We should check that the value in $birth_date
is actually defined and an object before we go and call isa()
on it! Leaving out those checks means our data validation code could actually cause our program to die. Oops.
Note that if we add a superclass to Person we'll have to change the constructor to account for that.
(As an aside, getting all the little details of what Moose does for you just right in this example was really not easy, which emphasizes the point of the example. Moose saves you a lot of work!)
Now let's see User:
package
User;
use
strict;
use
warnings;
use
Email::Valid;
sub
new {
my
$class
=
shift
;
my
%p
=
ref
$_
[0] ? %{
$_
[0] } :
@_
;
exists
$p
{email_address}
or confess
'email_address is a required attribute'
;
$class
->_validate_email_address(
$p
{email_address} );
my
$self
=
$class
->SUPER::new(
%p
);
$self
->{email_address} =
$p
{email_address};
return
$self
;
}
sub
_validate_email_address {
shift
;
my
$email_address
=
shift
;
local
$Carp::CarpLevel
=
$Carp::CarpLevel
+ 1;
defined
$email_address
or confess
'email_address must be a string'
;
Email::Valid->address(
$email_address
)
or confess
"$email_address is not a valid email address"
;
}
sub
email_address {
my
$self
=
shift
;
if
(
@_
) {
$self
->_validate_email_address(
$_
[0] );
$self
->{email_address} =
$_
[0];
}
return
$self
->{email_address};
}
That one was shorter, but it only has one attribute.
Between the two classes, we have a whole lot of code that doesn't do much. We could probably simplify this by defining some sort of "attribute and validation" hash, like this:
package
Person;
my
%Attr
= (
name
=> {
required
=> 1,
validate
=>
sub
{
defined
$_
},
},
birth_date
=> {
required
=> 1,
validate
=>
sub
{ blessed
$_
&&
$_
->isa(
'DateTime'
) },
},
shirt_size
=> {
required
=> 1,
validate
=>
sub
{
defined
$_
&&
$_
=~ /^(?:s|m|l|xl|xxl)$/i },
}
);
Then we could define a base class that would accept such a definition, and do the right thing. Keep that sort of thing up and we're well on our way to writing a half-assed version of Moose!
Of course, there are CPAN modules that do some of what Moose does, like Class::Accessor
, Class::Meta
, and so on. But none of them put together all of Moose's features along with a layer of declarative sugar, nor are these other modules designed for extensibility in the same way as Moose. With Moose, it's easy to write a MooseX module to replace or extend a piece of built-in functionality.
Moose is a complete OO package in and of itself, and is part of a rich ecosystem of extensions. It also has an enthusiastic community of users, and is being actively maintained and developed.
AUTHOR
Dave Rolsky <autarch@urth.org>
COPYRIGHT AND LICENSE
Copyright 2009 by Infinity Interactive, Inc.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.