plucky (3) MooseX::AttributeShortcuts.3pm.gz
NAME
MooseX::AttributeShortcuts - Shorthand for common attribute options
VERSION
This document describes version 0.037 of MooseX::AttributeShortcuts - released November 20, 2017 as part
of MooseX-AttributeShortcuts.
SYNOPSIS
package Some::Class;
use Moose;
use MooseX::AttributeShortcuts;
# same as:
# is => 'ro', lazy => 1, builder => '_build_foo'
has foo => (is => 'lazy');
# same as: is => 'ro', writer => '_set_foo'
has foo => (is => 'rwp');
# same as: is => 'ro', builder => '_build_bar'
has bar => (is => 'ro', builder => 1);
# same as: is => 'ro', clearer => 'clear_bar'
has bar => (is => 'ro', clearer => 1);
# same as: is => 'ro', predicate => 'has_bar'
has bar => (is => 'ro', predicate => 1);
# works as you'd expect for "private": predicate => '_has_bar'
has _bar => (is => 'ro', predicate => 1);
# extending? Use the "Shortcuts" trait alias
extends 'Some::OtherClass';
has '+bar' => (traits => [Shortcuts], builder => 1, ...);
DESCRIPTION
Ever find yourself repeatedly specifying writers and builders, because there's no good shortcut to
specifying them? Sometimes you want an attribute to have a read-only public interface, but a private
writer. And wouldn't it be easier to just say "builder => 1" and have the attribute construct the
canonical "_build_$name" builder name for you?
This package causes an attribute trait to be applied to all attributes defined to the using class. This
trait extends the attribute option processing to handle the above variations. All attribute options as
described in Moose or Class::MOP::Attribute remain usable, just as when this trait is not applied.
Some Notes On History
Moose has long had a lazy_build attribute option. It was once considered a best practice, but that has,
ah, changed. This trait began as a desire to still leverage bits of "lazy_build" (and a tacit
acknowledgment that fat-finger bugs rank among the most embarrassing, right up there with "the TV was
unplugged the entire time").
This author does not recommend you use "lazy_build", unless you know exactly what you're doing (probably)
and that it's a good idea (probably not).
Nonetheless, this "lazy_build" option is why we set certain options the way we do below; while
"lazy_build" in its entirety is not optimal, it had the right idea: regular, predictable accessor names
for regular, predictable attribute options.
As an example, just looking at the below it doesn't seem logical that:
has _foo => (is => 'ro', clearer => 1);
...becomes:
has _foo => (is => 'ro', clearer => '_clear_foo');
After reading the lazy_build attribute option, however, we see that the choice had already been made for
us.
USAGE
This package automatically applies an attribute metaclass trait. Simply using this package causes the
trait to be applied by default to your attribute's metaclasses.
EXTENDING A CLASS
If you're extending a class and trying to extend its attributes as well, you'll find out that the trait
is only applied to attributes defined locally in the class. This package exports a trait shortcut
function "Shortcuts" that will help you apply this to the extended attribute:
has '+something' => (traits => [Shortcuts], ...);
NEW ATTRIBUTE OPTIONS
Unless specified here, all options defined by Moose::Meta::Attribute and Class::MOP::Attribute remain
unchanged.
Want to see additional options? Ask, or better yet, fork on GitHub and send a pull request. If the
shortcuts you're asking for already exist in Moo or Mouse or elsewhere, please note that as it will carry
significant weight.
For the following, $name should be read as the attribute name; and the various prefixes should be read
using the defaults.
is => 'rwp'
Specifying "is => 'rwp'" will cause the following options to be set:
is => 'ro'
writer => "_set_$name"
rwp can be read as "read + write private".
is => 'lazy'
Specifying "is => 'lazy'" will cause the following options to be set:
is => 'ro'
builder => "_build_$name"
lazy => 1
NOTE: Since 0.009 we no longer set "init_arg => undef" if no "init_arg" is explicitly provided. This is
a change made in parallel with Moo, based on a large number of people surprised that lazy also made one's
"init_def" undefined.
is => 'lazy', default => ...
Specifying "is => 'lazy'" and a default will cause the following options to be set:
is => 'ro'
lazy => 1
default => ... # as provided
That is, if you specify "is => 'lazy'" and also provide a "default", then we won't try to set a builder,
as well.
builder => 1
Specifying "builder => 1" will cause the following options to be set:
builder => "_build_$name"
builder => sub { ... }
Passing a coderef to builder will cause that coderef to be installed in the class this attribute is
associated with the name you'd expect, and "builder => 1" to be set.
e.g., in your class (or role),
has foo => (is => 'ro', builder => sub { 'bar!' });
...is effectively the same as...
has foo => (is => 'ro', builder => '_build_foo');
sub _build_foo { 'bar!' }
The behaviour of this option in roles changed in 0.030, and the builder methods will be installed in the
role itself. This means you can alias/exclude/etc builder methods in roles, just as you can with any
other method.
clearer => 1
Specifying "clearer => 1" will cause the following options to be set:
clearer => "clear_$name"
or, if your attribute name begins with an underscore:
clearer => "_clear$name"
(that is, an attribute named "_foo" would get "_clear_foo")
predicate => 1
Specifying "predicate => 1" will cause the following options to be set:
predicate => "has_$name"
or, if your attribute name begins with an underscore:
predicate => "_has$name"
(that is, an attribute named "_foo" would get "_has_foo")
init_arg => 1 / -1
This is a somewhat esoteric shortcut; you probably don't want to use this (or even read this section).
Specifying "init_arg => 1" will cause the following options to be set:
# attribute: "name"
init_arg => 'name'
# or, attribute: "_name"
init_arg => '_name'
...while "init_arg => -1" will cause the following options to be set:
# attribute: "name"
init_arg => '_name'
# or, attribute: "_name"
init_arg => 'name'
trigger => 1
Specifying "trigger => 1" will cause the attribute to be created with a trigger that calls a named method
in the class with the options passed to the trigger. By default, the method name the trigger calls is
the name of the attribute prefixed with "_trigger_".
e.g., for an attribute named "foo" this would be equivalent to:
trigger => sub { shift->_trigger_foo(@_) }
For an attribute named "_foo":
trigger => sub { shift->_trigger__foo(@_) }
This naming scheme, in which the trigger is always private, is the same as the builder naming scheme
(just with a different prefix).
handles => { foo => sub { ... }, ... }
Creating a delegation with a coderef will now create a new, "custom accessor" for the attribute. These
coderefs will be installed and called as methods on the associated class (just as readers, writers, and
other accessors are), and will have the attribute metaclass available in $_. Anything the accessor is
called with it will have access to in @_, just as you'd expect of a method.
e.g., the following example creates an attribute named "bar" with a standard reader accessor named "bar"
and two custom accessors named "foo" and "foo_too".
has bar => (
is => 'ro',
isa => 'Int',
handles => {
foo => sub {
my $self = shift @_;
return $_->get_value($self) + 1;
},
foo_too => sub {
my $self = shift @_;
return $self->bar + 1;
},
# ...as you'd expect.
bar => 'bar',
},
);
...and later,
Note that in this example both foo() and foo_too() do effectively the same thing: return the attribute's
current value plus 1. However, foo() accesses the attribute value directly through the metaclass, the
pros and cons of which this author leaves as an exercise for the reader to determine.
You may choose to use the installed accessors to get at the attribute's value, or use the direct
metaclass access, your choice.
ANONYMOUS SUBTYPING AND COERCION
"Abusus non tollit usum."
Note that we create new, anonymous subtypes whenever the constraint or coercion options are specified in
such a way that the Shortcuts trait (this one) is invoked. It's fully supported to use both constraint
and coerce options at the same time.
This facility is intended to assist with the creation of one-off type constraints and coercions. It is
not possible to deliberately reuse the subtypes we create, and if you find yourself using a particular
isa / constraint / coerce option triplet in more than one place you should really think about creating a
type that you can reuse. MooseX::Types provides the facilities to easily do this, or even a simple
constant definition at the package level with an anonymous type stashed away for local use.
isa => sub { ... }
has foo => (
is => 'rw',
# $_ == $_[0] == the value to be validated
isa => sub { die unless $_[0] == 1 },
);
# passes constraint
$thing->foo(1);
# fails constraint
$thing->foo(5);
Given a coderef, create a type constraint for the attribute. This constraint will fail if the coderef
dies, and pass otherwise.
Astute users will note that this is the same way Moo constraints work; we use
MooseX::Meta::TypeConstraint::Mooish to implement the constraint.
isa_instance_of => ...
Given a package name, this option will create an "isa" type constraint that requires the value of the
attribute be an instance of the class (or a descendant class) given. That is,
has foo => (is => 'ro', isa_instance_of => 'SomeThing');
...is effectively the same as:
use Moose::TypeConstraints 'class_type';
has foo => (
is => 'ro',
isa => class_type('SomeThing'),
);
...but a touch less awkward.
isa => ..., constraint => sub { ... }
Specifying the constraint option with a coderef will cause a new subtype constraint to be created, with
the parent type being the type specified in the "isa" option and the constraint being the coderef
supplied here.
For example, only integers greater than 10 will pass this attribute's type constraint:
# value must be an integer greater than 10 to pass the constraint
has thinger => (
isa => 'Int',
constraint => sub { $_ > 10 },
# ...
);
Note that if you supply a constraint, you must also provide an "isa".
isa => ..., constraint => sub { ... }, coerce => 1
Supplying a constraint and asking for coercion will "Just Work", that is, any coercions that the "isa"
type has will still work.
For example, let's say that you're using the "File" type constraint from MooseX::Types::Path::Class, and
you want an additional constraint that the file must exist:
has thinger => (
is => 'ro',
isa => File,
constraint => sub { !! $_->stat },
coerce => 1,
);
"thinger" will correctly coerce the string "/etc/passwd" to a "Path::Class:File", and will only accept
the coerced result as a value if the file exists.
coerce => [ Type => sub { ...coerce... }, ... ]
Specifying the coerce option with a hashref will cause a new subtype to be created and used (just as with
the constraint option, above), with the specified coercions added to the list. In the passed hashref,
the keys are Moose types (well, strings resolvable to Moose types), and the values are coderefs that will
coerce a given type to our type.
has bar => (
is => 'ro',
isa => 'Str',
coerce => [
Int => sub { "$_" },
Object => sub { 'An instance of ' . ref $_ },
],
);
INTERACTIONS WITH OTHER ATTRIBUTE TRAITS
Sometimes attribute traits interact in surprising ways. This trait is well behaved; if you have
discovered any interactions with other traits (good, bad, indifferent, etc), please report this
<https://github.com/RsrchBoy/moosex-attributeshortcuts/issues/new> so that it can be worked around,
fixed, or documented, as appropriate.
MooseX::SemiAffordanceAccessor
MooseX::SemiAffordanceAccessor changes how the "is => 'rw'" and "accessor => ..." attribute options work.
If our trait detects that an attribute has had the MooseX::SemiAffordanceAccessor attribute trait
applied, then we change our behaviour to conform to its expectations:
• "is => 'rwp'"
This:
has foo => (is => 'rwp');
has _bar => (is => 'rwp');
...is now effectively equivalent to:
has foo => (is => 'ro', writer => '_set_foo');
has _bar => (is => 'ro', writer => '_set_bar')
• "-writer_prefix" is ignored
...as MooseX::SemiAffordanceAccessor has its own specific ideas as to how writers should look.
SEE ALSO
Please see those modules/websites for more information related to this module.
• Moo
• MooseX::Types
• MooseX::SemiAffordanceAccessor
BUGS
Please report any bugs or feature requests on the bugtracker website
<https://github.com/RsrchBoy/moosex-attributeshortcuts/issues>
When submitting a bug or request, please include a test-file or a patch to an existing test-file that
illustrates the bug or desired feature.
AUTHOR
Chris Weyl <cweyl@alumni.drew.edu>
CONTRIBUTORS
• David Steinbrunner <dsteinbrunner@pobox.com> • Graham Knop <haarg@haarg.org> • Karen Etheridge <ether@cpan.org> • Olaf Alders <olaf@wundersolutions.com>
COPYRIGHT AND LICENSE
This software is Copyright (c) 2017, 2015, 2014, 2013, 2012, 2011 by Chris Weyl.
This is free software, licensed under:
The GNU Lesser General Public License, Version 2.1, February 1999