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MooseX::AttributeShortUser(Contributed Perl DocumMooseX::AttributeShortcuts(3)

       MooseX::AttributeShortcuts - Shorthand for common attribute options

       This document describes version 0.037 of	MooseX::AttributeShortcuts -
       released	November 20, 2017 as part of MooseX-AttributeShortcuts.

	   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, ...);

       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);


	   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.

       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.

       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

	   has '+something' => (traits => [Shortcuts], ...);

       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!' }); 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;

		   # 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.

	   "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

	   # fails constraint

       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

   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'); 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 $_ },

       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
       <> so
       that it can be worked around, fixed, or documented, as appropriate.

       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:

       o   "is => 'rwp'"


	       has  foo	=> (is => 'rwp');
	       has _bar	=> (is => 'rwp'); now effectively equivalent to:

	       has foo	=> (is => 'ro',	writer => '_set_foo');
	       has _bar	=> (is => 'ro',	writer => '_set_bar')

       o   "-writer_prefix" is ignored MooseX::SemiAffordanceAccessor	has its	own specific ideas as
	   to how writers should look.

       Please see those	modules/websites for more information related to this

       o   Moo

       o   MooseX::Types

       o   MooseX::SemiAffordanceAccessor

       Please report any bugs or feature requests on the bugtracker website

       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.

       Chris Weyl <>

       o   David Steinbrunner <>

       o   Graham Knop <>

       o   Karen Etheridge <>

       o   Olaf	Alders <>

       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

perl v5.32.0			  2017-11-20	 MooseX::AttributeShortcuts(3)


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