NAME
MooseX::AttributeShortcuts - Shorthand for common attribute options
VERSION
This document describes version 0.035 of MooseX::AttributeShortcuts -
released September 22, 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)
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