/ 
MooseX-hasn-t-0.003
River stage zero No dependents
/ MooseX::hasn::t

NAME

MooseX::hasn't - syntactic sugar to complement "has"

SYNOPSIS

{
  package Person;
  use Moose;
  has name => (is => "ro", writer => "_rename", required => 1);
}

{
  package AnonymousPerson;
  use Moose;
  use MooseX::hasn't;
  extends "Person";
  hasn't name => ();
}

my $dude  = AnonymousPerson->new;
say($dude->can('_rename') ? 'true' : 'false');  # false
say($dude->name);                               # croaks

DESCRIPTION

hasn't is a counter-part for Moose's has.

It tries to stop a child class inheriting something (an attribute or a method) from its parent class - though it's not always 100% successful.

FAQ

Doesn't this break polymorphism?

The idea behind polymorphism is that if Bar inherits from Foo, then I should be able to use an object of type Bar wherever I'd normally use Foo.

In particular, if I can do:

Foo->new()->some_method();

then I should be able to do:

Bar->new()->some_method();

But if Bar can explicitly indicate that it hasn't got method some_method then this breaks. So, yes, this module does break polymorphism.

But observe that it's not especially difficult to break polymorphism manually:

{
  package Foo;
  use Moose;
  sub some_method {}
}

{
  package Bar;
  use Moose;
  extends 'Foo';
  sub some_method { die "some_method not found in package Bar" }
}

This module just makes it easier and more declarative.

How exactly is this achieved?

For hasn't $method, it simply adds an override method modifier to the given method that croaks.

For hasn't $attribute, it finds the names of the accessor, reader, writer, clearer, predicate and initializer methods for that attribute (if any) and overrides them all.

In both cases, it overrides the class' can method too.

What about required attributes?

If the parent class has an attribute which is required and has a default, then you can use hasn't in a child class safely.

If the parent class has an attribute which is required but has no default, then you must explicitly specify a default in the child class:

hasn't name => (default => 'anon');

This latter technique is probably not foolproof. Defaults may be coderefs, like in has.

BUGS AND LIMITATIONS

  • hasn't $attr (default => sub {}) will execute the coderef as a function with no arguments, not as a method.

  • $object->meta can still see attributes and methods which have been "hasn'ted". Some serious Class::MOP fu is needed to fix this.

Report anything else here:

http://rt.cpan.org/Dist/Display.html?Queue=Moose-hasn-t.

SEE ALSO

Moose.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.