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Cory G Watson > MooseX-UndefTolerant-0.06 > MooseX::UndefTolerant



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Module Version: 0.06   Source   Latest Release: MooseX-UndefTolerant-0.21


MooseX::UndefTolerant - Make your attribute(s) tolerant to undef initialization


  package My::Class;

  use Moose;
  use MooseX::UndefTolerant;

  has 'name' => (
    is => 'ro',
    isa => 'Str',
    predicate => 'has_name'

  # Meanwhile, under the city...

  # Doesn't explode
  my $class = My::Class->new(name => undef);
  $class->has_name # False!

Or, if you only want one attribute to have this behaviour:

  package My:Class;
  use Moose;

  use MooseX::UndefTolerant::Attribute;

  has 'bar' => (
      traits => [ qw(MooseX::UndefTolerant::Attribute)],
      is => 'ro',
      isa => 'Num',
      predicate => 'has_bar'


Loading this module in your Moose class makes initialization of your attributes tolerant of undef. If you specify the value of undef to any of the attributes they will not be initialized. Effectively behaving as if you had not provided a value at all.


I often found myself in this quandry:

  package My:Class;
  use Moose;

  has 'foo' => (
    is => 'ro',
    isa => 'Str',

  # ... then

  my $foo = ... # get the param from something

  my $class = My:Class->new(foo => $foo, bar => 123);

What if foo is undefined? I didn't want to change my attribute to be Maybe[Str] and I still want my predicate (has_foo) to work. The only real solution was:

  if(defined($foo)) {
    $class = My:Class->new(foo => $foo, bar => 123);
  } else {
    $class = My:Class->new(bar => 123);

Or some type of codemulch using ternarys. This module allows you to make your attributes more tolerant of undef so that you can keep the first example: have your cake and eat it too!


See MooseX::UndefTolerant::Attribute.


Cory G Watson, <gphat at>


Many thanks to the crew in #moose who talked me through this module:

Hans Dieter Pearcey (confound)

Jesse Luehrs (doy)

Tomas Doran (t0m)

Dylan Hardison (dylan)

Jay Shirley (jshirley)

Mike Eldridge (diz)


Copyright 2009 Cory G Watson.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See for more information.

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