MooseX::UndefTolerant(3) Make your attribute(s) tolerant to undef initialization


version 0.20


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.

You can also apply the 'UndefTolerant' trait to individual attributes. See MooseX::UndefTolerant::Attribute for details.

There will be no change in behaviour to any attribute with a type constraint that accepts undef values (for example "Maybe" types), as it is presumed that since the type is already ``undef tolerant'', there is no need to avoid initializing the attribute value with "undef".

As of Moose 1.9900, this module can also be used in a role, in which case all of that role's attributes will be undef-tolerant.


I often found myself in this quandary:

  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 ternary conditionals. 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.


This extension does not currently work in immutable classes when applying the trait to some (but not all) attributes in the class. This is because the inlined constructor initialization code currently lives in Moose::Meta::Class, not Moose::Meta::Attribute. The good news is that this is expected to be changing shortly.


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)


Cory G Watson <gphat at>



This software is copyright (c) 2011 by Cory G Watson.

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