Using with Moo, part 3


Type Registries

In all the examples so far, we have imported a collection of type constraints into each class:

  package Horse {
    use Moo;
    use Types::Standard qw( Str ArrayRef HashRef Int Any InstanceOf );
    use Types::Common::Numeric qw( PositiveInt );
    use Types::Common::String qw( NonEmptyStr );
    has name    => ( is => 'ro', isa => Str );
    has father  => ( is => 'ro', isa => InstanceOf["Horse"] );

This creates a bunch of subs in the Horse namespace, one for each type. We've used namespace::autoclean to clean these up later.

But it is also possible to avoid pulling all these into the Horse namespace. Instead we'll use a type registry:

  package Horse {
    use Moo;
    use Type::Registry qw( t );
    t->alias_type('InstanceOf["Horse"]' => 'Horsey');
    has name     => ( is => 'ro', isa => t('Str') );
    has father   => ( is => 'ro', isa => t('Horsey') );
    has mother   => ( is => 'ro', isa => t('Horsey') );
    has children => ( is => 'ro', isa => t('ArrayRef[Horsey]') );

You don't even need to import the t() function. Types::Registry can be used in an entirely object-oriented way.

  package Horse {
    use Moo;
    use Type::Registry;
    my $reg = Type::Registry->for_me;
    $reg->alias_type('InstanceOf["Horse"]' => 'Horsey');
    has name => ( is => 'ro', isa => $reg->lookup('Str') );

You could create two registries with entirely different definitions for the same named type.

  my $dracula = Aristocrat->new(name => 'Dracula');
  package AristocracyTracker {
    use Type::Registry;
    my $reg1 = Type::Registry->new;
    $reg1->alias_type('PositiveInt' => 'Count');
    my $reg2 = Type::Registry->new;
    $reg2->alias_type('InstanceOf["Aristocrat"]' => 'Count');

Type::Registry uses AUTOLOAD , so things like this work:

  $reg->ArrayRef->of( $reg->Int );

Although you can create as many registries as you like, Type::Registry will create a default registry for each package.

  # Create a new empty registry.
  my $reg = Type::Registry->new;
  # Get the default registry for my package.
  # It will be pre-populated with any types we imported using `use`.
  my $reg = Type::Registry->for_me;
  # Get the default registry for some other package.
  my $reg = Type::Registry->for_class("Horse");

Type registries are a convenient place to store a bunch of types without polluting your namespace. They are not the same as type libraries though. Types::Standard , Types::Common::String , and Types::Common::Numeric are type libraries; packages that export types for others to use. We will look at how to make one of those later.

For now, here's the best way to think of the difference:

Importing Functions

We've seen how, for instance, Types::Standard exports a sub called Int that returns the Int type object.

  use Types::Standard qw( Int );
  my $type = Int;
  $type->check($value) or die $type->get_message($value);

Type libraries are also capable of exporting other convenience functions.


This is a shortcut for checking a value meets a type constraint:

  use Types::Standard qw( is_Int );
  if ( is_Int $value ) {

Calling is_Int($value) will often be marginally faster than calling Int->check($value) because it avoids a method call. (Method calls in Perl end up slower than normal function calls.)

Using things like is_ArrayRef in your code might be preferable to ref($value) eq "ARRAY" because it's neater, leads to more consistent type checking, and might even be faster. (Type::Tiny can be pretty fast; it is sometimes able to export these functions as XS subs.)

If checking type constraints like is_ArrayRef or is_InstanceOf , there's no way to give a parameter. is_ArrayRef[Int]($value) doesn't work, and neither does is_ArrayRef(Int, $value) nor is_ArrayRef($value, Int) . For some types like is_InstanceOf , this makes them fairly useless; without being able to give a class name, it just acts the same as is_Object . See "Exporting Parameterized Types" for a solution. Also, check out isa .

There also exists a generic is function.

  use Types::Standard qw( ArrayRef Int );
  use Type::Utils qw( is );
  if ( is ArrayRef[Int], \@numbers ) {


While is_Int($value) returns a boolean, assert_Int($value) will throw an error if the value does not meet the constraint, and return the value otherwise. So you can do:

  my $sum = assert_Int($x) + assert_Int($y);

And you will get the sum of integers $x and $y , and an explosion if either of them is not an integer!

Assert is useful for quick parameter checks if you are avoiding Type::Params for some strange reason:

  sub add_numbers {
    my $x = assert_Num(shift);
    my $y = assert_Num(shift);
    return $x + $y;

You can also use a generic assert function.

  use Type::Utils qw( assert );
  sub add_numbers {
    my $x = assert Num, shift;
    my $y = assert Num, shift;
    return $x + $y;


This is a shortcut for coercion:

  my $truthy = to_Bool($value);

It trusts that the coercion has worked okay. You can combine it with an assertion if you want to make sure.

  my $truthy = assert_Bool(to_Bool($value));

Shortcuts for exporting functions

This is a little verbose:

  use Types::Standard qw( Bool is_Bool assert_Bool to_Bool );

Isn't this a little bit nicer?

  use Types::Standard qw( +Bool );

The plus sign tells a type library to export not only the type itself, but all of the convenience functions too.

You can also use:

  use Types::Standard -types;   # export Int, Bool, etc
  use Types::Standard -is;      # export is_Int, is_Bool, etc
  use Types::Standard -assert;  # export assert_Int, assert_Bool, etc
  use Types::Standard -to;      # export to_Bool, etc
  use Types::Standard -all;     # just export everything!!!

So if you imagine the functions exported by Types::Standard are like this:

    Str             is_Str          assert_Str
    Num             is_Num          assert_Num
    Int             is_Int          assert_Int
    Bool            is_Bool         assert_Bool     to_Bool
    ArrayRef        is_ArrayRef     assert_ArrayRef
  # ... and more

Then "+" exports a horizonal group of those, and "-" exports a vertical group.

Exporting Parameterized Types

It's possible to export parameterizable types like ArrayRef , but it is also possible to export parameterized types.

  use Types::Standard qw( ArrayRef Int );
  use Types::Standard (
    '+ArrayRef' => { of => Int, -as => 'IntList' },
  has numbers => (is => 'ro', isa => IntList);

Using is_IntList($value) should be significantly faster than ArrayRef->of(Int)->check($value) .

This trick only works for parameterized types that have a single parameter, like ArrayRef , HashRef , InstanceOf , etc. (Sorry, Dict and Tuple !)

Lexical imports

Type::Tiny 2.0 combined with Perl 5.37.2+ allows lexically scoped imports. So:

  my $is_ok = do {
    use Types::Standard -lexical, qw( Str ArrayRef );
    ArrayRef->of( Str )->check( \@things );
  # The Str and ArrayRef types aren't defined here.

Do What I Mean!

  use Type::Utils qw( dwim_type );

dwim_type will look up a type constraint from a string and attempt to guess what you meant.

If it's a type constraint that you seem to have imported with use , then it should find it. Otherwise, if you're using Moose or Mouse, it'll try asking those. Or if it's in Types::Standard, it'll look there. And if it still has no idea, then it will assume dwim_type("Foo") means dwim_type("InstanceOf['Foo']").

It just does a big old bunch of guessing.

The is function will use dwim_type if you pass it a string as a type.

  use Type::Utils qw( is );
  if ( is "ArrayRef[Int]", \@numbers ) {


Notice that in a lot of examples we're importing one or two functions each from a few different modules:

  use Types::Common::Numeric qw( PositiveInt );
  use Types::Common::String qw( NonEmptyStr );
  use Types::Standard qw( ArrayRef Slurpy );
  use Type::Params qw( signature );

A module called Types::Common exists which acts as a single place you can use for importing most of Type::Tiny's commonly used types and functions.

  use Types::Common qw(
    PositiveInt NonEmptyStr ArrayRef Slurpy

Types::Common provides:

Next Steps

You now know pretty much everything there is to know about how to use type libraries.

Here's your next step: