Type::Tiny - tiny, yet Moo(se)-compatible type constraint


   use Scalar::Util qw(looks_like_number);
   use Type::Tiny;
   my $NUM = "Type::Tiny"->new(
      name       => "Number",
      constraint => sub { looks_like_number($_) },
      message    => sub { "$_ ain't a number" },
   package Ermintrude {
      use Moo;
      has favourite_number => (is => "ro", isa => $NUM);
   package Bullwinkle {
      use Moose;
      has favourite_number => (is => "ro", isa => $NUM);
   package Maisy {
      use Mouse;
      has favourite_number => (is => "ro", isa => $NUM);


This module is covered by the Type-Tiny stability policy.


Type::Tiny is a tiny class for creating Moose-like type constraint objects which are compatible with Moo, Moose and Mouse.

Maybe now we won't need to have separate MooseX, MouseX and MooX versions of everything? We can but hope...

This documents the internals of Type::Tiny. Type::Tiny::Manual is a better starting place if you're new.



Moose-style constructor function.


Attributes are named values that may be passed to the constructor. For each attribute, there is a corresponding reader method. For example:

   my $type = Type::Tiny->new( name => "Foo" );
   print $type->name, "\n";   # says "Foo"

Important attributes

These are the attributes you are likely to be most interested in providing when creating your own type constraints, and most interested in reading when dealing with type constraint objects.


Coderef to validate a value ($_) against the type constraint. The coderef will not be called unless the value is known to pass any parent type constraint (see parent below).

Defaults to sub { 1 } - i.e. a coderef that passes all values.


Optional attribute; parent type constraint. For example, an "Integer" type constraint might have a parent "Number".

If provided, must be a Type::Tiny object.


A coderef which returns a string of Perl code suitable for inlining this type. Optional.

If constraint (above) is a coderef generated via Sub::Quote, then Type::Tiny may be able to automatically generate inlined for you.


The name of the type constraint. These need to conform to certain naming rules (they must begin with an uppercase letter and continue using only letters, digits 0-9 and underscores).

Optional; if not supplied will be an anonymous type constraint.


A name to display for the type constraint when stringified. These don't have to conform to any naming rules. Optional; a default name will be calculated from the name.


The package name of the type library this type is associated with. Optional. Informational only: setting this attribute does not install the type into the package.


Coderef that returns an error message when $_ does not validate against the type constraint. Optional (there's a vaguely sensible default.)


A Type::Coercion object associated with this type.

Generally speaking this attribute should not be passed to the constructor; you should rely on the default lazily-built coercion object.

You may pass coercion => 1 to the constructor to inherit coercions from the constraint's parent. (This requires the parent constraint to have a coercion.)


Experimenal hashref of additional methods that can be called on the type constraint object.

The following additional attributes are used for parameterizable (e.g. ArrayRef) and parameterized (e.g. ArrayRef[Int]) type constraints. Unlike Moose, these aren't handled by separate subclasses.


Coderef that generates a new constraint coderef based on parameters. Alternatively, the constraint generator can return a fully-formed Type::Tiny object, in which case the name_generator, inline_generator, and coercion_generator attributes documented below are ignored.

Optional; providing a generator makes this type into a parameterizable type constraint.


A coderef which generates a new display_name based on parameters. Optional; the default is reasonable.


A coderef which generates a new inlining coderef based on parameters.


A coderef which generates a new Type::Coercion object based on parameters.


This API is not finalized. Coderef used by Error::TypeTiny::Assertion to peek inside parameterized types and figure out why a value doesn't pass the constraint.


In parameterized types, returns an arrayref of the parameters.

Lazy generated attributes

The following attributes should not be usually passed to the constructor; unless you're doing something especially unusual, you should rely on the default lazily-built return values.


Coderef to validate a value ($_[0]) against the type constraint. This coderef is expected to also handle all validation for the parent type constraints.


A complementary type for this type. For example, the complementary type for an integer type would be all things that are not integers, including floating point numbers, but also alphabetic strings, arrayrefs, filehandles, etc.

moose_type, mouse_type

Objects equivalent to this type constraint, but as a Moose::Meta::TypeConstraint or Mouse::Meta::TypeConstraint.

It should rarely be necessary to obtain a Moose::Meta::TypeConstraint object from Type::Tiny because the Type::Tiny object itself should be usable pretty much anywhere a Moose::Meta::TypeConstraint is expected.


Predicate methods

These methods return booleans indicating information about the type constraint. They are each tightly associated with a particular attribute. (See "Attributes".)

has_parent, has_library, has_inlined, has_constraint_generator, has_inline_generator, has_coercion_generator, has_parameters, has_message, has_deep_explanation

Simple Moose-style predicate methods indicating the presence or absence of an attribute.


Predicate method with a little extra DWIM. Returns false if the coercion is a no-op.


Returns true iff the type constraint does not have a name.

is_parameterized, is_parameterizable

Indicates whether a type has been parameterized (e.g. ArrayRef[Int]) or could potentially be (e.g. ArrayRef).

Validation and coercion

The following methods are used for coercing and validating values against a type constraint:


Returns true iff the value passes the type constraint.


Returns the error message for the value; returns an explicit undef if the value passes the type constraint.


Like check($value) but dies if the value does not pass the type constraint.

Yes, that's three very similar methods. Blame Moose::Meta::TypeConstraint whose API I'm attempting to emulate. :-)


Like assert_valid($value) but returns the value if it passes the type constraint.

This seems a more useful behaviour than assert_valid($value). I would have just changed assert_valid($value) to do this, except that there are edge cases where it could break Moose compatibility.


Returns the error message for the value; even if the value passes the type constraint.

validate_explain($value, $varname)

Like validate but instead of a string error message, returns an arrayref of strings explaining the reasoning why the value does not meet the type constraint, examining parent types, etc.

The $varname is an optional string like '$foo' indicating the name of the variable being checked.


Attempt to coerce $value to this type.


Attempt to coerce $value to this type. Throws an exception if this is not possible.

Child type constraint creation and parameterization

These methods generate new type constraint objects that inherit from the constraint they are called upon:


Construct a new Type::Tiny object with this object as its parent.


The class that create_child_type will construct by default.


Creates a new parameterized type; throws an exception if called on a non-parameterizable type.

plus_coercions($type1, $code1, ...)

Shorthand for creating a new child type constraint with the same coercions as this one, but then adding some extra coercions (at a higher priority than the existing ones).

plus_fallback_coercions($type1, $code1, ...)

Like plus_coercions, but added at a lower priority.

minus_coercions($type1, ...)

Shorthand for creating a new child type constraint with fewer type coercions.


Shorthand for creating a new child type constraint with no coercions at all.

Type relationship introspection methods

These methods allow you to determine a type constraint's relationship to other type constraints in an organised hierarchy:

equals($other), is_subtype_of($other), is_supertype_of($other), is_a_type_of($other)

Compare two types. See Moose::Meta::TypeConstraint for what these all mean. (OK, Moose doesn't define is_supertype_of, but you get the idea, right?)

Note that these have a slightly DWIM side to them. If you create two Type::Tiny::Class objects which test the same class, they're considered equal. And:

   my $subtype_of_Num = Types::Standard::Num->create_child_type;
   my $subtype_of_Int = Types::Standard::Int->create_child_type;
   $subtype_of_Int->is_subtype_of( $subtype_of_Num );  # true
strictly_equals($other), is_strictly_subtype_of($other), is_strictly_supertype_of($other), is_strictly_a_type_of($other)

Stricter versions of the type comparison functions. These only care about explicit inheritance via parent.

   my $subtype_of_Num = Types::Standard::Num->create_child_type;
   my $subtype_of_Int = Types::Standard::Int->create_child_type;
   $subtype_of_Int->is_strictly_subtype_of( $subtype_of_Num );  # false

Returns a list of all this type constraint's ancestor constraints. For example, if called on the Str type constraint would return the list (Value, Defined, Item, Any).

Due to a historical misunderstanding, this differs from the Moose implementation of the parents method. In Moose, parents only returns the immediate parent type constraints, and because type constraints only have one immediate parent, this is effectively an alias for parent. The extension module MooseX::Meta::TypeConstraint::Intersection is the only place where multiple type constraints are returned; and they are returned as an arrayref in violation of the base class' documentation. I'm keeping my behaviour as it seems more useful.


Loops through the parent type constraints including the invocant itself and returns the nearest ancestor type constraint where the coderef evaluates to true. Within the coderef the ancestor currently being checked is $_. Returns undef if there is no match.

In list context also returns the number of type constraints which had been looped through before the matching constraint was found.


Return a type constraint which is the union of type constraints that can be coerced to this one (including this one). If this type constraint has no coercions, returns itself.


In parameterized type constraints, returns the first item on the list of parameters; otherwise returns undef. For example:

   ( ArrayRef[Int] )->type_parameter;    # returns Int
   ( ArrayRef[Int] )->parent;            # returns ArrayRef

Note that parameterizable type constraints can perfectly legitimately take multiple parameters (several off the parameterizable type constraints in Types::Standard do). This method only returns the first such parameter. "Attributes related to parameterizable and parameterized types" documents the parameters attribute, which returns an arrayref of all the parameters.

Inlining methods

The following methods are used to generate strings of Perl code which may be pasted into stringy evaluated subs to perform type checks:


Returns boolean indicating if this type can be inlined.


Creates a type constraint check for a particular variable as a string of Perl code. For example:

   print( Types::Standard::Num->inline_check('$foo') );

prints the following output:

   (!ref($foo) && Scalar::Util::looks_like_number($foo))

For Moose-compat, there is an alias _inline_check for this method.


Much like inline_check but outputs a statement of the form:

   die ... unless ...;

Note that if this type has a custom error message, the inlined code will ignore this custom message!!

Other methods


For non-anonymous type constraints that have a library, returns a qualified "MyLib::MyType" sort of name. Otherwise, returns the same as name.

isa($class), can($method), AUTOLOAD(@args)

If Moose is loaded, then the combination of these methods is used to mock a Moose::Meta::TypeConstraint.

If Mouse is loaded, then isa mocks Mouse::Meta::TypeConstraint.


Overridden to advertise support for various roles.

See also Type::API::Constraint, etc.


These are provided as hooks that wrap Type::Tie. (Type::Tie is distributed separately, and can be used with non-Type::Tiny type constraints too.) They allow the following to work:

   use Types::Standard qw(Int);
   tie my @list, Int;
   push @list, 123, 456;   # ok
   push @list, "Hello";    # dies

The following methods exist for Moose/Mouse compatibility, but do not do anything useful.



  • Stringification is overloaded to return the qualified name.

  • Boolification is overloaded to always return true.

  • Coderefification is overloaded to call assert_return.

  • On Perl 5.10.1 and above, smart match is overloaded to call check.

  • The == operator is overloaded to call equals.

  • The < and > operators are overloaded to call is_subtype_of and is_supertype_of.

  • The ~ operator is overloaded to call complementary_type.

  • The | operator is overloaded to build a union of two type constraints. See Type::Tiny::Union.

  • The & operator is overloaded to build the intersection of two type constraints. See Type::Tiny::Intersection.

Previous versions of Type::Tiny would overload the + operator to call plus_coercions or plus_fallback_coercions as appropriate. Support for this was dropped after 0.040.



Indicates whether the smart match overload is supported on your version of Perl.

Package Variables


This undef by default but may be set to a coderef that Type::Tiny and related modules will use to dump data structures in things like error messages.

Otherwise Type::Tiny uses it's own routine to dump data structures. $DD may then be set to a number to limit the lengths of the dumps. (Default limit is 72.)

This is a package variable (rather than get/set class methods) to allow for easy localization.


Please report any bugs to


IRC: support is available through in the #moops channel on


Type::Tiny::Manual, Type::API.

Type::Library, Type::Utils, Types::Standard, Type::Coercion.

Type::Tiny::Class, Type::Tiny::Role, Type::Tiny::Duck, Type::Tiny::Enum, Type::Tiny::Union, Type::Tiny::Intersection.

Moose::Meta::TypeConstraint, Mouse::Meta::TypeConstraint.



Toby Inkster <>.


Thanks to Matt S Trout for advice on Moo integration.


This software is copyright (c) 2013-2014 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.