Specio - Type constraints and coercions for Perl


version 0.48


    package MyApp::Type::Library;

    use Specio::Declare;
    use Specio::Library::Builtins;

        parent => t('Int'),
        inline => sub {
            $_[0]->parent->inline_check( $_[1] )
                . ' && ( '
                . $_[1]
                . ' > 0 )';

    # or ...

        parent => t('Int'),
        where  => sub { $_[0] > 0 },

        parent => t(
            of => t('PositiveInt'),

        from  => t('PositiveInt'),
        using => sub { [ $_[0] ] },

        methods => [ 'duck_walk', 'quack' ],



The Specio distribution provides classes for representing type constraints and coercion, along with syntax sugar for declaring them.

Note that this is not a proper type system for Perl. Nothing in this distribution will magically make the Perl interpreter start checking a value's type on assignment to a variable. In fact, there's no built-in way to apply a type to a variable at all.

Instead, you can explicitly check a value against a type, and optionally coerce values to that type.

My long-term goal is to replace Moose's built-in types and MooseX::Types with this module.


At it's core, a type is simply a constraint. A constraint is code that checks a value and returns true or false. Most constraints are represented by Specio::Constraint::Simple objects. However, there are other type constraint classes for specialized kinds of constraints.

Types can be named or anonymous, and each type can have a parent type. A type's constraint is optional because sometimes you may want to create a named subtype of some existing type without adding additional constraints.

Constraints can be expressed either in terms of a simple subroutine reference or in terms of an inline generator subroutine reference. The former is easier to write but the latter is preferred because it allow for better optimization.

A type can also have an optional message generator subroutine reference. You can use this to provide a more intelligent error message when a value does not pass the constraint, though the default message should suffice for most cases.

Finally, you can associate a set of coercions with a type. A coercion is a subroutine reference (or inline generator, like constraints), that takes a value of one type and turns it into a value that matches the type the coercion belongs to.


This distribution ships with a set of builtin types representing the types provided by the Perl interpreter itself. They are arranged in a hierarchy as follows:

      Maybe (of `a)
              ScalarRef (of `a)
              ArrayRef (of `a)
              HashRef (of `a)

The Item type accepts anything and everything.

The Bool type only accepts undef, 0, or 1.

The Undef type only accepts undef.

The Defined type accepts anything except undef.

The Num and Int types are stricter about numbers than Perl is. Specifically, they do not allow any sort of space in the number, nor do they accept "Nan", "Inf", or "Infinity".

The ClassName type constraint checks that the name is valid and that the class is loaded.

The FileHandle type accepts either a glob, a scalar filehandle, or anything that isa IO::Handle.

All types accept overloaded objects that support the required operation. See below for details.


Perl's overloading is horribly broken and doesn't make much sense at all.

However, unlike Moose, all type constraints allow overloaded objects where they make sense.

For types where overloading makes sense, we explicitly check that the object provides the type overloading we expect. We do not simply try to use the object as the type in question and hope it works. This means that these checks effectively ignore the fallback setting for the overloaded object. In other words, an object that overloads stringification will not pass the Bool type check unless it also overloads boolification.

Most types do not check that the overloaded method actually returns something that matches the constraint. This may change in the future.

The Bool type accepts an object that implements bool overloading.

The Str type accepts an object that implements string (q{""}) overloading.

The Num type accepts an object that implements numeric ('0+'}) overloading. The Int type does as well, but it will check that the overloading returns an actual integer.

The ClassName type will accept an object with string overloading that returns a class name.

To make this all more confusing, the Value type will never accept an object, even though some of its subtypes will.

The various reference types all accept objects which provide the appropriate overloading. The FileHandle type accepts an object which overloads globification as long as the returned glob is an open filehandle.


Any type followed by a type parameter of `a in the hierarchy above can be parameterized. The parameter is itself a type, so you can say you want an "ArrayRef of Int", or even an "ArrayRef of HashRef of ScalarRef of ClassName".

When they are parameterized, the ScalarRef and ArrayRef types check that the value(s) they refer to match the type parameter. For the HashRef type, the parameter applies to the values (keys are never checked).


The Maybe type is a special parameterized type. It allows for either undef or a value. All by itself, it is meaningless, since it is equivalent to "Maybe of Item", which is equivalent to Item. When parameterized, it accepts either an undef or the type of its parameter.

This is useful for optional attributes or parameters. However, you're probably better off making your code simply not pass the parameter at all This usually makes for a simpler API.


Types are local to each package where they are used. When you "import" types from some other library, you are actually making a copy of that type.

This means that a type named "Foo" in one package may not be the same as "Foo" in another package. This has potential for confusion, but it also avoids the magic action at a distance pollution that comes with a global type naming system.

The registry is managed internally by the Specio distribution's modules, and is not exposed to your code. To access a type, you always call t('TypeName').

This returns the named type or dies if no such type exists.

Because types are always copied on import, it's safe to create coercions on any type. Your coercion from Str to Int will not be seen by any other package, unless that package explicitly imports your Int type.

When you import types, you import every type defined in the package you import from. However, you can overwrite an imported type with your own type definition. You cannot define the same type twice internally.


By default, all types created inside a package are invisible to other packages. If you want to create a type library, you need to inherit from Specio::Exporter package:

  package MyApp::Type::Library;

  use parent 'Specio::Exporter';

  use Specio::Declare;
  use Specio::Library::Builtins;

      parent => t('Str'),
      where  => sub { $_[0] =~ /foo/i },

Now the MyApp::Type::Library package will export a single type named Foo. It does not re-export the types provided by Specio::Library::Builtins.

If you want to make your library re-export some other libraries types, you can ask for this explicitly:

  package MyApp::Type::Library;

  use parent 'Specio::Exporter';

  use Specio::Declare;
  use Specio::Library::Builtins -reexport;

  declare( 'Foo, ... );

Now MyApp::Types::Library exports any types it defines, as well as all the types defined in Specio::Library::Builtins.


Use the Specio::Declare module to declare types. It exports a set of helpers for declaring types. See that module's documentation for more details on these helpers.


This should just work. Use a Specio type anywhere you'd specify a type.


Using Specio with Moo is easy. You can pass Specio constraint objects as isa parameters for attributes. For coercions, simply call $type->coercion_sub.

    package Foo;

    use Specio::Declare;
    use Specio::Library::Builtins;
    use Moo;

    my $str_type = t('Str');
    has string => (
       is  => 'ro',
       isa => $str_type,

    my $ucstr = declare(
        parent => t('Str'),
        where  => sub { $_[0] =~ /^[A-Z]+$/ },

        from  => t('Str'),
        using => sub { return uc $_[0] },

    has ucstr => (
        is     => 'ro',
        isa    => $ucstr,
        coerce => $ucstr->coercion_sub,

The subs returned by Specio use Sub::Quote internally and are suitable for inlining.


See Specio::Constraint::Simple for the API that all constraint objects share.

Moose, MooseX::Types, and Specio

This module aims to supplant both Moose's built-in type system (see Moose::Util::TypeConstraints aka MUTC) and MooseX::Types, which attempts to patch some of the holes in the Moose built-in type design.

Here are some of the salient differences:

  • Types names are strings, but they're not global

    Unlike Moose and MooseX::Types, type names are always local to the current package. There is no possibility of name collision between different modules, so you can safely use short type names.

    Unlike MooseX::Types, types are strings, so there is no possibility of colliding with existing class or subroutine names.

  • No type auto-creation

    Types are always retrieved using the t() subroutine. If you pass an unknown name to this subroutine it dies. This is different from Moose and MooseX::Types, which assume that unknown names are class names.

  • Anon types are explicit

    With Moose and MooseX::Types, you use the same subroutine, subtype(), to declare both named and anonymous types. With Specio, you use declare() for named types and anon() for anonymous types.

  • Class and object types are separate

    Moose and MooseX::Types have class_type and duck_type. The former type requires an object, while the latter accepts a class name or object.

    With Specio, the distinction between accepting an object versus object or class is explicit. There are six declaration helpers, object_can_type, object_does_type, object_isa_type, any_can_type, any_does_type, and any_isa_type.

  • Overloading support is baked in

    Perl's overloading is quite broken but ignoring it makes Moose's type system frustrating to use in many cases.

  • Types can either have a constraint or inline generator, not both

    Moose and MooseX::Types types can be defined with a subroutine reference as the constraint, an inline generator subroutine, or both. This is purely for backwards compatibility, and it makes the internals more complicated than they need to be.

    With Specio, a constraint can have either a subroutine reference or an inline generator, not both.

  • Coercions can be inlined

    I simply never got around to implementing this in Moose.

  • No crazy coercion features

    Moose has some bizarre (and mostly) undocumented features relating to coercions and parameterizable types. This is a misfeature.


There are several optional prereqs that if installed will make this distribution better in some way.

  • Ref::Util

    Installing this will speed up a number of type checks for built-in types.

  • XString

    If this is installed it will be loaded instead of the B module if you have Perl 5.10 or greater. This module is much more memory efficient than loading all of B.

  • Sub::Util or Sub::Name

    If one of these is installed then stack traces that end up in Specio code will have much better subroutine names for any frames.


This distro was originally called "Type", but that's an awfully generic top level namespace. Specio is Latin for for "look at" and "spec" is the root for the word "species". It's short, relatively easy to type, and not used by any other distro.


Eventually I'd like to see this distro replace Moose's internal type system, which would also make MooseX::Types obsolete.


Bugs may be submitted at


The source code repository for Specio can be found at


If you'd like to thank me for the work I've done on this module, please consider making a "donation" to me via PayPal. I spend a lot of free time creating free software, and would appreciate any support you'd care to offer.

Please note that I am not suggesting that you must do this in order for me to continue working on this particular software. I will continue to do so, inasmuch as I have in the past, for as long as it interests me.

Similarly, a donation made in this way will probably not make me work on this software much more, unless I get so many donations that I can consider working on free software full time (let's all have a chuckle at that together).

To donate, log into PayPal and send money to, or use the button at


Dave Rolsky <>


  • Chris White <>

  • cpansprout <>

  • Graham Knop <>

  • Karen Etheridge <>

  • Vitaly Lipatov <>


This software is Copyright (c) 2012 - 2022 by Dave Rolsky.

This is free software, licensed under:

  The Artistic License 2.0 (GPL Compatible)

The full text of the license can be found in the LICENSE file included with this distribution.