/ 
Type-0.03-TRIAL
/ Type

NAME

Type - Type constraints and coercions for Perl

VERSION

version 0.03

SYNOPSIS

package MyApp::Type::Library;

use Type::Declare;
use Type::Library::Builtins;

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

# or ...

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

declare(
    'ArrayRefOfPositiveInt',
    parent => t(
        'ArrayRef',
        of => t('PositiveInt'),
    ),
);

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

any_can_type(
    'Duck',
    methods => [ 'duck_walk', 'quack' ],
);

object_isa_type('MyApp::Person');

DESCRIPTION

WARNING: This thing is very alpha.

The Type 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.

WHAT IS A TYPE?

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 Type::Constraint::Simple objects, though there are some other type constraint classes for specialized constraint types.

Types can be named or anonymous, and they can have a parent type. A type has an optional constraint. The constraint is optional because sometimes you want to make a named subtype of some existing parent 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, since it allow for much more extensive optimizations.

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 more 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.

BUILTIN TYPES

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:

Item
    Bool
    Maybe (of `a)
    Undef
    Defined
        Value
            Str
                Num
                    Int
                ClassName
        Ref
            ScalarRef (of `a)
            ArrayRef (of `a)
            HashRef (of `a)
            CodeRef
            RegexpRef
            GlobRef
            FileHandle
            Object

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.

Overloading

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 and question and hope it works. This means that these checks effective 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 provides bool overloading.

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

The Num type accepts an object that provides 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 it's 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.

PARAMETERIZABLE TYPES

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).

Maybe

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 its parameter.

This is useful for optional attributes or parameters. However, whenever possible, you're often better off making the parameter not required at all. This usually makes for a simpler API.

REGISTRIES AND IMPORTING

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 Type 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.

CREATING A TYPE LIBRARY

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 Type::Exporter package:

package MyApp::Type::Library;

use parent 'Type::Exporter';

use Type::Declare;
use Type::Library::Builtins;

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

Now the MyApp::Type::Library package will export a single type named Foo. It does not (yet) re-export the types provided by Type::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 'Type::Exporter';

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

declare( 'Foo, ... );

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

DECLARING TYPES

Use the Type::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.

Moose, MooseX::Types, and Type

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:

  • Type 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 types names for code.

    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.

  • Exceptions are objects

    The $type->validate_or_die() method throws a Type::Exception object on failure, not a string.

  • Anon types are explicit

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

    In Type, the distinction between accepting an object versus object or class is explicit. There are four declaration helpers, object_can_type, object_isa_type, any_can_type, and any_isa_type.

  • Overloading support is baked in

    Perl's overloading is broken as hell, but ignoring it makes Moose's type system frustrating.

  • 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 Type, 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.

LONG-TERM PLANS

Eventually I'd like to see this distro replace Moose's internal type system, which would also make MooseX::Types obsolete. This almost certainly means rewriting this distro to not use Moose itself (or any modules which use Moose, like Throwable).

For now, the current code is a proof of concept for the design.

BUGS

Please report any bugs or feature requests to bug-type@rt.cpan.org, or through the web interface at http://rt.cpan.org. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

DONATIONS

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, which seems unlikely at best.

To donate, log into PayPal and send money to autarch@urth.org or use the button on this page: http://www.urth.org/~autarch/fs-donation.html

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

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

This is free software, licensed under:

The Artistic License 2.0 (GPL Compatible)