Type::Tiny::Manual::Libraries(3) how to build a type library with Type::Tiny, Type::Library and Type::Utils

SYNOPSIS

A type library is a collection of type constraints, optionally with coercions.

The following is an example type library:


package Example::Types;

use Type::Library
-base,
-declare => qw( Datetime DatetimeHash EpochHash );
use Type::Utils -all;
use Types::Standard -types;

class_type Datetime, { class => "DateTime" };

declare DatetimeHash,
as Dict[
year => Int,
month => Optional[ Int ],
day => Optional[ Int ],
hour => Optional[ Int ],
minute => Optional[ Int ],
second => Optional[ Int ],
nanosecond => Optional[ Int ],
time_zone => Optional[ Str ],
];

declare EpochHash,
as Dict[ epoch => Int ];

coerce Datetime,
from Int, via { "DateTime"->from_epoch(epoch => $_) },
from Undef, via { "DateTime"->now },
from DatetimeHash, via { "DateTime"->new(%$_) },
from EpochHash, via { "DateTime"->from_epoch(%$_) };

1;

DESCRIPTION

Here's a line by line description of what's going on in the type library. 

   package Example::Types;

Type libraries are packages. It is recommended that re-usable type libraries be given a name in the "Types::*" namespace. For application-specific type libraries, assuming your application's namespace is "MyApp::*" then name the type library "MyApp::Types", or if more than one is needed, use the "MyApp::Types::*" namespace. 

   use Type::Library
      -base,
      -declare => qw( Datetime DatetimeHash EpochHash );

The "-base" part is used to establish inheritance. It makes "Example::Types" a child class of "Type::Library".

Declaring the types we're going to define ahead of their definition allows us to use them as barewords later on. (Note that in code which uses our type library, the types will always be available as barewords. The declaration above just allows us to use them within the library itself.) 

   use Type::Utils -all;

Imports some utility functions from Type::Utils. These will be useful for defining our types and the relationships between them. 

   use Types::Standard -types;

Here we import a standard set of type constraints from Types::Standard. There is no need to do this, but it's often helpful to have a base set of types which we can define our own in terms of.

Note that although we've imported the types to be able to use in our library, we haven't added the types to our library. We've imported "Str", but other people won't be able to re-import "Str" from our library. If you actually want your library to extend another library, do this instead: 

   BEGIN { extends "Types::AnotherLibrary" };

(Note: if your code breaks here when you upgrade from version 0.006 or below, saying that the 'extends' keyword has not been declared, just add '-all' after use Type::Utils.)

OK, now we're ready to declare a few types. 

   class_type Datetime, { class => "DateTime" };

This creates a type constraint named ``Datetime'' which is all objects blessed into the DateTime package. Because this type constraint is not anonymous (it has a name), it will be automagically installed into the type library.

The next two statements declare two further types constraints, using type constraints from the Types::Standard library. Let's look at "EpochHash" in more detail. This is a hashref with one key called ``epoch'' and a value which is an integer. 

   declare EpochHash,
      as Dict[ epoch => Int ];

"EpochHash" inherits from the "Dict" type defined in Types::Standard. It equally could have been defined as: 

   declare EpochHash,
      as HashRef[Int],
      where { scalar(keys(%$_))==1 and exists $_->{epoch} };

Or even: 

   declare EpochHash,
      where {
             ref($_) eq "HASH"
         and scalar(keys(%$_))==1
         and exists $_->{epoch}
      };

Lastly we set up coercions. It's best to define all your types before you define any coercions. 

   coerce Datetime,
      from Int,          via { "DateTime"->from_epoch(epoch => $_) },
      from Undef,        via { "DateTime"->now },
      from DatetimeHash, via { "DateTime"->new(%$_) },
      from EpochHash,    via { "DateTime"->from_epoch(%$_) };

These are simply coderefs that will be fired when you want a Datetime, but are given something else. For more information on coercions, see Type::Tiny::Manual::Coercions.

Using Your Library

Use a custom types library just like you would Types::Standard: 

   packge MyClass;
   use Moose;
   use DateTime;
   use Example::Types qw( Datetime ); # import the custom type
   
   has 'sometime' => (
     is      => 'rw',
     isa     => Datetime,
     coerce  => 1,
   );

Type libraries defined with Type::Library are also able to export some convenience functions: 

   use Example::Types qw( is_Datetime to_Datetime assert_Datetime );
   
   my $dt = Foo::get_datetime;
   
   unless ( is_Datetime $dt )
   {
      $dt = to_Datetime $dt;
   }
   
   assert_Datetime $dt;

These functions act as shortcuts for: 

   use Example::Types qw( Datetime );
   
   my $dt = Foo::get_datetime;
   
   unless ( Datetime->check($dt) )
   {
      $dt = Datetime->coerce($dt);
   }
   
   Datetime->assert_return($dt);

Pick whichever style you think is clearer!

Type::Library-based libraries provide a shortcut for importing a type constraint along with all its associated convenience functions: 

   # Shortcut for qw( DateTime is_Datetime to_Datetime assert_Datetime )
   #
   use Example::Types qw( +Datetime );

See Type::Tiny::Manual for other ways to make use of type libraries.

ADVANCED TOPICS

Messages

It is sometimes nice to be able to emit a more useful error message than the standard: 

   Value "Foo" did not pass type constraint "Bar"

It is possible to define custom error messages for types. 

   declare MediumInteger, as Integer,
      where   { $_ >= 10 and $_ < 20 },
      message {
         return Integer->get_message($_) if !Integer->check($_);
         return "$_ is too small!"       if $_ < 10;
         return "$_ is so very, very big!";
      };

Parameterized Constraints

Parameterized type constraints are those that can generate simple child type constraints by passing parameters to their "parameterize" method. For example, ArrayRef in Types::Standard: 

   use Types::Standard;
   
   my $ArrayRef         = Types::Standard::ArrayRef;
   my $Int              = Types::Standard::Int;
   my $ArrayRef_of_Ints = $ArrayRef->parameterize($Int);

Type libraries provide some convenient sugar for this: 

   use Types::Standard qw( ArrayRef Int );
   
   my $ArrayRef_of_Ints = ArrayRef[Int];

Unlike Moose which has separate meta classes for parameterizable, parameterized and non-parameterizable type constraints, Type::Tiny handles all that in one.

To create a parameterizable type constraint, you'll need to pass an extra named parameter to "declare". Let's imagine that we want to make our earlier "NonEmptyHash" constraint accept a parameter telling it the minimum size of the hash. For example "NonEmptyHash[4]" would need to contain at least four key-value pairs. Here's how you'd do it: 

   declare NonEmptyHash, as HashLike,
      where     { scalar values %$_ },
      inline_as {
         my ($constraint, $varname) = @_;
         return sprintf(
            '%s and scalar values %%{%s}',
            $constraint->parent->inline_check($varname),
            $varname,
         );
      },
      # Generate a new "where" coderef...
      constraint_generator => sub {
         my ($minimum) = @_;
         die "parameter must be positive" unless int($minimum) > 0;
         return sub {
             scalar(values(%$_)) >= int($minimum);
         };
      },
      # Generate a new "inline_as" coderef...
      inline_generator => sub {
         my ($minimum) = @_;
         return sub {
            my ($constraint, $varname) = @_;
            return sprintf(
               '%s and scalar(values(%%{%s})) >= %d',
               $constraint->parent->inline_check($varname),
               $varname,
               $minimum,
            );
         };
      };

AUTHOR

Toby Inkster <[email protected]>.

COPYRIGHT AND LICENCE

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.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

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