Toby Inkster > Type-Tiny-0.036 > Type::Tiny::Manual::Libraries

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NAME ^

Type::Tiny::Manual::Libraries - 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 Types::Datetime;
   
   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 Types::Datetime;

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 Types::Datetime 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.

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!";
      };

Inlining

If your type constraint can be inlined, this can not only speed up Type::Tiny's own checks and coercions, it may also allow your type constraint to be inlined into generated methods such as Moose attribute accessors.

All of the constraints from Types::Standard can be inlined, as can enum, class_type, role_type and duck_type constraints. Union and intersection constraints can be inlined if their sub-constraints can be. So if you can define your own types purely in terms of these types, you automatically get inlining:

   declare HashLike, as union [
      Ref["HASH"],
      Overload["&{}"],
   ];

However, sometimes these base types are not powerful enough and you'll need to write a constraint coderef:

   declare NonEmptyHash, as HashLike,
      where     { scalar values %$_ };

... and you've suddenly sacrificed a lot of speed.

Inlining to the rescue! You can define an inlining coderef which will be passed two parameters: the constraint itself and a variable name as a string. For example, the variable name might be '$_' or '$_[0]'. Your coderef should return a Perl expression string, interpolating that variable name.

   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,
         );
      };

The Perl expression could be inlined within a function or a if clause or potentially anywhere, so it really must be an expression, not a statement. It should not return or exit and probably shouldn't die. (If you need loops and so on, you can output a do block.)

Note that if you're subtyping an existing type constraint, your inline_as block is also responsible for checking the parent type's constraint. This can be done quite easily, as shown in the example above.

Note that defining a type constraint in terms of a constraint coderef and an inlining coderef can be a little repetitive. Sub::Quote provides an alternative that reduces repetition (though the inlined code might not be as compact/good/fast).

   declare NonEmptyHash, as HashLike,
      constraint => quote_sub q{ scalar values %$_ };

Aside: it's been pointed out that "might not be as fast" above is a bit hand-wavy. When Type::Tiny does inlining from Sub::Quote coderefs, it needs to inline all the ancestor type constraints, and smush them together with &&. This may result in duplicate checks. For example, if 'MyArray' inherits from 'MyRef' which inherits from 'MyDef', the inlined code might end up as:

   defined($_)              # check MyDef
   && ref($_)               # check MyRef
   && ref($_) eq 'ARRAY'    # check MyArray

When just the last check would have been sufficient. A custom inline_as allows you finer control over how the type constraint is inlined.

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,
            );
         };
      };

SEE ALSO ^

Some type libraries on CPAN:

AUTHOR ^

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE ^

This software is copyright (c) 2013 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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