Type::Tiny::Duck(3)

NAME
Type::Tiny::Duck - type constraints based on the "can" method

SYNOPSIS
Using via Types::Standard:

  package Logger {
    use Moo;
    use Types::Standard qw( HasMethods Bool );

    has debugging => ( is => 'rw', isa => Bool, default => 0 );
    has output    => ( is => 'ro', isa => HasMethods[ 'print' ] );

    sub warn {
      my ( $self, $message ) = @_;
      $self->output->print( "[WARNING] $message\n" );
    }

    sub debug {
      my ( $self, $message ) = @_;
      $self->output->print( "[DEBUG] $message\n" ) if $self->debugging;
    }
  }

Using Type::Tiny::Duck's export feature:

  package Logger {
    use Moo;
    use Types::Standard qw( Bool );
    use Type::Tiny::Duck Printable => [ 'print' ];

    has debugging => ( is => 'rw', isa => Bool, default => 0 );
    has output    => ( is => 'ro', isa => Printable );

    sub warn {
      my ( $self, $message ) = @_;
      $self->output->print( "[WARNING] $message\n" );
    }

    sub debug {
      my ( $self, $message ) = @_;
      $self->output->print( "[DEBUG] $message\n" ) if $self->debugging;
    }
  }

Using Type::Tiny::Duck's object-oriented interface:

  package Logger {
    use Moo;
    use Types::Standard qw( Bool );
    use Type::Tiny::Duck;

    my $Printable = Type::Type::Duck->new(
      name    => 'Printable',
      methods => [ 'print' ],
    );

    has debugging => ( is => 'rw', isa => Bool, default => 0 );
    has output    => ( is => 'ro', isa => $Printable );

    sub warn {
      my ( $self, $message ) = @_;
      $self->output->print( "[WARNING] $message\n" );
    }

    sub debug {
      my ( $self, $message ) = @_;
      $self->output->print( "[DEBUG] $message\n" ) if $self->debugging;
    }
  }

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

DESCRIPTION
Type constraints of the general form "{ $_->can("method") }".

The name refers to the saying, "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck". Duck typing can be a more flexible way of testing objects than relying on "isa", as it allows people to easily substitute mock objects.

This package inherits from Type::Tiny; see that for most documentation. Major differences are listed below:

  Constructors
The "new" constructor from Type::Tiny still works, of course. But there is also:

"new_intersection( type_constraints => \@ducklings, %opts )"
    Creates a new duck type constraint which is the intersection of existing duck type constraints. This allows the intersection of "HasMethods['read']" and "HasMethods['write']" to be simply "HasMethods['read', 'write']" instead of a complex union type constraint with two duck type constraints as its children.

  Attributes

"methods"
    An arrayref of method names.

"constraint"
    Unlike Type::Tiny, you cannot pass a constraint coderef to the constructor. Instead rely on the default.

"inlined"
    Unlike Type::Tiny, you cannot pass an inlining coderef to the constructor. Instead rely on the default.

"parent"
    Parent is always Types::Standard::Object, and cannot be passed to the constructor.

  Methods

stringifies_to($constraint)
    See Type::Tiny::ConstrainedObject.

numifies_to($constraint)
    See Type::Tiny::ConstrainedObject.

"with_attribute_values($attr1 => $constraint1, ...)"
    See Type::Tiny::ConstrainedObject.

  Exports
Type::Tiny::Duck can be used as an exporter.

  use Type::Tiny::Duck HttpClient => [ 'get', 'post' ];

This will export the following functions into your namespace:

"HttpClient"
is_HttpClient( $value )
assert_HttpClient( $value )
to_HttpClient( $value )

Multiple types can be exported at once:

  use Type::Tiny::Duck (
    HttpClient   => [ 'get', 'post' ],
    FtpClient    => [ 'upload', 'download' ],
  );

BUGS
Please report any bugs to <https://github.com/tobyink/p5-type-tiny/issues>.

SEE ALSO
Type::Tiny::Manual.

Type::Tiny.

Moose::Meta::TypeConstraint::DuckType.

AUTHOR
Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE
This software is copyright (c) 2013-2014, 2017-2025 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.