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Man Pages


Manual Reference Pages  -  SCALAR::DOES (3)

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NAME

Scalar::Does - like ref() but useful

CONTENTS

SYNOPSIS



  use Scalar::Does qw( -constants );
 
  my $object = bless {}, Some::Class;
 
  does($object, Some::Class);   # true
  does($object, %{});           # true
  does($object, HASH);            # true
  does($object, ARRAY);           # false



DESCRIPTION

It has long been noted that Perl would benefit from a does() built-in. A check that ref($thing) eq ARRAY doesn’t allow you to accept an object that uses overloading to provide an array-like interface.

    Functions

does($scalar, $role) Checks if a scalar is capable of performing the given role. The following (case-sensitive) roles are predefined:
o <B>SCALARB> or <B>${}B>

Checks if the scalar can be used as a scalar reference.

Note: this role does not check whether a scalar is a scalar (which is obviously true) but whether it is a reference to another scalar.

o <B>ARRAYB> or <B>@{}B>

Checks if the scalar can be used as an array reference.

o <B>HASHB> or <B>%{}B>

Checks if the scalar can be used as a hash reference.

o <B>CODEB> or <B>&{}B>

Checks if the scalar can be used as a code reference.

o <B>GLOBB> or <B>*{}B>

Checks if the scalar can be used as a glob reference.

o <B>REFB>

Checks if the scalar can be used as a ref reference (i.e. a reference to another reference).

o <B>LVALUEB>

Checks if the scalar is a reference to a special lvalue (e.g. the result of substr or splice).

o <B>IOB> or <B><>B>

Uses IO::Detect to check if the scalar is a filehandle or file-handle-like object.

(The <> check is slightly looser, allowing objects which overload <>, though overloading <> well can be a little tricky.)

o <B>VSTRINGB>

Checks if the scalar is a vstring reference.

o <B>FORMATB>

Checks if the scalar is a format reference.

o <B>RegexpB> or <B>qrB>

Checks if the scalar can be used as a quoted regular expression.

o <B>boolB>

Checks if the scalar can be used as a boolean. (It’s pretty rare for this to not be true.)

o <B>""B>

Checks if the scalar can be used as a string. (It’s pretty rare for this to not be true.)

o <B>0+B>

Checks if the scalar can be used as a number. (It’s pretty rare for this to not be true.)

Note that this is far looser than looks_like_number from Scalar::Util. For example, an unblessed arrayref can be used as a number (it numifies to its reference address); the string Hello World can be used as a number (it numifies to 0).

o <B>~~B>

Checks if the scalar can be used on the right hand side of a smart match.

If the given role is blessed, and provides a check method, then does delegates to that.

Otherwise, if the scalar being tested is blessed, then $scalar->DOES($role) is called, and does returns true if the method call returned true.

If the scalar being tested looks like a Perl class name, then $scalar->DOES($role) is also called, and the string 0E0 is returned for success, which evaluates to 0 in a numeric context but true in a boolean context.

does($role) Called with a single argument, tests $_. Yes, this works with lexical $_.



  given ($object) {
     when(does ARRAY)  { ... }
     when(does HASH)   { ... }
  }



Note: in Scalar::Does 0.007 and below the single-argument form of does returned a curried coderef. This was changed in Scalar::Does 0.008.

overloads($scalar, $role) A function overloads (which just checks overloading) is also available.
overloads($role) Called with a single argument, tests $_. Yes, this works with lexical $_.

Note: in Scalar::Does 0.007 and below the single-argument form of overloads returned a curried coderef. This was changed in Scalar::Does 0.008.

blessed($scalar), reftype($scalar), looks_like_number($scalar) For convenience, this module can also re-export these functions from Scalar::Util. looks_like_number is generally more useful than does($scalar, q[0+]).
make_role $name, where { BLOCK } Returns an anonymous role object which can be used as a parameter to does. The block is arbitrary code which should check whether $_[0] does the role.
where { BLOCK } Syntactic sugar for make_role. Compatible with the where function from Moose::Util::TypeConstraints, so don’t worry about conflicts.

    Constants

The following constants may be exported for convenience:
SCALAR
ARRAY
HASH
CODE
GLOB
REF
LVALUE
IO
VSTRING
FORMAT
REGEXP
BOOLEAN
STRING
NUMBER
SMARTMATCH

    Export

By default, only does is exported. This module uses Exporter::Tiny, so functions can be renamed:



  use Scalar::Does does => { -as => performs_role };



Scalar::Does also plays some tricks with namespace::clean to ensure that any functions it exports to your namespace are cleaned up when you’re finished with them. This ensures that if you’re writing object-oriented code does and overloads will not be left hanging around as methods of your classes. Moose::Object provides a does method, and you should be able to use Scalar::Does without interfering with that.

You can import the constants (plus does) using:



  use Scalar::Does -constants;



The make_role and where functions can be exported like this:



  use Scalar::Does -make;



Or list specific functions/constants that you wish to import:



  use Scalar::Does qw( does ARRAY HASH STRING NUMBER );



    Custom Role Checks



  use Scalar::Does
    custom => { -as => does_array, -role => ARRAY },
    custom => { -as => does_hash,  -role => HASH  };
 
  does_array($thing);
  does_hash($thing);



BUGS

Please report any bugs to <http://rt.cpan.org/Dist/Display.html?Queue=Scalar-Does>.

SEE ALSO

Scalar::Util.

<http://perldoc.perl.org/5.10.0/perltodo.html#A-does()-built-in>.

    Relationship to Moose roles

Scalar::Does is not dependent on Moose, and its role-checking is not specific to Moose’s idea of roles, but it does work well with Moose roles.

Moose::Object overrides DOES, so Moose objects and Moose roles should just work with Scalar::Does.



  {
    package Transport;
    use Moose::Role;
  }
 
  {
    package Train;
    use Moose;
    with qw(Transport);
  }
 
  my $thomas = Train->new;
  does($thomas, Train);          # true
  does($thomas, Transport);      # true
  does($thomas, Transport->meta);  # not yet supported!



Mouse::Object should be compatible enough to work as well.

See also: Moose::Role, Moose::Object, UNIVERSAL.

    Relationship to Moose type constraints

Moose::Meta::TypeConstraint objects, plus the constants exported by MooseX::Types libraries all provide a check method, so again, should just work with Scalar::Does. Type constraint strings are not supported however.



  use Moose::Util::TypeConstraints qw(find_type_constraint);
  use MooseX::Types qw(Int);
  use Scalar::Does qw(does);
 
  my $int = find_type_constraint("Int");
 
  does( "123", $int );     # true
  does( "123", Int );      # true
  does( "123", "Int" );    # false



Mouse::Meta::TypeConstraints and MouseX::Types should be compatible enough to work as well.

See also: Moose::Meta::TypeConstraint, Moose::Util::TypeConstraints, MooseX::Types, Scalar::Does::MooseTypes.

    Relationship to Type::Tiny type constraints

Types built with Type::Tiny and Type::Library can be used exactly as Moose type constraint objects above.



  use Types::Standard qw(Int);
  use Scalar::Does qw(does);
 
  does(123, Int);   # true



In fact, Type::Tiny and related libraries are used extensively in the internals of Scalar::Does 0.200+.

See also: Type::Tiny, Types::Standard.

    Relationship to Role::Tiny and Moo roles

Roles using Role::Tiny 1.002000 and above provide a DOES method, so should work with Scalar::Does just like Moose roles. Prior to that release, Role::Tiny did not provide DOES.

Moo’s role system is based on Role::Tiny.

See also: Role::Tiny, Moo::Role.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012-2014 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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perl v5.20.3 SCALAR::DOES (3) 2014-04-05

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