bionic (3) Scalar::Does.3pm.gz
NAME
Scalar::Does - like ref() but useful
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:
• SCALAR or ${}
Checks if the scalar can be used as a scalar reference.
Note: this role does not check whether a scalar is a scalar (which is obviously true) but whether
it is a reference to another scalar.
• ARRAY or @{}
Checks if the scalar can be used as an array reference.
• HASH or %{}
Checks if the scalar can be used as a hash reference.
• CODE or &{}
Checks if the scalar can be used as a code reference.
• GLOB or *{}
Checks if the scalar can be used as a glob reference.
• REF
Checks if the scalar can be used as a ref reference (i.e. a reference to another reference).
• LVALUE
Checks if the scalar is a reference to a special lvalue (e.g. the result of "substr" or
"splice").
• IO or <>
Uses IO::Detect to check if the scalar is a filehandle or file-handle-like object.
(The "<>" check is slightly looser, allowing objects which overload "<>", though overloading "<>"
well can be a little tricky.)
• VSTRING
Checks if the scalar is a vstring reference.
• FORMAT
Checks if the scalar is a format reference.
• Regexp or qr
Checks if the scalar can be used as a quoted regular expression.
• bool
Checks if the scalar can be used as a boolean. (It's pretty rare for this to not be true.)
• ""
Checks if the scalar can be used as a string. (It's pretty rare for this to not be true.)
• 0+
Checks if the scalar can be used as a number. (It's pretty rare for this to not be true.)
Note that this is far looser than "looks_like_number" from Scalar::Util. For example, an
unblessed arrayref can be used as a number (it numifies to its reference address); the string
"Hello World" can be used as a number (it numifies to 0).
• ~~
Checks if the scalar can be used on the right hand side of a smart match.
If the given role is blessed, and provides a "check" method, then "does" delegates to that.
Otherwise, if the scalar being tested is blessed, then "$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, 2017 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.