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) 2012-2013 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.