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 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.