NAME

Types::Bool - Booleans as objects for Perl

VERSION

version 2.98003

SYNOPSIS

use Types::Bool qw(true false is_bool to_bool);

$true  = true;
$false = false;

is_bool(true);     # true
is_bool('xxx');    # false

to_bool(1);        # Types::Bool::true()
to_bool('');       # Types::Bool::false()

DESCRIPTION

This is meant as a draft for a standard interface to boolean objects for Perl. This is an alternative to bool.pm draft in http://blogs.perl.org/users/tinita/2018/05/my-report-of-the-perl-toolchain-summit-2018-in-oslo.html.

Perl has no native representation for booleans. Most of the time the Perl concept of truth is enough. But when dealing with serialization of formats which support booleans, it is desirable to keep the booleans intact on round trips, eg. when writing after loading. And there are other good reasons for that, like strict validation via various mechanisms, like schemas, OpenAPI, type hierarchies, etc.

A solution for that was adopted for JSON modules around 2012 by using references to 1 or 0 blessed into JSON::PP::Boolean which was the chosen canonical package for these objects.

The problem with that was the coupling with JSON::PP for no apparent good reason. Booleans are independent of JSON and this association makes little sense when loading documents in formats like YAML, MessagePack, BSON, etc. However, the integration of the concept of boolean for all these applications is quite convenient.

Marc Lehmann's Types::Serialiser approached this problem by creating a common interface used by JSON::XS and CBOR::XS modules. This module lifts this core concept (including idea, implementation and documentation) into an isolated treatment for booleans only – so this can be proposed as the common ground for interoperability on booleans as objects for Perl modules.

The implementation keeps the compatibility with the previous agreement on JSON::PP::Boolean by making Types::Bool stash an alias for JSON::PP::Boolean. That means

Types::Bool::true->isa('JSON::PP::Boolean');

but also

Types::Bool::true->isa('Types::Bool');

This also allows an optimization to an isa test by direct comparison of stash pointers.

INTERFACE

Types::Bool has two ready-to-use instances for true and false.

Types::Bool::true()

Types::Bool::false()

Types::Bool true values are represented as a reference to a scalar containing 1 – implementations are allowed to directly test for this. For example, one can tell if a value is a Types::Bool true by using this:

Types::Bool::is_bool($value) && $$value

Types::Bool false values are represented as a reference to a scalar containing 0 – implementations are allowed to directly test for this.

One can test if a value is a Types::Bool with

Types::Bool::is_bool($value);

Converting from a Perl true or false value into Types::Bool can be done with

Types::Bool::to_bool($value);

Also part of this interface is a few overloaded operators for Type::Bool objects.

# bool
true  ? 'yes' : 'no';    # 'yes'
false ? 'yes' : 'no';    # 'no'

# 0+
0+true;        # 1
0+false;       # 0

# ""
true . '';     # 1
false . '';    # 0

This minimum set of functionality allows for easy and efficient implementation of other operations, like negating booleans:

Types::Bool::to_bool( !$value );

CORE CODE

The code of this module would look as below, if not for the efforts to play nice with JSON::PP, Types::Serialiser and Cpanel::JSON::XS modules.

package Types::Bool;

use 5.006;
use Exporter 1.57 'import';
use Scalar::Util ();

use overload (
    '0+' => sub { ${ $_[0] } },
    '++' => sub { $_[0] = ${ $_[0] } + 1 },
    '--' => sub { $_[0] = ${ $_[0] } - 1 },
    fallback => 1,
);

our @EXPORT_OK = qw(true false is_bool to_bool);

use constant true  => bless \( my $dummy = 1 ), 'Types::Bool';
use constant false => bless \( my $dummy = 0 ), 'Types::Bool';

sub is_bool ($) { Scalar::Util::blessed( $_[0] ) and $_[0]->isa('Types::Bool') }

sub to_bool ($) { $_[0] ? true : false }

FUNCTIONS

Types::Bool implements the following functions, which can be imported individually.

false

$false = Types::Bool::false;

Return the canonical "false" value.

This function has a () prototype and works as a constant (suitable for inlining by the Perl interpreter).

is_bool

$is_bool = Types::Bool::is_bool($value);

Return true if given a Types::Bool object. Return false otherwise.

This function has a ($) prototype.

to_bool

$bool = Types::Bool::to_bool($value);

Turns a true or false Perl value into Types::Bool::true or Types::Bool::false.

This function has a ($) prototype.

true

$true = Types::Bool::true;

Return the canonical "true" value.

This function has a () prototype and works as a constant (suitable for inlining by the Perl interpreter).

BUGS

The use of overload makes this module heavier. See "BUGS" in Types::Serializer.

ACKNOWLEDGMENTS

Original idea and code came from JSON::XS::Boolean written by Marc Lehmann.

SEE ALSO

Types::Serialiser

JSON::XS

JSON::PP

Cpanel::JSON::XS

AUTHOR

Adriano Ferreira <ferreira@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2018 by Adriano Ferreira.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

cpanm Types::Bool

perl -MCPAN -e shell
install Types::Bool