/ 
Types-Bool-2.98006
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/ Types::Bool

NAME

Types::Bool - Booleans as objects for Perl

VERSION

version 2.98006

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

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

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

COMPATIBILITY

Besides the agreement on using JSON::PP::Boolean as the canonical package for booleans, as of May 2018, the main JSON modules all step on each others' toes with varying degrees of intensity when it comes to decide what is implemented.

What we consider here the "main JSON modules" are: JSON::PP, Types::Serialiser, and Cpanel::JSON::XS. These are the original sources of ideas, implementations, and package names related to boolean support and the most relevant CPAN modules on the dependency chain.

For example,

As these modules are loaded by a Perl program, these actions are all replayed over the JSON::PP::Boolean package. (The last one loaded is basically who wins on the final code at runtime.) This is almost harmless, because their implementations mostly agree with each other. But it is not hard to see this getting out of hand if they start to diverge.

The additional interface for boolean support provided by these modules, whether functions, variables and methods, live on their own packages:

  • JSON::PP: true, false, is_bool under JSON::PP

  • Types::Serialiser: true, $true, false, $false, is_bool, is_true, is_false under Types::Serialiser

  • Cpanel::JSON::XS: true, $true, false, $false, is_bool under Cpanel::JSON::XS.

They all have subtle differences related to prototypes, enabled pragmas, and stricter or more lenient behavior. Since those don't dwell in the same base package, JSON::PP::Boolean, they are safe from the point of view of other consumers of booleans and relevant to the back-compatibility history of each of these modules. But they all contribute to the lack of consistency on the interface to deal with booleans.

Those issues mainly stem from the lack of a consensus on ownership of JSON::PP::Boolean. With the advent of Types::Bool, this may or may not be mitigated depending on the adoption of a unified treatment of sorts.

When Types::Bool gets added to this list as yet another module playing with JSON::PP::Boolean, it takes the most conservative approach: it does not touch a function or overloaded method which is already there. That means it will still provide all of the interface described before, because it will implement anything that is missing. But it may suboptimally accept what has been defined before by a previous module. And it does not fight with modules which get loaded after either.

Ideally, the best scenario will be if / when the main JSON modules start depending on Types::Bool. Even if that does not happen, this module still stands as a compatible alternative to support booleans as objects for Perl.

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::PP

JSON::XS

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.