NAME
boolean - Boolean support for Perl
SYNOPSIS
use boolean ':all';
do &always if true;
do &never if false;
$guess = int(rand(2)) % 2 ? true : false;
do &something if isTrue($guess);
do &something_else if isFalse($guess);DESCRIPTION
Most programming languages have a native Boolean data type. Perl does not.
Perl has a simple and well known Truth System. The following scalar values are false:
$false1 = undef;
$false2 = 0;
$false3 = 0.0;
$false4 = '';
$false5 = '0';Every other scalar value is true.
A lesser known aspect of Perl is that its C API has two constant scalar values:
PL_sv_yes-
This is the "true" SV. It is a Perl internals constant scalar with a value of 1. See perlapi for more info.
PL_sv_no-
This is the "false" SV. It is a Perl internals constant scalar with a value of ''. See perlapi for more info.
The boolean module exposes these obscure values as the subroutines true and false. Finding meaningful ways to use these in a Perl program is left as an exercise for the reader.
IMPORTABLE SUBROUTINES
This module does not export any subroutines by default. It exports all of the following subroutines if you use the following invocation:
use boolean ':all';true
true()This function always returns a scalar whose value is the Perl internals constant PL_sv_yes. The function acts like a constant and thus takes no input parameters.
false
false()This function always returns a scalar whose value is the Perl internals constant PL_sv_no. The function acts like a constant and thus takes no input parameters.
isTrue
isTrue($scalar)Returns true if the scalar passed to it is actually PL_sv_yes. Returns false otherwise.
isFalse
isFalse($scalar)Returns true if the scalar passed to it is actually PL_sv_no. Returns false otherwise.
isBoolean
isBoolean($scalar)Returns true if the scalar passed to it is actually PL_sv_yes or PL_sv_no. Returns false otherwise.
OTHER IMPORTABLE SUBROUTINES
It turns out that the value for undef works out the same as true and false in Perl internals. All scalar values set to undef actually point to the same scalar value internally, PL_sv_undef.
This module exposes corresponding subroutines for the undef value, giving it the name "null" to match common programming language terminology.
true()
This function always returns a scalar whose value is the Perl internals constant PL_sv_undef. The function acts like a constant and thus takes no input parameters.
isNull($scalar)
Returns true if the scalar passed to it is actually PL_sv_undef. Returns false otherwise.
YAML AND JSON
It is true by definition that all valid JSON streams are valid YAML streams. The YAML and JSON specification writers have worked together to ensure this.
This leads to the following implications for Boolean values.
JSON supports 3 data types and null values:
{
"string type": "all strings are double quoted in JSON",
"number type": [1, 2, 3.1415],
"boolean type": [true, false],
"null type": null
}Strings are always double quoted. All hash keys are strings (thus quoted). Only numbers (integer and floating point) boolean values and null values are unquoted.
I recommend that all Perl YAML and JSON implementations always Dump scalar strings with the value "true", "false" or "null", using quotes.
Unquoted values of this set can be Loaded into Perl using the same constants as boolean.pm does. When these values are Dumped they are unquoted. Pure Perl YAML and JSON implementations can use boolean.pm to accomplish this.
BUGS
Currently, assigning a boolean value to a variable causes the booleanity to be lost. This code dies:
$x = true;
isBoolean($x) || isTrue($x) or die;If you know how to solve this using SV magic or otherwise, please let me know. I'm trying to fix it myslf now. Hoping to not need to resort to:
setTrue($x);AUTHOR
Ingy döt Net <ingy@cpan.org>
COPYRIGHT
Copyright (c) 2007. Ingy döt Net. All rights reserved.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
See http://www.perl.com/perl/misc/Artistic.html