Why does Perl use the empty string to represent the boolean false value?

Question

When evaluating an expression in a scalar (boolean) context, Perl uses the explicit value 1 as a result if the expression evaluates to true and the empty string if the expression evaluates to false. I'm curious why Perl uses the empty string to represent boolean false value and not 0 which seems more intuitive.

Note that I'm not concerned with Perl treating the empty string as a false in scalar (boolean) context.

Answer 1

It's not just "" that's false in Perl. As for why... it's either because Perl is awesome or terrible -- depending on your personal preferences :)

Answer 2

Both number 0 and empty string ultimately evaluate as false in Perl. I think this is a matter of language design. When writing your own code, you can of course assume any which one false encoding convention.

For further details, check out "How do I use boolean variables in Perl?".

Answer 3

The various logical operators don't return an empty string, they return a false or true value in all three simple scalar types. It just looks like it returns an empty string because print forces a string context on its arguments:

#!/usr/bin/perl

use strict;
use warnings;

use Devel::Peek;

my $t = 5 > 4;
my $f = 5 < 4;

Dump $t;
Dump $f;

Output:

SV = PVNV(0x100802c20) at 0x100827348
  REFCNT = 1
  FLAGS = (PADMY,IOK,NOK,POK,pIOK,pNOK,pPOK)
  IV = 1
  NV = 1
  PV = 0x100201e60 "1"\0
  CUR = 1
  LEN = 16
SV = PVNV(0x100802c40) at 0x100827360
  REFCNT = 1
  FLAGS = (PADMY,IOK,NOK,POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x100208ca0 ""\0
  CUR = 0
  LEN = 16

For those not familiar with the Perl 5 internals, a PVNV is a scalar structure that holds all three simple scalar types (integer IV, double precision float NV, and string PV). The flags IOK, NOK, and POK mean that the integer, double, and string values are all in sync (for some definition of in sync) so any one of them may be used (i.e. no conversions need to take place if you use it as an integer, double, or string).

I assume the empty string was chosen for the false string because it is smaller and is more in keeping with the idea of a false string than "0". Ignore my statement about it being smaller, both "" and "1" are the same size: sixteen characters. It says so right in the dump. Perl 5 adds extra space to strings to allow them to grow quickly.

Oh, and I hate you. In researching this I have found that I have lied in perlopquick and will now have to find a way to fix it. If only you had been like all of the other sheep and just accepted Perl 5's surface weirdness as fact, I would have less work to do.

Answer 4

You can overload the stringification of true, false and undef, like this:

&Internals::SvREADONLY( \ !!1, 0);    # make !!1 writable
${ \ !!1 } = 'true';                  # change the string value of true
&Internals::SvREADONLY( \ !!1, 1);    # make !!1 readonly again
print 42 == (6*7);                    # prints 'true'

&Internals::SvREADONLY( \ !!0, 0);    # make !!0 writable
${ \ !!0 } = 'false';                 # change the string value of false
&Internals::SvREADONLY( \ !!0, 1);    # make !!0 readonly again
print 42 == (6*6);                    # prints 'false'