Re: TRUE

[James E. Tilley]

On Wed, Jan 21, 2004 at 07:48:21PM +0200, Gaal Yahas wrote:
> 8.2 is not TRUE as Andrew defined it, but it is true under Perl's idea
> of truth. That is in fact my point: using the non-native notion of what
> truth is, you can't use any code from other people because you can't
> assume it returns what you expect to be called true to signal truth.

Somebody had to do it. . . .

#!/usr/bin/perl -w
use strict;

BEGIN
{
  package True;

  use overload
    'eq' => \&equals,
    '==' => \&equals,
    bool => sub { !!1 },
    '!'  => sub { False->new() },
  ;

  use base 'Exporter';
  our @EXPORT = qw( &TRUE );

  sub new
  {
    my ($class) = @_;
    my $self = 1;
    return bless \$self, $class;
  }

  sub TRUE
  {
    return True->new();
  }

  sub equals
  {
    my ($x, $y, $swap) = @_;

    $y ? True->new() : False->new();
  }
}

BEGIN
{
  package False;

  use overload
    'eq' => \&equals,
    '==' => \&equals,
    bool => sub { !!0 },
    '!'  => sub { True->new() },
  ;

  use base 'Exporter';
  our @EXPORT = qw( FALSE );

  sub new
  {
    my ($class) = @_;
    my $self = 0;
    return bless \$self, $class;
  }

  sub FALSE
  {
    return False->new();
  }

  sub equals
  {
    my ($x, $y, $swap) = @_;

    return $y ? False->new() : True->new();
  }
}

BEGIN
{
  import True;
  import False;
}

my $n;
BEGIN { $n = 0 }
use Test;
plan tests => $n;

BEGIN { $n += 2 }
ok TRUE;
ok not FALSE;

BEGIN { $n += 4 }
ok not TRUE == FALSE;
ok not TRUE eq FALSE;
ok not FALSE == TRUE;
ok not FALSE eq TRUE;

BEGIN { $n += 4 }
{
  my $a = "true";

  ok $a;
  ok $a == TRUE;
  ok $a eq TRUE;
  ok $a, TRUE;
}

BEGIN { $n += 4 }
{
  my $b = 0;

  ok !$b;
  ok $b eq FALSE;
  ok $b == FALSE;
  ok $b, FALSE;
}

BEGIN { $n += 1 }
{
  my $c;

  if ($c) { ok $c == TRUE }
  else    { ok $c == FALSE }
}

BEGIN { $n += 8 }
{
  ok TRUE == TRUE;
  ok FALSE == FALSE;
  ok FALSE == FALSE; TRUE;
  ok TRUE == FALSE, FALSE;
  ok FALSE == TRUE, FALSE;
  ok((TRUE == TRUE) == TRUE);
  ok((FALSE == FALSE) == TRUE);
  ok((TRUE == FALSE) == FALSE);
}

BEGIN { $n += 4 }
{
  ok 8.2 == TRUE;
  ok 8.2, TRUE;
  ok 8.2, not FALSE;
  ok 8.2, not not TRUE;
}

__END__

LP^>