I have use for a boolean class which is lazy evaluated. That is, it will take no value until needed, after which its value is memoized.

I want this for testing complicated nests of logical operators, but a similar construction would be useful for expensive functions.

I've implemented this in terms of a closure in the constructor and overload 'bool'. I'm not very satisfied with the implementation. The $foo->{peek}{} notation for the closures is really ugly, but I like the way overloading works here. I'd appreciate any suggestions for a cleaner interface. Is there an idiom for this? Would tie help?

Here's the module and a demo script:

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

package LazyBool;

use constant THE_BIT => 1 << 9;
use constant RAND_SIZE => 1 << 16;

use overload 'bool' => sub {
    my $self = shift;
    $self->{value}();
};

sub new {
    my $class = shift;
    my ($value,$self) = (undef,{});

    $self->{value} = sub {
        defined $value ?
        $value :
        $value = THE_BIT & rand(RAND_SIZE) ? 1 : 0;
    };

    $self->{peek} = sub {
        $value;
    };

    bless $self, $class;
}

1;

[download]
lazybool.pl:
#!/usr/bin/perl -w
use strict;

use LazyBool;

my $foo = LazyBool->new;

print '$foo is ',
    defined $foo->{peek}() ? $foo->{peek}() : "not defined",
    '.',
    $/;

print $foo,$/;

print '$foo is ',
    defined $foo->{peek}()? $foo->{peek}() : "not defined",
    '.',
    $/;


=pod

$ perl lazybool.pl
$foo is not defined.
0
$foo is 0.
$ perl lazybool.pl
$foo is not defined.
0
$foo is 0.
$ perl lazybool.pl
$foo is not defined.
1
$foo is 1.

=cut

[download]

After Compline,
Zaxo

In reply to A Lazy Class by Zaxo

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