Perl Hacks

Hack #93. Return Active Values

Return values that automatically change as you use them.

The Contextual::Return module [Hack #92] has another very powerful trick up its sleeve. The scalar values it returns don't have to be constants; they can be "active." An active value is one that adapts itself each time it is evaluated. This is useful for performing initialization, cleanup, or error-handling code without forcing the caller to do anything special.

The Hack

For example, you can create a subroutine that returns a value that automatically tracks the elapsed time between events:

use Contextual::Return;
use Time::HiRes qw( sleep time );      # Allow subsecond timing

# Subroutine returns an active timer value...
sub timer
{
    my $start = time;                  # Set initial start time

    return VALUE                       # Return an active value that...
    {                     
        my $elapsed = time - $start;   #    1. computes elapsed time
        $start      = time;            #    2. resets start time
        return $elapsed;               #    3. returns elapsed time
    }
}

# Create an active value...
my $process_timer = timer( );

# Use active value...
while (1)
{
    do_some_long_process( );
    print "Process took $process_timer seconds\\n";
}

Because the timer( ) subroutine returns a contextual value that is computed within the VALUE block itself, that returned value becomes active. Each time the value of $process_timer is reevaluated (in the print statement), the value's VALUE block executes, recomputing and resetting the value stored in $process_timer.

Running the Hack

Of course, the real advantage here is that you ...