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(â); # Use active value... while (1) { do_some_long_process(â); print "Process took $process_timer seconds\\n"; }
Because the timer(â)
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 ...
Get Perl Hacks now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.