Santa's little helpers had just completed an upgrade to his fleet of sleighs. Ruldoph was getting tired of lighting the way each night, and wanted a year off, so they'd finally fitted some headlights. They'd also updated their code to turn them on and off again automatically (so the batteries didn't wear out) whenever Santa took off:

sub fly_to_next_house {
  my $self = shift;

  $self->sleigh->lights(1);

  $self->gps->set_destination( shift(@{ $self->nice_list })->address );
  $self->sleigh->fly_to_destination( $self->gps );

  $self->sleigh->lights(0);
}

"I like it," said Santa, "but what happens though if the GPS doesn't know where the address is?"

"Hmmm", said the wise old elf, "well, it is running Apple Maps so I guess there might be a problem". "But", he continued, "not to worry. The set_destination method throws an exception and there's some terribly complicated code that catches it and deals with it in the routine that calls fly_to_next_house."

"Ha! The elves just look it up on Google Maps you mean.", Santa laughed, "Though that's not what I'm on about. Look: If the GPS throws an exception, can't you see the lights never get turned off because the code to do so won't be executed?"

"Oh crumbs", the elf conceded, "well, I guess we could use a localized variable to set the lights. Those are automatically unset at the end of the current scope no matter what - even if you do exit by an exception!"

sub fly_to_next_house {
  my $self = shift;

  local $Sliegh::Lights = 1;

  $self->gps->set_destination( shift(@{ $self->nice_list })->address );
  $self->sleigh->fly_to_destination( $self->gps );
}

Santa stroked his beard for a few minutes. Then he shook his head. "No, that's not going to work. For a starters you've mistyped 'Sleigh,' and since there's no error with misspelled fully qualified variables, the sleigh will end up flying me in the dark! Even if you fix that, this isn't going to work with one single variable controlling all the lights on every one of my sleighs."

"Oh, good point, "how about this then?"

sub fly_to_next_house {
  my $self = shift;

  $self->sleigh->turn_lights_on_till_end_of_scope;

  $self->gps->set_destination( shift(@{ $self->nice_list })->address );
  $self->sleigh->fly_to_destination( $self->gps );
}

"That's great! How did you do that?"

"Well, that's why they call me the Wise Old Elf"

package Sleigh;
...

use Scope::Upper qw(reap UP);

sub turn_lights_on_till_end_of_scope {
  my $self = shift;

  # turn on the lights
  $self->lights(1);

  # and turn them off again we exit our caller's scope
  reap { $self->lights(0) } UP;
}

See Also

• Scope::Upper