2.1 The Cure for the Common Code

Let's say a famous sailor (we'll call him "the Skipper") uses Perl to help navigate his ocean-going vessel (call it "the Minnow"). The Skipper writes many Perl programs to provide navigation for all the common ports of call for the Minnow. He finds himself cutting and pasting a very common routine into each program:

sub turn_towards_heading {
  my $new_heading = shift;
  my $current_heading = current_heading(  );
  print "Current heading is ", $current_heading, ".\n";
  print "Come about to $new_heading ";
  my $direction = "right";
  my $turn = ($new_heading - $current_heading) % 360;
  if ($turn > 180) { # long way around
    $turn = 360 - $turn;
    $direction = "left";
  }
  print "by turning $direction $turn degrees.\n";
}

This routine gives the shortest turn to make from the current heading (returned by the subroutine current_heading( )) to a new heading (given as the first parameter to the subroutine).

The first line of this subroutine might have read instead:

my ($new_heading) = @_;

This is mostly a style call: in both cases, the first parameter ends up in $new_heading. However, in later chapters, you'll see that removing the items from @_ as they are identified does have some advantages. So, this book sticks (mostly) with the "shifting" style of argument parsing. Now back to the matter at hand...

Suppose that after having written a dozen programs using this routine, the Skipper realizes that the output is excessively chatty when he's already taken the time to steer the proper course (or perhaps simply started drifting in the proper direction). After all, if the current heading is 234 degrees and he needs to turn to 234 degrees, you see:

Current heading is 234.
Come about to 234 by turning right 0 degrees.

How annoying! The Skipper decides to fix this problem by checking for a zero turn value:

sub turn_towards_heading {
  my $new_heading = shift;
  my $current_heading = current_heading(  );
  print "Current heading is ", $current_heading, ".\n";
  my $direction = "right";
  my $turn = ($new_heading - $current_heading) % 360;
  unless ($turn) {
    print "On course (good job!).\n";
    return;
  }
  print "Come about to $new_heading ";
  if ($turn > 180) { # long way around
    $turn = 360 - $turn;
    $direction = "left";
  }
  print "by turning $direction $turn degrees.\n";
}

Great. The new subroutine works nicely in the current navigation program. However, because it had previously been cut-and-pasted into a half dozen other navigation programs, those other programs will still annoy the Skipper with extraneous turning messages.

You need a way to write the code in one place and then share it among many programs. And like most things in Perl, there's more than one way to do it.