Learning Perl Objects, References, and Modules

Chapter 2. Building Larger Programs

This chapter looks at how to break up a program into pieces and includes some of the concerns that arise when you put those pieces back together again, or when many people work together on the same program.

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” ...