#!/usr/bin/perl

require 'navigation.pl';

sub turn_toward_port {
  turn_toward_heading(compute_heading_to_island( ));
}

sub compute_heading_to_island {
  .. code here ..
}

.. more program here ..

Gilligan then has his program debugged (perhaps with the aid of a smart person whom we'll call "the Professor"), and everything works well.

However, now the Skipper decides to modify his navigation.pl library, adding a routine called turn_toward_port that makes a 45-degree turn toward the left (known as "port" in nautical jargon).

Gilligan's program will fail in a catastrophic way, as soon as he tries to head to port: he'll start steering the ship in circles! The problem is that the Perl compiler first compiles turn_toward_port from Gilligan's main program, then when the require is evaluated at runtime, the definition for turn_toward_port is redefined as the Skipper's definition. Sure, if Gilligan has warnings enabled, he'll notice something is wrong, but why should he have to count on that?

The problem is that Gilligan defined turn_toward_port as meaning "turn toward the port on the island," while the Skipper defined it as "turn toward the left." How do you resolve this?

#!/usr/bin/perl

require 'navigation.pl';

sub turn_toward_port {
  navigation_turn_toward_heading(compute_heading_to_island( ));
}

sub compute_heading_to_island {
  .. code here ..
}

.. more program here ..

Clearly, the navigation_turn_toward_heading comes from the navigation.pl file. This is great for Gilligan, but awkward for the Skipper, as his file now becomes:

sub navigation_turn_toward_heading {
  .. code here ..
}

sub navigation_turn_toward_port {
  .. code here ..
}

1;