Instead of giving you the theory, which is probably covered much better elwsewhere, let me give you a practical example of something I did rcently which put a smile on my face.

Once of the first modules I wrote did some simple manipulations with dates (this was in early 1999, before I knew what CPAN was all about). Since it does what I want and I know how it works, I pulled it off of a dusty shelf and cleaned up the code so I could re-use it for a project. The work I did included changing one subroutine call from

Adjust ( $Date, $DeltaYears, $DeltaMonths, $DeltaDays );
[download]

to use a hash for passing the parameters. Originally, if I wanted to call using only some of the parameters, I'd have to say

Adjust ( $Date, 0, 0, 15 );  # or
Adjust ( $Date, 0, -1, 0 );  # or
Adjust ( $Date, -3, 0, -1 );
[download]

Now I pass in an anonymous hash reference so that those examples become:

Adjust ( { date => $Date, day => 15 } );  # or
Adjust ( { date => $Date, month => -1 } );  # or
Adjust ( { date => $Date, year => -3, day => -1 } );
[download]

The great thing about this technique is that I only pass in the values I need to. If the values aren't there, the subroutine will assume some default value. How?

sub Adjust
{
  my ( $Args ) = @_;

  my $Year  = $Args->{ year }  || 0;
  my $Month = $Args->{ month } || 0;
  my $Day   = $Args->{ day }   || 0;

  #  More code ..
}
[download]

The double bar zero at the end of the three lines allows an undefined value to be interpreted as a zero -- this allows you to specify whatever default value you want, 5, 42, "U.S." or whatever.

Obviously, this becomes much cooler the more parameters you pass in -- and another great thing is that you can expand the subroutine's functionality without worrying about how it affects old code. In an old script I wrote, I had a subroutine that accepted a dozen or so parameters; adding a new one meant searching through the other scripts that called that script and updating all of them.

This technique is much easier to maintain; in my example, I would have had to set up a default value for the new parameter -- and I'd be done!

--t. alex

"Excellent. Release the hounds." -- Monty Burns.

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