my $status = login( -username => $user,
-password => $pass,
-host => $host,
);
This assumes we have already obtained values for $user and $pass from
the user input or the command line (or wherever), and we have a
function named login() that apparently is going to log us in to $host.
Without worrying about the specifics of the function itself, let's look
at how it can process the arguments:
sub login {
my %args = @_;
# do stuff
}
Easy right, so what benefit do we get? First, our calling code is
rather self-documenting (even if we chose horrible names for the
variables themselves, the hash keys give it all away). Second, we no
longer have to worry about the order we give the arguments, and neither
does our subroutine! We can call it like above, or like either of these:
my $status = login( -password => $pass,
-host => $host,
-username => $user,
);
my $status = login( -password => $pass,
-username => $user,
-host => $host,
);
As a bonus, if we want a subroutine where some or all of the arguments
are optional (in any order), then we can do that too:
sub draw_rect3D {
my %args = shift;
$args{-length} ||= 1;
$args{-width} ||= 1;
$args{-height} ||= 1;
$args{-units} = $args{-units} eq 'in'? 'in' : 'cm';
# get to work
}
Now, we can call this and supply any of the parameters, all of the
parameters, or none of the parameters (the default will be a 1
centimeter cube):
draw_rect3D(-length => 2, -units => 'in'); # 2 x 1 x 1 inches
draw_rect3D(-length => 5, -wdith => 2); # 5 x 2 x 1 centimeters
draw_rect3D(-length => 5,
-height => 10,
-units => 'in',
-width => 3,
); # 5 x 3 x 10 inches
This is quite unlike our regular way of just using the argument array
as an array -- in that case, the order you use must always be the same,
and optional arguments must be at the end.
You will see this style of function used often for Object constructor
methods (and other OO methods) and it is widely used in the various Tk
routines and methods (for GUI programming). However, you can use this
technique with any regular subroutine -- all we are doing is using the
argument list as a hash instead of a list or array.
