Re: Default subroutine parameters

[stephen]

in reply to Default subroutine parameters

The "defined" method is the best way, but there's a way to have your cake and eat it too, for a price... just define a sub that processes your arguments, filling in as necessary. Like so:

##
## _fd()
##
##   Arguments:
##     ARGUMENTS: arrayref -- The arguments to a subroutine
##     DEFAULTS: arrayref -- The defaults for those arguments
##
##   Returns:
##     list -- The elements of ARGUMENTS. Any elements in ARGUMENTS ar
+e replaced
##             by the corresponding element in DEFAULTS.
##
sub _fd {
  my ($args, $defaults) = @_;

  my @filled = ();

  foreach (0 .. max($#$args, $#$defaults)) {
    push @filled, ( defined( $args->[$_] ) ? $args->[$_] : $defaults->
+[$_] );
  }

  return @filled;
}

sub max {
  ($_[0] > $_[1]) ? $_[0] : $_[1];
}

Then, in the rest of your code, you can just run your subroutines through _fd(), like so:

##
## test_default()
##
##  Arguments:
##    $name: string -- Somebody's name. (Optional)
##
## Prints "Hello my name is $name". Name defaults to "ben".
##
sub test_default {
  my ($name) = _fd(\@_, ["ben"]);

  print "Hello my name is $name\n";
}

Then, test_default('luke') prints "Hello my name is luke", while test_default() prints "Hello my name is ben". There's some overhead to calling subroutines and doing list processing all the time, but you can use it most of the time.

Update: After thinking about this for a while, I figured out a way to use theDamian's Attribute::Handlers and a small amount of symbol-table work to give Perl a default attribute: Attribute::Default.

use base 'Attribute::Default';

sub test_default : default('ben') {
  my ($name) = @_;

  print "Hello my name is $name\n";
}

stephen