NAME

Attribute::Default - Perl extension to assign default values to subroutine arguments

SYNOPSIS

package MyPackage;
use base 'Attribute::Default';

# Makes person's name default to "jimmy"
sub introduce : Default("jimmy") {
   my ($name) = @_;
   print "My name is $name\n";
}

# Make age default to 14, sex default to male
sub vitals : Default({age => 14, sex => 'male'}) {
   my %vitals = @_;
   print "I'm $vitals{'sex'}, $vitals{'age'} years old, and am from $vitals{'location'}\n";
}

DESCRIPTION

You've probably seen it a thousand times: a subroutine begins with a complex series of defined($blah) or $blah = 'fribble' statements designed to provide reasonable default values for optional parameters. They work fine, but every once in a while one wishes that perl 5 had a simple mechanism to provide default values to subroutines.

This module attempts to fill that gap.

SIMPLE DEFAULTS

If you would like to have a subroutine that takes three parameters, but the second two should default to 'Mister Morton' and 'walked', you can declare it like this:

package WhateverPackage;
use base 'Attribute::Default';

sub what_happened : Default(undef, 'Mister Morton', 'walked down the street') {
  my ($time, $subject, $verb) = @_;

  print "At $time, $subject $verb\n";
}

and $subject and $verb will automatically be filled in when someone calls the what_happened() subroutine with only a single argument.

# prints "At 12AM, Mister Morton walked down the street"
what_happened('12AM');

# prints "At 3AM, Interplanet Janet walked down the street"
what_happened('3AM', 'Interplanet Janet');

# prints "At 6PM, a bill got passed into law"
what_happened('6PM', 'a bill', 'got passed into law');

# prints "At 7:03 PM, Mister Morton grew flowers for Perl"
what_happened("7:03 PM", undef, "grew flowers for Perl");

You can also use the default mechanism to handle the named parameter style of coding. Just pass a hash reference as the value of Default(), like so:

package YetAnotherPackage;
use base 'Attribute::Default';

sub found_pet : Default({name => 'Rufus Xavier Sarsaparilla', pet => 'kangaroo'}) {
  my %args = @_;
  my ($first_name) = split(/ /, $args{'name'}, 2);
  print "$first_name found a $args{'pet'} that followed $first_name home\n"; 
  print "And now that $args{'pet'} belongs...\n";
  print "To $args{'name'}.\n\n";
}

# Prints "Rufus found a kangaroo that followed Rufus home"...
found_pet();

# Prints "Rafaella found a kangaroo that followed Rafaella home"...
found_pet(name => 'Rafaella Gabriela Sarsaparilla');

# Or...
found_pet(name => 'Rafaella Gabriela Sarsaparilla', pet => undef);

# Prints "Albert found a rhinoceros that followed Albert home"...
found_pet(name => 'Albert Andreas Armadillo', pet => 'rhinoceros');

DEFAULTING REFERENCES

If you prefer to pass around your arguments as references, rather than full lists, Attribute::Default can accomodate you. Simply use Defaults() instead of Default(), and your reference parameters will have defaults added wherever necessary. For example:

package StillAnotherPackage;
use base 'Attribute::Default';

sub lally : Defaults({part_of_speech => 'adverbs', place => 'here'}, 3) {
  my ($in, $number) = @_;
  print join(' ', ('lally') x $number), ", get your $in->{part_of_speech} $in->{'place'}...\n";
}

# Prints "lally lally lally, get your adverbs here"
lally();

# Prints "lally, get your nouns here"
lally({part_of_speech => 'nouns'}, 1);

If an argument reference's type does not match an expected default type, then it is passed along without any attempt at defaulting.

BUGS

An alpha module; may change. Based on (The) Damian Conway's Attribute::Handlers, so shares whatever bugs may be found there.

AUTHOR

Stephen Nelson, <steven@jubal.com>

SEE ALSO

Attribute::Handlers.