NAME

Hash::DefaultValue - create a hash where the default value ain't undef

SYNOPSIS

  use 5.010;
  use Hash::DefaultValue;
  
  tie my %hash, 'Hash::DefaultValue', 42;
  say $hash{the_answer};  # says 42

DESCRIPTION

Normally, if you try fetching a value from a hash where the key does not exist, you get undef.

  my %hash;
  if (defined $hash{foobar}) {
    say "this will not happen!";
  }

Hash::DefaultValue allows you to choose another value instead of undef. It tried to avoid changing any other part of the hash's behaviour. For example, it doesn't automatically create any hash keys that Perl wouldn't normally autovivify.

  tie my %hash, 'Hash::DefaultValue', 42;
  say $hash{the_answer};                    # says 42
  my $exists = exists $hash{the_answer};    # false
  say keys %hash;                           # says nothing

And of course you can still store explicit values in the hash, as you'd expect:

  tie my %hash, 'Hash::DefaultValue', 42;
  $hash{monkey} = 'Bobo';
  say $hash{the_answer};     # says 42
  say $hash{monkey};         # says "Bobo"

Delete operations on the hash are vaguely interesting:

  tie my %hash, 'Hash::DefaultValue', 42;
  $hash{monkey} = 'Bobo';
  my $the_answer = delete $hash{the_answer};  # undef
  my $monkey     = delete $hash{monkey};      # "Bobo"

Allowed Default Values

Any non-reference scalar can be used as a default value.

Coderefs can be used too, in which case when a default value is being fetched the coderef will be evaluated (in scalar context) and the return value used as the default. The coderef will have a reference to the tied hash, and the key being fetched passed as arguments. Additionally, the key will be available in $_ which often makes for nicer looking code.

  tie my %hash, 'Hash::DefaultValue', sub { $_ + 10 };
  say $hash{32};     # says 42
  say $hash{monkey}; # says 10

Other references are disallowed, which provides a handy extensibility point in the future. If you want to use some other reference, then wrap it in a coderef.

  tie my %hash, 'Hash::DefaultValue', sub { \@foo };

Alias

The aliased module allows you to define aliases for class names, and works great for tie implementations.

  use aliased 'Hash::DefaultValue' => 'HDV';
  tie my %hash, HDV, 42;

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=Hash-DefaultValue.

SEE ALSO

Hash::Missing is a subclass of this module.

Hash::WithDefaults allows you to default particular keys by providing a template hashref.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

cpanm Hash::DefaultValue

perl -MCPAN -e shell
install Hash::DefaultValue