NAME

Tie::Discovery - Perl extension for `Discovery' hashes

SYNOPSIS

  use Tie::Discovery;
  my %config = ();
  my $obj = tie %config, "Tie::Discovery";

  sub discover_os { ... }
  $obj->register("os",\&discover_os);

  $config{os};

DESCRIPTION

A `discovery' hash is a hash that's designed to help you solve the data dependency problem. It's based on the principle of least work; some times, you may spend a lot of time in your program finding out paths, filenames, operating system specifics, network information and so on that you may not end up using. Discovery hashes allow you to get the data when you need it, and only when you need it.

To use a discovery hash, first tie a hash as shown above. You will want to keep hold of the object returned by tie. You can then add things to discover by calling the register method as shown above. The above code $obj->register("os",\&discover_os); means that when (and only when!) the value $config{os} is fetched, the sub &discover_os will be called to find it. The return value of that sub will then be cached to save a look-up next time.

The real power comes from the fact that you may refer to the tied hash inside of the discovery subroutines. This allows for fast, neat and flexible top-down programming, and helps you avoid hard-coding values. For instance, let us find the OS by calling the uname program.

        $obj->register( os => sub { `$config{path_to_uname}` } );

Now we need code to find the program itself:

        use File::Spec::Functions;
        $obj->register( path_to_uname => sub {
                foreach (path) {
                        return catfile($_,"uname") if -x catfile($_,"uname")
                }
                die "Couldn't even find uname"
        };

Fetching $config{os} may now need a further call to fetch $config{path_to_uname} unless the path is already cached. And, of course, we needn't stop at two levels.

Fuller examples of this technique are seen in the forthcoming CTAN and SysConf modules.

METHODS

Aside from the usual hash methods, the following are available:

register(name,sub)

Registers name as an entry in the hash, to be discovered by running sub

store(name,value)

Stores value directly into the hash under the name key. The only time you should need to do this is to set the value of the debug key; if set, this shows a trace of the discovery process.

EXPORT

Nothing.

LIMITATIONS

At present, since a subroutine reference signifies something to look up, you can't usefully return one from your discovery subroutine.

AUTHOR

Simon Cozens <simon@cpan.org>

SEE ALSO

perl(1), Tie::Hash