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Tie::Discovery - Lazily evaluated "discovery" hashes


This document describes version 1.11 of Tie::Discovery, released January 28, 2009.


    use Tie::Discovery;
    my %info = ();
    my $obj = tie %info, 'Tie::Discovery';

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

    print $info{os};


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 $info{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 {
        # Here $self is the same as $obj above
        my $self = shift;
        my $uname = $self->FETCH('path_to_uname');
        return `$uname`;
    } );

Alternatively, if the tied %info is still in scope, this will also do:

    $obj->register( os => sub {
        my $uname = $info{path_to_uname};
        return `$uname`;
    } );

Now we need code to find the program itself:

    use Config;
    use File::Spec::Functions;
    $obj->register( path_to_uname => sub {
        my $self = shift;
        foreach (split($Config{path_sep}, $ENV{PATH})) {
            return catfile($_, 'uname') if -x catfile($_, 'uname');
        die "Couldn't even find uname";

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

Note that, since version 1.10, as long as the discovery function returns a code reference, it will be invoked repeatedly, until a final value is produced.


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

register($name, \&code)

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.


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


Scalar::Defer, in particular its lazy() function that provides a viable alternative to this module.



Simon Cozens <>, Audrey Tang <>


Copyright 2004-2009 by Simon Cozens <>, Audrey Tang <>.

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