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only-0.10
River stage zero No dependents
3 ++
/ only

NAME

only - Load specific module versions; Install many

SYNOPSIS

    # Install version 0.30 of MyModule
    cd MyModule-0.30
    perl Makefile.PL
    make test
    perl -Monly=install    # substitute for 'make install' 
    
    # Only use MyModule version 0.30
    use only MyModule => '0.30';

    # Only use MyModule if version is between 0.30 and 0.50
    # but not 0.36; or if version is >= to 0.55.
    use only MyModule => '0.30-0.50 !0.36 0.55-', qw(:all);

    # Don't export anything!
    use only MyModule => '0.30', [];

    # Version dependent arguments
    use only MyModule =>
        [ '0.20-0.27', qw(f1 f2 f3 f4) ],
        [ '0.30-',     qw(:all) ];

USAGE

    # Note: <angle brackets> mean "optional".

    # To load a specific module
    use only MODULE => 'CONDITION SPEC' <, ARGUMENTS>;

    # For multiple argument sets
    use only MODULE => 
        ['CONDITION SPEC 1' <, ARGUMENTS1>],
        ['CONDITION SPEC 2' <, ARGUMENTS2>],
        ...
        ;

    # To install an alternate version of a module
    perl -Monly=install <- VERSION>        # instead of 'make install'

DESCRIPTION

The only.pm facility allows you to load a MODULE only if it satisfies a given CONDITION. Normally that condition is a version. If you just specify a single version, 'only' will only load the module matching that version. If you specify multiple versions, the module can be any of those versions. See below for all the different conditions you can use with only.

only.pm will also allow you to load a particular version of a module, when many versions of the same module are installed. See below for instructions on how to easily install many different versions of the same module.

CONDITION SPECS

A condition specification is a single string containing a list of zero or more conditions. The list of conditions is separated by spaces. Each condition can take one of the following forms:

  • plain version

    This is the most basic form. The loaded module must match this version string or be loaded from a version directory that uses the version string. Mulitiple versions means one or the other.

        use only MyModule => '0.11';
    use only MyModule => '0.11 0.15';

  • version range

    This is two single versions separated by a dash. The end points are inclusive in the range. If either end of the range is ommitted, then the range is open ended on that side.

        use only MyModule => '0.11-0.12';
    use only MyModule => '0.13-';
    use only MyModule => '-0.10';
    use only MyModule => '-';       # Means any version

    Note that a completely open range (any version) is not the same as just saying:

        use MyModule;

    because the only module will search all the various version libs before searhing in the regular @INC paths.

    Also note that an empty string or no string means the same thing as '-'.

        # All of these mean "use any version"
    use only MyModule => '-';
    use only MyModule => '';
    use only 'MyModule';

  • complement version or range

    Any version or range beginning with a '!' is considered to mean the inverse of that specification. A complement takes precedence over all other specifications. If a module version matches a complement, that version is immediately rejected without further inspection.

        use only MyModule => '!0.31';
    use only MyModule => '0.30-0.40 !0.31-0.33';

The search works by searching the version-lib directories (found in only::config) for a module that meets the condition specification. If more than one version is found, the highest version is used. If no module meets the specification, then a normal @INC style require is performed.

If the condition is a subroutine reference, that subroutine will be called and passed an only object. If the subroutine returns a false value, the program will die. See below for a list of public methods that may be used upon the only object.

ARGUMENTS

All of the arguments following the CONDITION specification, will be passed to the module being loaded.

Normally you can pass an empty list to use to turn off Exporting. To do this with only, use an empty array ref.

    use only MyModule => '0.30';       # Default exporting
    use only MyModule => '0.30', [];   # No exporting
    use only MyModule => '0.30', qw(export list);  # Specific export

If you need pass different arguments depending on which version is used, simply wrap each condition spec and arguments with an array ref.

    use only MyModule =>
        [ '0.20-0.27', qw(f1 f2 f3 f4) ],
        [ '0.30-',     qw(:all) ];

INSTALLING MULTIPLE MODULE VERSIONS

The only.pm module also has a facility for installing more than one version of a particular module. Using this facility you can install an older version of a module and use it with the 'use only' syntax.

It works like this; when installing a module, do the familiar:

    perl Makefile.PL
    make
    make test

But instead of make install, do this:

    perl -Monly=install

This will attempt to determine what version the module should be installed under. In some cases you may need to specify the version yourself. Do the following:

    perl -Monly=install - 0.55

NOTE: Also works with Module::Build style modules.

NOTE: The perl you use for this must be the same perl as the one used to do perl Makefile.PL or perl Build.PL. While this seems obvious, you may run into problems with sudo perl -Monly=install, since the root account may have a different perl in its path. If this happens, just use the full path to your perl.

INSTALLATION LOCATION

When you install the only module, you can tell it where to install alternate versions of modules. These paths get stored into only::config. The default location to install things is parallel to your sitelib. For instance if your sitelib was:

    /usr/lib/perl5/site_perl

only would default to:

    /usr/lib/perl5/version

This keeps your normal install trees free from any potential complication with version modules.

If you install version 0.24 and 0.26 of MyModule and version 0.26 of Your::Module, they will end up here:

    /usr/lib/perl5/version/0.24/My/Module.pm
    /usr/lib/perl5/version/0.26/My/Module.pm
    /usr/lib/perl5/version/0.26/Your/Module.pm

HOW IT WORKS

only.pm is kind of like lib.pm on Koolaid! Instead of adding a search path to @INC, it adds a search object to @INC. This object is actually the only.pm object itself. The object keeps track of all of the modules related to a given package installation, and takes responsibility for loading those modules. This is very important because if you say:

    use only Goodness => '0.23';

and then later:

    require Goodness::Gracious;

you want to be sure that the correct version of the second module gets loaded. Especially when another module is doing the loading.

THE FINE PRINT ON VERSIONING

The only.pm module loads a module by the following process:

 1) Look for the highest suitable version of the module in the version
    libraries specified in only::config.
 
 else:
 
 2) Do a normal require() of the module, and check to make sure the 
    version is in the range specified.

It is important to understand that the versions used in these two different steps come from different places and might not be the same.

In the first step the version used is the version of the distribution that the module was installed from. This is grepped out of the Makefile and saved as metadata for that module.

In the second step, the version is taken from $VERSION of that module. This is the same process used when you do something like:

     use MyModule '0.50';

Unfortunately, there is no way to know what the distribution version is for a normally installed module.

Fortunately, $VERSION is usually the same as the distribution version. That's because the popular VERSION_FROM Makefile.PL option makes it happen. Authors are encouraged to use this option.

The conclusion here is that only.pm usually gets things right. Always check %INC, if you suspect that the wrong versions are being pulled in. If this happens, use more 'use only' statements to pull in the right versions.

One failsafe solution is to make sure that all module versions in question are installed into the version libraries.

LOADING MULTIPLE MODULE VERSIONS (at the same time)

You can't do that! Are you crazy? Well I am. I can't do this yet but I'd really like to. I'm working on it. If you have ideas on how this might be accomplished, send me an email. If you don't have a good idea, send me some coffee.

BUGS AND CAVEATS

  • There is currently no way to install documentation for multiple modules.

  • This module only works with Perl 5.6.1 and higher. That's because earlier versions of Perl don't support putting objects in @INC.

AUTHOR

Brian Ingerson <INGY@cpan.org>

COPYRIGHT

Copyright (c) 2003. Brian Ingerson. All rights reserved.

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

See http://www.perl.com/perl/misc/Artistic.html