NAME

Importer - Alternative but compatible interface to modules that export symbols.

DESCRIPTION

This module acts as a layer between Exporter and modules which consume exports. It is feature-compatible with Exporter, plus some much needed extras. You can use this to import symbols from any exporter that follows Exporters specification. The exporter modules themselves do not need to use or inherit from the Exporter module, they just need to set @EXPORT and/or other variables.

SYNOPSIS

    # Import defaults
    use Importer 'Some::Module';
    # Import a list
    use Importer 'Another::Module' => qw/foo bar baz/;
    # Import a specific version:
    use Importer 'That::Module' => '1.00';
    # Require a sepcific version of Importer
    use Importer 0.001, 'Foo::Bar' => qw/a b c/;
    foo()
    bar()
    baz()
    # Remove all subroutines imported by Importer
    no Importer;
    # Import symbols into variables
    my $croak = Importer->get_one(Carp => qw/croak/);
    $croak->("This will croak");
    my $CARP = Importer->get(Carp => qw/croak confess cluck/);
    $CARP->{croak}->("This will croak");
    $CARP->{cluck}->("This will cluck");
    $CARP->{confess}->("This will confess");

WHY?

There was recently a discussion on p5p about adding features to Exporter. This conversation raised some significant concerns, those are listed here, in addition to others.

The burden is on export consumers to specify a version of Exporter

Adding a feature to Exporter means that any consumer module that relies on the new features must depend on a specific version of Exporter. This seems somewhat backwards since Exporter is used by the module you are importing from.

Exporter.pm is really old/crazy code

Not much more to say here. It is very old, it is very crazy, and if you break it you break EVERYTHING.

Using a modules import() for exporting makes it hard to give it other purposes

It is not unusual for a module to want to export symbols and provide import behaviors. It is also not unusual for a consumer to only want 1 or the other. Using this module you can import symbols without also getting the "import()" side effects.

In addition, moving forward, modules can specify exports and have a custom "import()" without conflating the two. A module can tell you to use Importer to get the symbols, and to use the module directly for behaviors. A module could also use Importer within its own "import()" method without the need to subclass Exporter, or bring in its "import()" method.

There are other exporter modules on cpan

This module normally assumes an exporter uses Exporter, so it looks for the variables and methods Exporter expects. However, other exporters on cpan can override this using the "IMPORTER_MENU()" hook.

COMPATIBILITY

This module aims for 100% compatibility with every feature of Exporter, plus added features such as import renaming.

If you find something that works differently, or not at all when compared to Exporter please report it as a bug, unless it is noted as an intentional feature (like import renaming).

IMPORT PARAMETERS

    use Importer $IMPORTER_VERSION, $FROM_MODULE, $FROM_MODULE_VERSION, \&SET_SYMBOL, @SYMBOLS;

$IMPORTER_VERSION (optional)

If you provide a numeric argument as the first argument it will be treated as a version number. Importer will do a version check to make sure it is at least at the requested version.

$FROM_MODULE (required)

This is the only required argument. This is the name of the module to import symbols from.

$FROM_MODULE_VERSION (optional)

Any numeric argument following the $FROM_MODULE will be treated as a version check against $FROM_MODULE.

\&SET_SYMBOL (optional)

Normally Importer will put the exports into your namespace. This is usually done via a more complex form of "*name = $ref". If you do NOT want this to happen then you can provide a custom sub to handle the assignment.

This is an example that uses this feature to put all the exports into a lexical hash instead of modifying the namespace (This is how the "get()" method is implemented).

    my %CARP;
    use Importer Carp => sub {
        my ($name, $ref) = @_;
        $CARP{$name} = $ref;
    };
    $CARP{cluck}->("This will cluck");
    $CARP{croak}->("This will croak");
    

The first two arguments to the custom sub are the name (no sigil), and the reference. The additional arguments are key/value pairs:

    sub set_symbol {
        my ($name, $ref, %info) = @_;
    }
    

@SYMBOLS (optional)

Symbols you wish to import. If no symbols are specified then the defaults will be used. You may also specify tags using the ':' prefix.

SUPPORTED FEATURES

    use Importer 'Some::Thing' => ':tag';

    use Importer 'Some::Thing' => '/oo/';
    use Importer 'Some::Thing' => qr/oo/;

    use Importer 'Some::Thing'
        '!foo',         # Exclude one specific symbol
        '!/pattern/',   # Exclude all matching symbols
        '!' => qr/oo/,  # Exclude all that match the following arg
        '!:tag';        # Exclude all in tag

    use Importer 'Some::Thing' => (
        foo => { -as => 'my_foo' },
    );

    use Importer 'Some::Thing' => (
        foo => { -prefix => 'my_' },
    );

UNIMPORT PARAMETERS

    no Importer;    # Remove all subs brought in with Importer
    no Importer qw/foo bar/;    # Remove only the specified subs

Only subs can be unimported.

You can only unimport subs imported using Importer.

SUPPORTED VARIABLES

    our @EXPORT = qw/foo bar &baz $BAT/;

    our @EXPORT_OK = qw/foo bar &baz $BAT/;

    use Importer 'Some::Thing'  => ':DEFAULT';
    use Importer 'Other::Thing' => ':some_tag';

    our %EXPORT_TAGS = (
        oos => [qw/foo boo zoo/],
        ees => [qw/fee bee zee/],
    );

    our @EXPORT_FAIL = qw/maybe_bad/;

    our %EXPORT_ANON = (
        '&foo' => sub { 'foo' }
        '$foo' => \$foo,
        ...
    );

    our %EXPORT_GEN = (
        '&foo' => sub {
            my $from_package = shift;
            my ($into_package, $symbol_name) = @_;
            ...
            return sub { ... };
        },
        ...
    );

    our %EXPORT_MAGIC = (
        foo => sub {
            my $from = shift;    # Should be the package doing the exporting
            my %args = @_;
            my $into      = $args{into};         # Package symbol was exported into
            my $orig_name = $args{orig_name};    # Original name of the export (in the exporter)
            my $new_name  = $args{new_name};     # Name the symbol was imported as
            my $ref       = $args{ref};          # The reference to the symbol
            ...; # whatever you want, return is ignored.
        },
    );

CLASS METHODS

Importer->import($from)

Importer->import($from, $version)

Importer->import($from, @imports)

Importer->import($from, $from_version, @imports)

Importer->import($importer_version, $from, ...)

This is the magic behind "use Importer ...".

Importer->import_into($from, $into, @imports)

Importer->import_into($from, $level, @imports)

You can use this to import symbols from $from into $into. $into may either be a package name, or a caller level to get the name from.

Importer->unimport()

Importer->unimport(@sub_name)

This is the magic behind "no Importer ...".

Importer->unimport_from($from, @sub_names)

Importer->unimport_from($level, @sub_names)

This lets you remove imported symbols from $from. $from my be a package name, or a caller level.

my $exports = Importer->get($from, @imports)

This returns hashref of "{ $name => $ref }" for all the specified imports.

$from should be the package from which to get the exports.

my @export_refs = Importer->get_list($from, @imports)

This returns a list of references for each import specified. Only the export references are returned, the names are not.

$from should be the package from which to get the exports.

$export_ref = Importer->get_one($from, $import)

This returns a single reference to a single export. If you provide multiple imports then only the LAST one will be used.

$from should be the package from which to get the exports.

USING WITH OTHER EXPORTER IMPLEMENTATIONS

If you want your module to work with Importer, but you use something other than Exporter to define your exports, you can make it work be defining the "IMPORTER_MENU" method in your package. As well other exporters can be updated to support Importer by putting this sub in your package. IMPORTER_MENU() must be defined in your package, not a base class!

    sub IMPORTER_MENU {
        my $class = shift;
        my ($into, $caller) = @_;
        return (
            export       => \@EXPORT,          # Default exports
            export_ok    => \@EXPORT_OK,       # Other allowed exports
            export_tags  => \%EXPORT_TAGS,     # Define tags
            export_fail  => \@EXPORT_FAIL,     # For subs that may not always be available
            export_anon  => \%EXPORT_ANON,     # Anonymous symbols to export
            export_magic => \%EXPORT_MAGIC,    # Magic to apply after a symbol is exported
            generate   => \&GENERATE,          # Sub to generate dynamic exports
                                               # OR
            export_gen => \%EXPORT_GEN,        # Hash of builders, key is symbol
                                               # name, value is sub that generates
                                               # the symbol ref.
        );
    }
    sub GENERATE {
        my ($symbol) = @_;
        ...
        return $ref;
    }

All exports must be listed in either @EXPORT or @EXPORT_OK, or be keys in %EXPORT_GEN or %EXPORT_ANON to be allowed. 'export_tags', 'export_fail', 'export_anon', 'export_gen', and 'generate' are optional. You cannot combine 'generate' and 'export_gen'.

Note: If your GENERATE sub needs the $class, $into, or $caller then your "IMPORTER_MENU()" method will need to build an anonymous sub that closes over them:

    sub IMPORTER_MENU {
        my $class = shift;
        my ($into, $caller) = @_;
        return (
            ...
            generate => sub { $class->GENERATE($into, $caller, @_) },
        );
    }

OO Interface

    use Importer;
    my $imp = Importer->new(from => 'Some::Exporter');
    $imp->do_import('Destination::Package');
    $imp->do_import('Another::Destination', @symbols);

Or, maybe more useful:

    my $imp = Importer->new(from => 'Carp');
    my $croak = $imp->get_one('croak');
    $croak->("This will croak");

FUNCTIONS

These can be imported:

    use Importer 'Importer' => qw/import optimal_import/;

$bool = optimal_import($from, $into, \@caller, @imports)

This function will attempt to import @imports from the $from package into the $into package. @caller needs to have a package name, filename, and line number. If this function fails then no exporting will actually happen.

If the import is successful this will return true.

If the import is unsuccessful this will return false, and no modifications to the symbol table will occur.

$class->import(@imports)

If you write class intended to be used with Importer, but also need to provide a legacy "import()" method for direct consumers of your class, you can import this "import()" method.

    package My::Exporter;
    # This will give you 'import()' much like 'use base "Exporter";'
    use Importer 'Importer' => qw/import/;
    ...
    

SOURCE

The source code repository for Importer can be found at <http://github.com/exodist/Importer>.

MAINTAINERS

Chad Granum <exodist@cpan.org>

AUTHORS

Chad Granum <exodist@cpan.org>

COPYRIGHT

Copyright 2015 Chad Granum <exodist7@gmail.com>.

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

See <http://dev.perl.org/licenses/>