path: root/README

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.

*** EXPERIMENTAL ***
    This module is still experimental. Anything can change at any time.

SYNOPSYS
        # 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;

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 backwords 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.

COMPATABILITY
    This module aims for 100% compatabilty 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, @SYMBOLS;

    $IMPORTER_VERSION (optional)
        If you provide a numeric argument as the first argument it will be
        treated as a version number. Importer will d 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.

    @SYMBOLS (optional)
        Symbols you wish to import. If no symbols are specified then the
        defaults will be used.

SUPPORTED FEATURES
  TAGS
    You can define/import subsets of symbols using predefined tags.

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

  /PATTERN/ or qr/PATTERN/
    You can import all symbols that match a pattern. The pattern can be
    supplied a string starting and ending with '/', or you can provide a
    "qr/../" reference.

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

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

  EXLUDING SYMBOLS
    You can exclude symbols by prefixing them with '!'.

        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

  RENAMING SYMBOLS AT IMPORT
    *This is a new feature,* Exporter *does not support this on its own.*

    You can rename symbols at import time using a specification hash
    following the import name:

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

    You can also add a prefix and/or postfix:

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

    Using this syntax to set prefix and/or postfix also works on tags and
    patterns that are specified for import, in which case the prefix/postfix
    is applied to all symbols from the tag/patterm.

  UNIMPORTING
    See "UNIMPORT PARAMETERS".

  ANONYMOUS EXPORTS
    See "%EXPORT_ANON".

  GENERATED EXPORTS
    See "%EXPORT_GEN".

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
  @EXPORT
    This is used exactly the way Exporter uses it.

    List of symbols to export. Sigil is optional for subs. Symbols listed
    here are exported by default. If possible you should put symbols in
    @EXPORT_OK instead.

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

  @EXPORT_OK
    This is used exactly the way Exporter uses it.

    List of symbols that can be imported. Sigil is optional for subs.
    Symbols listed here are not exported by default. This is preferred over
    @EXPORT.

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

  %EXPORT_TAGS
    This module supports tags exactly the way Exporter does.

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

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

  @EXPORT_FAIL
    This is used exactly the way Exporter uses it.

    Use this to list subs that are not available on all platforms. If
    someone tries to import one of these, Importer will hit your
    "$from->export_fail(@items)" callback to try to resolve the issue. See
    Exporter.pm for documentation of this feature.

        our @EXPORT_FAIL = qw/maybe_bad/;

  %EXPORT_ANON
    This is new to this module, Exporter does not support it.

    This allows you to export symbols that are not actually in your package
    symbol table. The keys should be the symbol names, the values are the
    references for the symbols.

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

  %EXPORT_GEN
    This is new to this module, Exporter does not support it.

    This allows you to export symbols that are generated on export. The key
    should be the name of a symbol. The value should be a coderef that
    produces a reference that will be exported.

    When the generators are called they will recieve 2 arguments, the
    package the symbol is being exported into, and the symbol being imported
    (name may or may not include sigil for subs).

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

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.

GIVING YOUR PACKAGE AN OLD-SCHOOL IMPORT METHOD
        package My::Exporter;
        use Importer Importer => ('exporter_import' => {-as => 'import'});

        ...

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:

        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

                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);

  OBJECT CONSTRUCTION
    $imp = Importer->new(from => 'Some::Exporter')
    $imp = Importer->new(from => 'Some::Exporter', caller => [$package,
    $file, $line])
        This is how you create a new Importer instance. "from =>
        'Some::Exporter'" is the only required argument. You may also
        specify the "caller => [...]" arrayref, which will be used only for
        error reporting. If you do not specify a caller then Importer will
        attempt to find the caller dynamically every time it needs it (this
        is slow and expensive, but necessary if you intend to re-use the
        object.)

  OBJECT METHODS
    $imp->do_import($into)
    $imp->do_import($into, @symbols)
        This will import from the objects "from" package into the $into
        package. You can provide a list of @symbols, or you can leave it
        empty for the defaults.

    $imp->do_unimport()
    $imp->do_unimport(@symbols)
        This will remove imported symbols from the objects "from" package.
        If you specify a list of @symbols then only the specified symbols
        will be removed, otherwise all symbols imported using Importer will
        be removed.

        Note: Please be aware fo the difference between "do_import()" and
        "do_unimport()". For import 'from' us used as the origin, in
        unimport it is used as the target. This means you cannot re-use an
        instance to import and then unimport.

    ($into, $versions, $exclude, $symbols) =
    $imp->parse_args('Dest::Package')
    ($into, $versions, $exclude, $symbols) =
    $imp->parse_args('Dest::Package', @symbols)
        This parses arguments. The first argument must be the destination
        package. Other arguments can be a mix of symbol names, tags,
        patterns, version numbers, and exclusions.

    $caller_ref = $imp->get_caller()
        This will find the caller. This is mainly used for error reporting.
        IF the object was constructed with a caller then that is what is
        returned, otherwise this will scan the stack looking for the first
        call that does not originate from a package that ISA Importer.

    $imp->carp($warning)
        Warn at the callers level.

    $imp->croak($exception)
        Die at the callers level.

    $from_package = $imp->from()
        Get the "from" package that was specified at construction.

    $file = $imp->from_file()
        Get the filename for the "from" package.

    $imp->load_from()
        This will load the "from" package if it has not been loaded already.
        This uses some magic to ensure errors in the load process are
        reported to the "caller".

    $menu_hr = $imp->menu($into)
        Get the export menu built from, or provided by the "from" package.
        This is cached after the first time it is called. Use
        "$imp->reload_menu()" to refresh it.

        The menu structure looks like this:

            $menu = {
                # every valid export has a key in the lookup hashref, value is always
                # 1, key always includes the sigil
                lookup => {'&symbol_a' => 1, '$symbol_b' => 1, ...},

                # most exports are listed here, symbol name with sigil is key, value is
                # a reference to the symbol. If a symbol is missing it may be generated.
                exports => {'&symbol_a' => \&symbol_a, '$symbol_b' => \$symbol_b, ...},

                # Hashref of tags, tag name (without ':' prefix) is key, value is an
                # arrayref of symbol names, subs may have a sigil, but are not required
                # to.
                tags => { DEFAULT => [...], foo => [...], ... },

                # This is a hashref just like 'lookup'. Keys are symbols which may not
                # always be available. If there are no symbols in this category then
                # the value of the 'fail' key will be undef instead of a hashref.
                fail => { '&iffy_symbol' => 1, '\&only_on_linux' => 1 },
                # OR fail => undef,

                # If present, this subroutine knows how to generate references for the
                # symbols listed in 'lookup', but missing from 'exports'. References
                # this returns are NEVER cached.
                generate => sub { my $sym_name = shift; ...; return $symbol_ref },
            };

    $imp->reload_menu($into)
        This will reload the export menu from the "from" package.

SOURCE
    The source code repository for symbol 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/