Importer(3pm)
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) = @_;
}
$info{from}
Package the symbol comes from.
$info{into}
Package to which the symbol should be added.
$info{sig}
The sigil that should be used.
$info{spec}
Extra details.
$info{symbol}
The original symbol name (with sigil) from the original
package.
@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
TAGS
You can define/import subsets of symbols using predefined tags.
use Importer 'Some::Thing' => ':tag';
Importer will automatically populate the ":DEFAULT" tag for you.
Importer will also give you an ":ALL" tag with ALL exports so long as
the exporter does not define a ":ALL" tag already.
/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/;
EXCLUDING 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_' },
);
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.
CUSTOM EXPORT ASSIGNMENT
This lets you provide an alternative to the "*name = $ref" export
assignment. See the list of parameters to "import()"
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 $BAT/;
%EXPORT_TAGS
This module supports tags exactly the way Exporter does.
use Importer 'Some::Thing' => ':DEFAULT';
use Importer 'Other::Thing' => ':some_tag';
Tags can be specified this way:
our %EXPORT_TAGS = (
oos => [qw/foo boo zoo/],
ees => [qw/fee bee zee/],
);
@EXPORT_FAIL
This is used exactly the way Exporter uses it.
Use this to list subs that are not available on all platforms. If
someone tries to import one of these, Importer will hit your
"$from->export_fail(@items)" callback to try to resolve the issue. See
Exporter 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 receive 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 $from_package = shift;
my ($into_package, $symbol_name) = @_;
...
return sub { ... };
},
...
);
%EXPORT_MAGIC
This is new to this module. Exporter does not support it.
This allows you to define custom actions to run AFTER an export has
been injected into the consumers namespace. This is a good place to
enable parser hooks like with Devel::Declare. These will NOT be run if
a consumer uses a custom assignment callback.
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");
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()
$imp->do_unimport(@symbols)
This will remove imported symbols from the objects "from" package.
If you specify a list of @symbols then only the specified symbols
will be removed, otherwise all symbols imported using Importer will
be removed.
Note: Please be aware of the difference between "do_import()" and
"do_unimport()". For import 'from' us used as the origin, in
unimport it is used as the target. This means you cannot re-use an
instance to import and then unimport.
($into, $versions, $exclude, $symbols, $set) =
$imp->parse_args('Dest::Package')
($into, $versions, $exclude, $symbols, $set) =
$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 => [...], ... },
# Magic to apply
magic => { foo => sub { ... }, ... },
# 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.
my $exports = $imp->get(@imports)
This returns hashref of "{ $name => $ref }" for all the specified
imports.
my @export_refs = $imp->get_list(@imports)
This returns a list of references for each import specified. Only
the export references are returned, the names are not.
$export_ref = $imp->get_one($import)
This returns a single reference to a single export. If you provide
multiple imports then only the LAST one will be used.
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/>
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