Exporter

NAME

SYNOPSIS

  package YourModule;
  require Exporter;
  @ISA = qw(Exporter);
  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request

or

  package YourModule;
  use Exporter 'import'; # gives you Exporter's import() method directly
  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request

In other files which wish to use YourModule:

  use ModuleName qw(frobnicate);      # import listed symbols
  frobnicate ($left, $right)          # calls YourModule::frobnicate

Take a look at ``Good Practices'' for some variants you will like to use in modern Perl code.  

DESCRIPTION

Perl automatically calls the "import" method when processing a "use" statement for a module. Modules and "use" are documented in perlfunc and perlmod. Understanding the concept of modules and how the "use" statement operates is important to understanding the Exporter.  

How to Export

    @EXPORT    = qw(afunc $scalar @array);   # afunc is a function
    @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc

If you are only exporting function names it is recommended to omit the ampersand, as the implementation is faster this way.  

Selecting What To Export

Do not export anything else by default without a good reason!

Exports pollute the namespace of the module user. If you must export try to use @EXPORT_OK in preference to @EXPORT and avoid short or common symbol names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the module using the ModuleName::item_name (or $blessed_ref->method) syntax. By convention you can use a leading underscore on names to informally indicate that they are 'internal' and not for public use.

(It is actually possible to get private functions by saying:

  my $subref = sub { ... };
  $subref->(@args);            # Call it as a function
  $obj->$subref(@args);        # Use it as a method

However if you use them for methods it is up to you to figure out how to make inheritance work.)

As a general rule, if the module is trying to be object oriented then export nothing. If it's just a collection of functions then @EXPORT_OK anything but use @EXPORT with caution. For function and method names use barewords in preference to names prefixed with ampersands for the export lists.

Other module design guidelines can be found in perlmod.  

How to Import

use ModuleName;

This imports all the symbols from ModuleName's @EXPORT into the namespace of the "use" statement.

use ModuleName ();

This causes perl to load your module but does not import any symbols.

use ModuleName qw(...);

This imports only the symbols listed by the caller into their namespace. All listed symbols must be in your @EXPORT or @EXPORT_OK, else an error occurs. The advanced export features of Exporter are accessed like this, but with list entries that are syntactically distinct from symbol names.

Unless you want to use its advanced features, this is probably all you need to know to use Exporter.  

Advanced features

Specialised Import Lists

    [!]name         This name only
    [!]:DEFAULT     All names in @EXPORT
    [!]:tag         All names in $EXPORT_TAGS{tag} anonymous list
    [!]/pattern/    All names in @EXPORT and @EXPORT_OK which match

A leading ! indicates that matching names should be deleted from the list of names to import. If the first specification is a deletion it is treated as though preceded by :DEFAULT. If you just want to import extra names in addition to the default set you will still need to include :DEFAULT explicitly.

e.g., Module.pm defines:

    @EXPORT      = qw(A1 A2 A3 A4 A5);
    @EXPORT_OK   = qw(B1 B2 B3 B4 B5);
    %EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);

    Note that you cannot use tags in @EXPORT or @EXPORT_OK.
    Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.

An application using Module can say something like:

    use Module qw(:DEFAULT :T2 !B3 A3);

Other examples include:

    use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
    use POSIX  qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);

Remember that most patterns (using //) will need to be anchored with a leading ^, e.g., "/^EXIT/" rather than "/EXIT/".

You can say "BEGIN { $Exporter::Verbose=1 }" to see how the specifications are being processed and what is actually being imported into modules.  

Exporting without using Exporter's import method

    MyPackage->export_to_level($where_to_export, $package, @what_to_export);

where $where_to_export is an integer telling how far up the calling stack to export your symbols, and @what_to_export is an array telling what symbols *to* export (usually this is @_). The $package argument is currently unused.

For example, suppose that you have a module, A, which already has an import function:

    package A;

    @ISA = qw(Exporter);
    @EXPORT_OK = qw ($b);

    sub import
    {
        $A::b = 1;     # not a very useful import method
    }

and you want to Export symbol $A::b back to the module that called package A. Since Exporter relies on the import method to work, via inheritance, as it stands Exporter::import() will never get called. Instead, say the following:

    package A;
    @ISA = qw(Exporter);
    @EXPORT_OK = qw ($b);

    sub import
    {
        $A::b = 1;
        A->export_to_level(1, @_);
    }

This will export the symbols one level 'above' the current package - ie: to the program or module that used package A.

Note: Be careful not to modify @_ at all before you call export_to_level - or people using your package will get very unexplained results!  

Exporting without inheriting from Exporter

  package YourModule;
  use Exporter qw( import );

which will export Exporter's own import() method into YourModule. Everything will work as before but you won't need to include Exporter in @YourModule::ISA.

Note: This feature was introduced in version 5.57 of Exporter, released with perl 5.8.3.  

Module Version Checking

The Exporter module supplies a default require_version method which checks the value of $VERSION in the exporting module.

Since the default require_version method treats the $VERSION number as a simple numeric value it will regard version 1.10 as lower than 1.9. For this reason it is strongly recommended that you use numbers with at least two decimal places, e.g., 1.09.  

Managing Unknown Symbols

The names of any symbols that cannot be exported should be listed in the @EXPORT_FAIL array.

If a module attempts to import any of these symbols the Exporter will give the module an opportunity to handle the situation before generating an error. The Exporter will call an export_fail method with a list of the failed symbols:

  @failed_symbols = $module_name->export_fail(@failed_symbols);

If the export_fail method returns an empty list then no error is recorded and all the requested symbols are exported. If the returned list is not empty then an error is generated for each symbol and the export fails. The Exporter provides a default export_fail method which simply returns the list unchanged.

Uses for the export_fail method include giving better error messages for some symbols and performing lazy architectural checks (put more symbols into @EXPORT_FAIL by default and then take them out if someone actually tries to use them and an expensive check shows that they are usable on that platform).  

Tag Handling Utility Functions

  %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);

  Exporter::export_tags('foo');     # add aa, bb and cc to @EXPORT
  Exporter::export_ok_tags('bar');  # add aa, cc and dd to @EXPORT_OK

Any names which are not tags are added to @EXPORT or @EXPORT_OK unchanged but will trigger a warning (with "-w") to avoid misspelt tags names being silently added to @EXPORT or @EXPORT_OK. Future versions may make this a fatal error.  

Generating combined tags

The simplest way to do this is:

  %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);

  # add all the other ":class" tags to the ":all" class,
  # deleting duplicates
  {
    my %seen;

    push @{$EXPORT_TAGS{all}},
      grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
  }

CGI.pm creates an ``:all'' tag which contains some (but not really all) of its categories. That could be done with one small change:

  # add some of the other ":class" tags to the ":all" class,
  # deleting duplicates
  {
    my %seen;

    push @{$EXPORT_TAGS{all}},
      grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
        foreach qw/html2 html3 netscape form cgi internal/;
  }

Note that the tag names in %EXPORT_TAGS don't have the leading ':'.  

AUTOLOADed Constants

Even if a prototype is available at compile time, the body of the subroutine is not (it hasn't been "AUTOLOAD"ed yet). perl needs to examine both the "()" prototype and the body of a subroutine at compile time to detect that it can safely replace calls to that subroutine with the constant value.

A workaround for this is to call the constants once in a "BEGIN" block:

   package My ;

   use Socket ;

   foo( SO_LINGER );     ## SO_LINGER NOT optimized away; called at runtime
   BEGIN { SO_LINGER }
   foo( SO_LINGER );     ## SO_LINGER optimized away at compile time.

This forces the "AUTOLOAD" for "SO_LINGER" to take place before SO_LINGER is encountered later in "My" package.

If you are writing a package that "AUTOLOAD"s, consider forcing an "AUTOLOAD" for any constants explicitly imported by other packages or which are usually used when your package is "use"d.  

Good Practices

Declaring @EXPORT_OK and Friends

  our @ISA = qw(Exporter);
  our @EXPORT_OK = qw(munge frobnicate);

If backward compatibility for Perls under 5.6 is important, one must write instead a "use vars" statement.

  use vars qw(@ISA @EXPORT_OK);
  @ISA = qw(Exporter);
  @EXPORT_OK = qw(munge frobnicate);

Playing Safe

The ideal (but a bit ugly) way to never have to think about that is to use "BEGIN" blocks. So the first part of the ``SYNOPSIS'' code could be rewritten as:

  package YourModule;

  use strict;
  use warnings;

  our (@ISA, @EXPORT_OK);
  BEGIN {
     require Exporter;
     @ISA = qw(Exporter);
     @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
  }

The "BEGIN" will assure that the loading of Exporter.pm and the assignments to @ISA and @EXPORT_OK happen immediately, leaving no room for something to get awry or just plain wrong.

With respect to loading "Exporter" and inheriting, there are alternatives with the use of modules like "base" and "parent".

  use base qw( Exporter );
  # or
  use parent qw( Exporter );

Any of these statements are nice replacements for "BEGIN { require Exporter; @ISA = qw(Exporter); }" with the same compile-time effect. The basic difference is that "base" code interacts with declared "fields" while "parent" is a streamlined version of the older "base" code to just establish the IS-A relationship.

For more details, see the documentation and code of base and parent.

Another thorough remedy to that runtime vs. compile-time trap is to use Exporter::Easy, which is a wrapper of Exporter that allows all boilerplate code at a single gulp in the use statement.

   use Exporter::Easy (
       OK => [ qw(munge frobnicate) ],
   );
   # @ISA setup is automatic
   # all assignments happen at compile time

What not to Export

*

method names (because you don't need to and that's likely to not do what you want),

*

anything by default (because you don't want to surprise your users... badly)

*

anything you don't need to (because less is more)

There's one more item to add to this list. Do not export variable names. Just because "Exporter" lets you do that, it does not mean you should.

  @EXPORT_OK = qw( $svar @avar %hvar ); # DON'T!

Exporting variables is not a good idea. They can change under the hood, provoking horrible effects at-a-distance, that are too hard to track and to fix. Trust me: they are not worth it.

To provide the capability to set/get class-wide settings, it is best instead to provide accessors as subroutines or class methods instead.  

SEE ALSO

    Exporter::Easy
    Exporter::Lite
    Exporter::Renaming
    Exporter::Tidy
    Sub::Exporter / Sub::Installer
    Perl6::Export / Perl6::Export::Attrs

LICENSE