IRIX Base Documentation 2001 May/SGI IRIX Base Documentation 2001 May.iso/usr/share/catman/p_man/cat3/perl5/Exporter.z/Exporter

Text File | 1998-10-30 | 10.9 KB | 331 lines

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     Exporter - Implements default import method for modules

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     In module ModuleName.pm:

       package ModuleName;
       require Exporter;
       @ISA = qw(Exporter);

       @EXPORT = qw(...);            # symbols to export by default
       @EXPORT_OK = qw(...);         # symbols to export on request
       %EXPORT_TAGS = tag => [...];  # define names for sets of symbols

     In other files which wish to use ModuleName:

       use ModuleName;               # import default symbols into my package

       use ModuleName qw(...);       # import listed symbols into my package

       use ModuleName ();            # do not import any symbols


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     The Exporter module implements a default import method which many modules
     choose to inherit rather than implement their own.

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

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     Do nnnnooootttt export method names!

     Do nnnnooootttt 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:






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       my $subref = sub { ... };
       &$subref;

     But there's no way to call that directly as a method, since a method must
     have a name in the symbol table.)

     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.

     Other module design guidelines can be found in the _p_e_r_l_m_o_d manpage.

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     If the first entry in an import list begins with !, : or / then the list
     is treated as a series of specifications which either add to or delete
     from the list of names to import. They are processed left to right.
     Specifications are in the form:

         [!]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/.





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

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     Exporter has a special method, 'export_to_level' which is used in
     situations where you can't directly call Export's import method. The
     export_to_level method looks like:

     MyPackage->_e_x_p_o_r_t__t_o__l_e_v_e_l($where_to_export, @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 @_).

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

     package A;

     @ISA = _q_w(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 _E_x_p_o_r_t_e_r::_i_m_p_o_r_t() will never get called.
     Instead, say the following:

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

     sub import {
         $A::b = 1;
         A->_e_x_p_o_r_t__t_o__l_e_v_e_l(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!

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     The Exporter module will convert an attempt to import a number from a
     module into a call to $module_name->_r_e_q_u_i_r_e__v_e_r_s_i_o_n($value). This can be
     used to validate that the version of the module being used is greater
     than or equal to the required version.






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     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.

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     In some situations you may want to prevent certain symbols from being
     exported. Typically this applies to extensions which have functions or
     constants that may not exist on some systems.

     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).

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     Since the symbols listed within %EXPORT_TAGS must also appear in either
     @EXPORT or @EXPORT_OK, two utility functions are provided which allow you
     to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:

       %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.




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