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 symbolsIn 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
Perl automatically calls the import method when processing a use statement for a module. Modules and use are documented in the perlfunc manpage and the perlmod manpage. Understanding the concept of modules and how the use statement operates is important to understanding the Exporter.
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;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 perlmod manpage.
[!]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 matchA 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 { $Exporter::Verbose=1 } to see how the specifications are being processed and what is actually being imported into modules.
MyPackage->export_to_level($where_to_export, @what_to_export);
where $where_to_export is an integer telling how far up the calling stack to export your symbols, and @what_to_export is an array telling what symbols *to* export (usually this is @_).
For example, suppose that you have a module, A, which already has an import function:
package A;
@ISA = qw(Exporter); @EXPORT_OK = qw ($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!
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.
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).
%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_OKAny 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.