exporter.3

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.\" ========================================================================
.\"
.IX Title "Exporter 3"
.TH Exporter 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
Exporter \- Implements default import method for modules
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
In module YourModule.pm:
.PP
.Vb 4
\&  package YourModule;
\&  require Exporter;
\&  @ISA = qw(Exporter);
\&  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
.Ve
.PP
or
.PP
.Vb 3
\&  package YourModule;
\&  use Exporter \*(Aqimport\*(Aq; # gives you Exporter\*(Aqs import() method directly
\&  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
.Ve
.PP
In other files which wish to use YourModule:
.PP
.Vb 2
\&  use ModuleName qw(frobnicate);      # import listed symbols
\&  frobnicate ($left, $right)          # calls YourModule::frobnicate
.Ve
.PP
Take a look at \*(L"Good Practices\*(R" for some variants
you will like to use in modern Perl code.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The Exporter module implements an \f(CW\*(C`import\*(C'\fR method which allows a module
to export functions and variables to its users' namespaces. Many modules
use Exporter rather than implementing their own \f(CW\*(C`import\*(C'\fR method because
Exporter provides a highly flexible interface, with an implementation optimised
for the common case.
.PP
Perl automatically calls the \f(CW\*(C`import\*(C'\fR method when processing a
\&\f(CW\*(C`use\*(C'\fR statement for a module. Modules and \f(CW\*(C`use\*(C'\fR are documented
in perlfunc and perlmod. Understanding the concept of
modules and how the \f(CW\*(C`use\*(C'\fR statement operates is important to
understanding the Exporter.
.Sh "How to Export"
.IX Subsection "How to Export"
The arrays \f(CW@EXPORT\fR and \f(CW@EXPORT_OK\fR in a module hold lists of
symbols that are going to be exported into the users name space by
default, or which they can request to be exported, respectively.  The
symbols can represent functions, scalars, arrays, hashes, or typeglobs.
The symbols must be given by full name with the exception that the
ampersand in front of a function is optional, e.g.
.PP
.Vb 2
\&    @EXPORT    = qw(afunc $scalar @array);   # afunc is a function
\&    @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
.Ve
.PP
If you are only exporting function names it is recommended to omit the
ampersand, as the implementation is faster this way.
.Sh "Selecting What To Export"
.IX Subsection "Selecting What To Export"
Do \fBnot\fR export method names!
.PP
Do \fBnot\fR export anything else by default without a good reason!
.PP
Exports pollute the namespace of the module user.  If you must export
try to use \f(CW@EXPORT_OK\fR in preference to \f(CW@EXPORT\fR and avoid short or
common symbol names to reduce the risk of name clashes.
.PP
Generally anything not exported is still accessible from outside the
module using the ModuleName::item_name (or \f(CW$blessed_ref\fR\->method)
syntax.  By convention you can use a leading underscore on names to
informally indicate that they are 'internal' and not for public use.
.PP
(It is actually possible to get private functions by saying:
.PP
.Vb 3
\&  my $subref = sub { ... };
\&  $subref\->(@args);            # Call it as a function
\&  $obj\->$subref(@args);        # Use it as a method
.Ve
.PP
However if you use them for methods it is up to you to figure out
how to make inheritance work.)
.PP
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
\&\f(CW@EXPORT_OK\fR anything but use \f(CW@EXPORT\fR with caution. For function and
method names use barewords in preference to names prefixed with
ampersands for the export lists.
.PP
Other module design guidelines can be found in perlmod.
.Sh "How to Import"
.IX Subsection "How to Import"
In other files which wish to use your module there are three basic ways for
them to load your module and import its symbols:
.ie n .IP """use ModuleName;""" 4
.el .IP "\f(CWuse ModuleName;\fR" 4
.IX Item "use ModuleName;"
This imports all the symbols from ModuleName's \f(CW@EXPORT\fR into the namespace
of the \f(CW\*(C`use\*(C'\fR statement.
.ie n .IP """use ModuleName ();""" 4
.el .IP "\f(CWuse ModuleName ();\fR" 4
.IX Item "use ModuleName ();"
This causes perl to load your module but does not import any symbols.
.ie n .IP """use ModuleName qw(...);""" 4
.el .IP "\f(CWuse ModuleName qw(...);\fR" 4
.IX Item "use ModuleName qw(...);"
This imports only the symbols listed by the caller into their namespace.
All listed symbols must be in your \f(CW@EXPORT\fR or \f(CW@EXPORT_OK\fR, 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.
.PP
Unless you want to use its advanced features, this is probably all you
need to know to use Exporter.
.SH "Advanced features"
.IX Header "Advanced features"
.Sh "Specialised Import Lists"
.IX Subsection "Specialised Import Lists"
If any of the entries 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:
.PP
.Vb 4
\&    [!]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
.Ve
.PP
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.
.PP
e.g., Module.pm defines:
.PP
.Vb 3
\&    @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.
.Ve
.PP
An application using Module can say something like:
.PP
.Vb 1
\&    use Module qw(:DEFAULT :T2 !B3 A3);
.Ve
.PP
Other examples include:
.PP
.Vb 2
\&    use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
\&    use POSIX  qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
.Ve
.PP
Remember that most patterns (using //) will need to be anchored
with a leading ^, e.g., \f(CW\*(C`/^EXIT/\*(C'\fR rather than \f(CW\*(C`/EXIT/\*(C'\fR.
.PP
You can say \f(CW\*(C`BEGIN { $Exporter::Verbose=1 }\*(C'\fR to see how the
specifications are being processed and what is actually being imported
into modules.
.Sh "Exporting without using Exporter's import method"
.IX Subsection "Exporting without using Exporter's import method"
Exporter has a special method, 'export_to_level' which is used in situations
where you can't directly call Exporter's import method. The export_to_level
method looks like:
.PP
.Vb 1
\&    MyPackage\->export_to_level($where_to_export, $package, @what_to_export);
.Ve
.PP
where \f(CW$where_to_export\fR is an integer telling how far up the calling stack
to export your symbols, and \f(CW@what_to_export\fR is an array telling what
symbols *to* export (usually this is \f(CW@_\fR).  The \f(CW$package\fR argument is
currently unused.
.PP