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588 lines
18 KiB
Perl
588 lines
18 KiB
Perl
package Exporter;
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require 5.006;
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# Be lean.
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#use strict;
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#no strict 'refs';
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our $Debug = 0;
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our $ExportLevel = 0;
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our $Verbose ||= 0;
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our $VERSION = '5.64_03';
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our (%Cache);
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sub as_heavy {
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require Exporter::Heavy;
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# Unfortunately, this does not work if the caller is aliased as *name = \&foo
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# Thus the need to create a lot of identical subroutines
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my $c = (caller(1))[3];
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$c =~ s/.*:://;
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\&{"Exporter::Heavy::heavy_$c"};
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}
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sub export {
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goto &{as_heavy()};
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}
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sub import {
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my $pkg = shift;
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my $callpkg = caller($ExportLevel);
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if ($pkg eq "Exporter" and @_ and $_[0] eq "import") {
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*{$callpkg."::import"} = \&import;
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return;
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}
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# We *need* to treat @{"$pkg\::EXPORT_FAIL"} since Carp uses it :-(
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my $exports = \@{"$pkg\::EXPORT"};
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# But, avoid creating things if they don't exist, which saves a couple of
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# hundred bytes per package processed.
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my $fail = ${$pkg . '::'}{EXPORT_FAIL} && \@{"$pkg\::EXPORT_FAIL"};
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return export $pkg, $callpkg, @_
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if $Verbose or $Debug or $fail && @$fail > 1;
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my $export_cache = ($Cache{$pkg} ||= {});
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my $args = @_ or @_ = @$exports;
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local $_;
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if ($args and not %$export_cache) {
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s/^&//, $export_cache->{$_} = 1
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foreach (@$exports, @{"$pkg\::EXPORT_OK"});
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}
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my $heavy;
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# Try very hard not to use {} and hence have to enter scope on the foreach
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# We bomb out of the loop with last as soon as heavy is set.
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if ($args or $fail) {
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($heavy = (/\W/ or $args and not exists $export_cache->{$_}
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or $fail and @$fail and $_ eq $fail->[0])) and last
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foreach (@_);
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} else {
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($heavy = /\W/) and last
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foreach (@_);
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}
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return export $pkg, $callpkg, ($args ? @_ : ()) if $heavy;
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local $SIG{__WARN__} =
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sub {require Carp; &Carp::carp} if not $SIG{__WARN__};
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# shortcut for the common case of no type character
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*{"$callpkg\::$_"} = \&{"$pkg\::$_"} foreach @_;
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}
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# Default methods
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sub export_fail {
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my $self = shift;
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@_;
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}
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# Unfortunately, caller(1)[3] "does not work" if the caller is aliased as
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# *name = \&foo. Thus the need to create a lot of identical subroutines
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# Otherwise we could have aliased them to export().
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sub export_to_level {
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goto &{as_heavy()};
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}
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sub export_tags {
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goto &{as_heavy()};
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}
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sub export_ok_tags {
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goto &{as_heavy()};
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}
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sub require_version {
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goto &{as_heavy()};
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}
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1;
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__END__
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=head1 NAME
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Exporter - Implements default import method for modules
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=head1 SYNOPSIS
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In module F<YourModule.pm>:
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package YourModule;
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require Exporter;
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@ISA = qw(Exporter);
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@EXPORT_OK = qw(munge frobnicate); # symbols to export on request
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or
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package YourModule;
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use Exporter 'import'; # gives you Exporter's import() method directly
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@EXPORT_OK = qw(munge frobnicate); # symbols to export on request
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In other files which wish to use C<YourModule>:
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use YourModule qw(frobnicate); # import listed symbols
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frobnicate ($left, $right) # calls YourModule::frobnicate
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Take a look at L</Good Practices> for some variants
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you will like to use in modern Perl code.
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=head1 DESCRIPTION
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The Exporter module implements an C<import> method which allows a module
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to export functions and variables to its users' namespaces. Many modules
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use Exporter rather than implementing their own C<import> method because
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Exporter provides a highly flexible interface, with an implementation optimised
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for the common case.
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Perl automatically calls the C<import> method when processing a
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C<use> statement for a module. Modules and C<use> are documented
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in L<perlfunc> and L<perlmod>. Understanding the concept of
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modules and how the C<use> statement operates is important to
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understanding the Exporter.
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=head2 How to Export
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The arrays C<@EXPORT> and C<@EXPORT_OK> in a module hold lists of
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symbols that are going to be exported into the users name space by
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default, or which they can request to be exported, respectively. The
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symbols can represent functions, scalars, arrays, hashes, or typeglobs.
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The symbols must be given by full name with the exception that the
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ampersand in front of a function is optional, e.g.
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@EXPORT = qw(afunc $scalar @array); # afunc is a function
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@EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
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If you are only exporting function names it is recommended to omit the
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ampersand, as the implementation is faster this way.
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=head2 Selecting What To Export
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Do B<not> export method names!
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Do B<not> export anything else by default without a good reason!
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Exports pollute the namespace of the module user. If you must export
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try to use C<@EXPORT_OK> in preference to C<@EXPORT> and avoid short or
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common symbol names to reduce the risk of name clashes.
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Generally anything not exported is still accessible from outside the
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module using the C<YourModule::item_name> (or C<< $blessed_ref->method >>)
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syntax. By convention you can use a leading underscore on names to
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informally indicate that they are 'internal' and not for public use.
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(It is actually possible to get private functions by saying:
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my $subref = sub { ... };
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$subref->(@args); # Call it as a function
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$obj->$subref(@args); # Use it as a method
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However if you use them for methods it is up to you to figure out
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how to make inheritance work.)
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As a general rule, if the module is trying to be object oriented
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then export nothing. If it's just a collection of functions then
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C<@EXPORT_OK> anything but use C<@EXPORT> with caution. For function and
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method names use barewords in preference to names prefixed with
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ampersands for the export lists.
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Other module design guidelines can be found in L<perlmod>.
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=head2 How to Import
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In other files which wish to use your module there are three basic ways for
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them to load your module and import its symbols:
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=over 4
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=item C<use YourModule;>
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This imports all the symbols from YourModule's C<@EXPORT> into the namespace
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of the C<use> statement.
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=item C<use YourModule ();>
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This causes perl to load your module but does not import any symbols.
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=item C<use YourModule qw(...);>
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This imports only the symbols listed by the caller into their namespace.
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All listed symbols must be in your C<@EXPORT> or C<@EXPORT_OK>, else an error
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occurs. The advanced export features of Exporter are accessed like this,
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but with list entries that are syntactically distinct from symbol names.
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=back
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Unless you want to use its advanced features, this is probably all you
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need to know to use Exporter.
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=head1 Advanced features
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=head2 Specialised Import Lists
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If any of the entries in an import list begins with !, : or / then
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the list is treated as a series of specifications which either add to
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or delete from the list of names to import. They are processed left to
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right. Specifications are in the form:
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[!]name This name only
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[!]:DEFAULT All names in @EXPORT
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[!]:tag All names in $EXPORT_TAGS{tag} anonymous list
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[!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
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A leading ! indicates that matching names should be deleted from the
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list of names to import. If the first specification is a deletion it
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is treated as though preceded by :DEFAULT. If you just want to import
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extra names in addition to the default set you will still need to
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include :DEFAULT explicitly.
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e.g., F<Module.pm> defines:
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@EXPORT = qw(A1 A2 A3 A4 A5);
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@EXPORT_OK = qw(B1 B2 B3 B4 B5);
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%EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
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Note that you cannot use tags in @EXPORT or @EXPORT_OK.
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Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
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An application using Module can say something like:
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use Module qw(:DEFAULT :T2 !B3 A3);
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Other examples include:
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use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
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use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
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Remember that most patterns (using //) will need to be anchored
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with a leading ^, e.g., C</^EXIT/> rather than C</EXIT/>.
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You can say C<BEGIN { $Exporter::Verbose=1 }> to see how the
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specifications are being processed and what is actually being imported
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into modules.
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=head2 Exporting without using Exporter's import method
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Exporter has a special method, 'export_to_level' which is used in situations
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where you can't directly call Exporter's import method. The export_to_level
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method looks like:
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MyPackage->export_to_level($where_to_export, $package, @what_to_export);
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where C<$where_to_export> is an integer telling how far up the calling stack
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to export your symbols, and C<@what_to_export> is an array telling what
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symbols *to* export (usually this is C<@_>). The C<$package> argument is
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currently unused.
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For example, suppose that you have a module, A, which already has an
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import function:
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package A;
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@ISA = qw(Exporter);
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@EXPORT_OK = qw ($b);
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sub import
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{
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$A::b = 1; # not a very useful import method
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}
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and you want to Export symbol C<$A::b> back to the module that called
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package A. Since Exporter relies on the import method to work, via
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inheritance, as it stands Exporter::import() will never get called.
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Instead, say the following:
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package A;
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@ISA = qw(Exporter);
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@EXPORT_OK = qw ($b);
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sub import
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{
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$A::b = 1;
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A->export_to_level(1, @_);
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}
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This will export the symbols one level 'above' the current package - ie: to
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the program or module that used package A.
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Note: Be careful not to modify C<@_> at all before you call export_to_level
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- or people using your package will get very unexplained results!
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=head2 Exporting without inheriting from Exporter
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By including Exporter in your C<@ISA> you inherit an Exporter's import() method
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but you also inherit several other helper methods which you probably don't
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want. To avoid this you can do
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package YourModule;
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use Exporter qw( import );
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which will export Exporter's own import() method into YourModule.
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Everything will work as before but you won't need to include Exporter in
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C<@YourModule::ISA>.
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Note: This feature was introduced in version 5.57
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of Exporter, released with perl 5.8.3.
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=head2 Module Version Checking
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The Exporter module will convert an attempt to import a number from a
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module into a call to C<< $module_name->require_version($value) >>. This can
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be used to validate that the version of the module being used is
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greater than or equal to the required version.
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The Exporter module supplies a default C<require_version> method which
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checks the value of C<$VERSION> in the exporting module.
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Since the default C<require_version> method treats the C<$VERSION> number as
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a simple numeric value it will regard version 1.10 as lower than
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1.9. For this reason it is strongly recommended that you use numbers
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with at least two decimal places, e.g., 1.09.
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=head2 Managing Unknown Symbols
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In some situations you may want to prevent certain symbols from being
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exported. Typically this applies to extensions which have functions
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or constants that may not exist on some systems.
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The names of any symbols that cannot be exported should be listed
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in the C<@EXPORT_FAIL> array.
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If a module attempts to import any of these symbols the Exporter
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will give the module an opportunity to handle the situation before
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generating an error. The Exporter will call an export_fail method
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with a list of the failed symbols:
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@failed_symbols = $module_name->export_fail(@failed_symbols);
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If the C<export_fail> method returns an empty list then no error is
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recorded and all the requested symbols are exported. If the returned
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list is not empty then an error is generated for each symbol and the
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export fails. The Exporter provides a default C<export_fail> method which
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simply returns the list unchanged.
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Uses for the C<export_fail> method include giving better error messages
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for some symbols and performing lazy architectural checks (put more
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symbols into C<@EXPORT_FAIL> by default and then take them out if someone
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actually tries to use them and an expensive check shows that they are
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usable on that platform).
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=head2 Tag Handling Utility Functions
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Since the symbols listed within C<%EXPORT_TAGS> must also appear in either
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C<@EXPORT> or C<@EXPORT_OK>, two utility functions are provided which allow
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you to easily add tagged sets of symbols to C<@EXPORT> or C<@EXPORT_OK>:
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%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
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Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
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Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
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Any names which are not tags are added to C<@EXPORT> or C<@EXPORT_OK>
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unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
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names being silently added to C<@EXPORT> or C<@EXPORT_OK>. Future versions
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may make this a fatal error.
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=head2 Generating combined tags
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If several symbol categories exist in C<%EXPORT_TAGS>, it's usually
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useful to create the utility ":all" to simplify "use" statements.
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The simplest way to do this is:
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%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
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# add all the other ":class" tags to the ":all" class,
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# deleting duplicates
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{
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my %seen;
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push @{$EXPORT_TAGS{all}},
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grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
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}
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F<CGI.pm> creates an ":all" tag which contains some (but not really
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all) of its categories. That could be done with one small
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change:
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# add some of the other ":class" tags to the ":all" class,
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# deleting duplicates
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{
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my %seen;
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push @{$EXPORT_TAGS{all}},
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grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
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foreach qw/html2 html3 netscape form cgi internal/;
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}
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Note that the tag names in C<%EXPORT_TAGS> don't have the leading ':'.
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=head2 C<AUTOLOAD>ed Constants
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Many modules make use of C<AUTOLOAD>ing for constant subroutines to
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avoid having to compile and waste memory on rarely used values (see
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L<perlsub> for details on constant subroutines). Calls to such
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constant subroutines are not optimized away at compile time because
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they can't be checked at compile time for constancy.
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Even if a prototype is available at compile time, the body of the
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subroutine is not (it hasn't been C<AUTOLOAD>ed yet). perl needs to
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examine both the C<()> prototype and the body of a subroutine at
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compile time to detect that it can safely replace calls to that
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subroutine with the constant value.
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A workaround for this is to call the constants once in a C<BEGIN> block:
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package My ;
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use Socket ;
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foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime
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BEGIN { SO_LINGER }
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foo( SO_LINGER ); ## SO_LINGER optimized away at compile time.
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This forces the C<AUTOLOAD> for C<SO_LINGER> to take place before
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SO_LINGER is encountered later in C<My> package.
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If you are writing a package that C<AUTOLOAD>s, consider forcing
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an C<AUTOLOAD> for any constants explicitly imported by other packages
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or which are usually used when your package is C<use>d.
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=head1 Good Practices
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=head2 Declaring C<@EXPORT_OK> and Friends
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When using C<Exporter> with the standard C<strict> and C<warnings>
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pragmas, the C<our> keyword is needed to declare the package
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variables C<@EXPORT_OK>, C<@EXPORT>, C<@ISA>, etc.
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our @ISA = qw(Exporter);
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our @EXPORT_OK = qw(munge frobnicate);
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If backward compatibility for Perls under 5.6 is important,
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one must write instead a C<use vars> statement.
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use vars qw(@ISA @EXPORT_OK);
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@ISA = qw(Exporter);
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@EXPORT_OK = qw(munge frobnicate);
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=head2 Playing Safe
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There are some caveats with the use of runtime statements
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like C<require Exporter> and the assignment to package
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variables, which can very subtle for the unaware programmer.
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This may happen for instance with mutually recursive
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modules, which are affected by the time the relevant
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constructions are executed.
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The ideal (but a bit ugly) way to never have to think
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about that is to use C<BEGIN> blocks. So the first part
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of the L</SYNOPSIS> code could be rewritten as:
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package YourModule;
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use strict;
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use warnings;
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our (@ISA, @EXPORT_OK);
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BEGIN {
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require Exporter;
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@ISA = qw(Exporter);
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@EXPORT_OK = qw(munge frobnicate); # symbols to export on request
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}
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The C<BEGIN> will assure that the loading of F<Exporter.pm>
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and the assignments to C<@ISA> and C<@EXPORT_OK> happen
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immediately, leaving no room for something to get awry
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or just plain wrong.
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With respect to loading C<Exporter> and inheriting, there
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are alternatives with the use of modules like C<base> and C<parent>.
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use base qw( Exporter );
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# or
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use parent qw( Exporter );
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Any of these statements are nice replacements for
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C<BEGIN { require Exporter; @ISA = qw(Exporter); }>
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with the same compile-time effect. The basic difference
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is that C<base> code interacts with declared C<fields>
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while C<parent> is a streamlined version of the older
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C<base> code to just establish the IS-A relationship.
|
|
|
|
For more details, see the documentation and code of
|
|
L<base> and L<parent>.
|
|
|
|
Another thorough remedy to that runtime vs.
|
|
compile-time trap is to use L<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
|
|
|
|
=head2 What not to Export
|
|
|
|
You have been warned already in L</Selecting What To Export>
|
|
to not export:
|
|
|
|
=over 4
|
|
|
|
=item *
|
|
|
|
method names (because you don't need to
|
|
and that's likely to not do what you want),
|
|
|
|
=item *
|
|
|
|
anything by default (because you don't want to surprise your users...
|
|
badly)
|
|
|
|
=item *
|
|
|
|
anything you don't need to (because less is more)
|
|
|
|
=back
|
|
|
|
There's one more item to add to this list. Do B<not>
|
|
export variable names. Just because C<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.
|
|
|
|
=head1 SEE ALSO
|
|
|
|
C<Exporter> is definitely not the only module with
|
|
symbol exporter capabilities. At CPAN, you may find
|
|
a bunch of them. Some are lighter. Some
|
|
provide improved APIs and features. Peek the one
|
|
that fits your needs. The following is
|
|
a sample list of such modules.
|
|
|
|
Exporter::Easy
|
|
Exporter::Lite
|
|
Exporter::Renaming
|
|
Exporter::Tidy
|
|
Sub::Exporter / Sub::Installer
|
|
Perl6::Export / Perl6::Export::Attrs
|
|
|
|
=head1 LICENSE
|
|
|
|
This library is free software. You can redistribute it
|
|
and/or modify it under the same terms as Perl itself.
|
|
|
|
=cut
|
|
|
|
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|