[perl5.git] / lib / Exporter.pm

package Exporter;

require 5.001;

#
# We go to a lot of trouble not to 'require Carp' at file scope,
#  because Carp requires Exporter, and something has to give.
#

$ExportLevel = 0;
$Verbose = 0 unless $Verbose;

sub export {

    # First make import warnings look like they're coming from the "use".
    local $SIG{__WARN__} = sub {
	my $text = shift;
	if ($text =~ s/ at \S*Exporter.pm line \d+.*\n//) {
	    require Carp;
	    local $Carp::CarpLevel = 1;	# ignore package calling us too.
	    Carp::carp($text);
	}
	else {
	    warn $text;
	}
    };
    local $SIG{__DIE__} = sub {
	require Carp;
	local $Carp::CarpLevel = 1;	# ignore package calling us too.
	Carp::croak("$_[0]Illegal null symbol in \@${1}::EXPORT")
	    if $_[0] =~ /^Unable to create sub named "(.*?)::"/;
    };

    my($pkg, $callpkg, @imports) = @_;
    my($type, $sym, $oops);
    *exports = *{"${pkg}::EXPORT"};

    if (@imports) {
	if (!%exports) {
	    grep(s/^&//, @exports);
	    @exports{@exports} = (1) x @exports;
	    my $ok = \@{"${pkg}::EXPORT_OK"};
	    if (@$ok) {
		grep(s/^&//, @$ok);
		@exports{@$ok} = (1) x @$ok;
	    }
	}

	if ($imports[0] =~ m#^[/!:]#){
	    my $tagsref = \%{"${pkg}::EXPORT_TAGS"};
	    my $tagdata;
	    my %imports;
	    my($remove, $spec, @names, @allexports);
	    # negated first item implies starting with default set:
	    unshift @imports, ':DEFAULT' if $imports[0] =~ m/^!/;
	    foreach $spec (@imports){
		$remove = $spec =~ s/^!//;

		if ($spec =~ s/^://){
		    if ($spec eq 'DEFAULT'){
			@names = @exports;
		    }
		    elsif ($tagdata = $tagsref->{$spec}) {
			@names = @$tagdata;
		    }
		    else {
			warn qq["$spec" is not defined in %${pkg}::EXPORT_TAGS];
			++$oops;
			next;
		    }
		}
		elsif ($spec =~ m:^/(.*)/$:){
		    my $patn = $1;
		    @allexports = keys %exports unless @allexports; # only do keys once
		    @names = grep(/$patn/, @allexports); # not anchored by default
		}
		else {
		    @names = ($spec); # is a normal symbol name
		}

		warn "Import ".($remove ? "del":"add").": @names "
		    if $Verbose;

		if ($remove) {
		   foreach $sym (@names) { delete $imports{$sym} } 
		}
		else {
		    @imports{@names} = (1) x @names;
		}
	    }
	    @imports = keys %imports;
	}

	foreach $sym (@imports) {
	    if (!$exports{$sym}) {
		if ($sym =~ m/^\d/) {
		    $pkg->require_version($sym);
		    # If the version number was the only thing specified
		    # then we should act as if nothing was specified:
		    if (@imports == 1) {
			@imports = @exports;
			last;
		    }
		    # We need a way to emulate 'use Foo ()' but still
		    # allow an easy version check: "use Foo 1.23, ''";
		    if (@imports == 2 and !$imports[1]) {
			@imports = ();
			last;
		    }
		} elsif ($sym !~ s/^&// || !$exports{$sym}) {
                    require Carp;
		    Carp::carp(qq["$sym" is not exported by the $pkg module]);
		    $oops++;
		}
	    }
	}
	if ($oops) {
	    require Carp;
	    Carp::croak("Can't continue after import errors");
	}
    }
    else {
	@imports = @exports;
    }

    *fail = *{"${pkg}::EXPORT_FAIL"};
    if (@fail) {
	if (!%fail) {
	    # Build cache of symbols. Optimise the lookup by adding
	    # barewords twice... both with and without a leading &.
	    # (Technique could be applied to %exports cache at cost of memory)
	    my @expanded = map { /^\w/ ? ($_, '&'.$_) : $_ } @fail;
	    warn "${pkg}::EXPORT_FAIL cached: @expanded" if $Verbose;
	    @fail{@expanded} = (1) x @expanded;
	}
	my @failed;
	foreach $sym (@imports) { push(@failed, $sym) if $fail{$sym} }
	if (@failed) {
	    @failed = $pkg->export_fail(@failed);
	    foreach $sym (@failed) {
                require Carp;
		Carp::carp(qq["$sym" is not implemented by the $pkg module ],
			"on this architecture");
	    }
	    if (@failed) {
		require Carp;
		Carp::croak("Can't continue after import errors");
	    }
	}
    }

    warn "Importing into $callpkg from $pkg: ",
		join(", ",sort @imports) if $Verbose;

    foreach $sym (@imports) {
	# shortcut for the common case of no type character
	(*{"${callpkg}::$sym"} = \&{"${pkg}::$sym"}, next)
	    unless $sym =~ s/^(\W)//;
	$type = $1;
	*{"${callpkg}::$sym"} =
	    $type eq '&' ? \&{"${pkg}::$sym"} :
	    $type eq '$' ? \${"${pkg}::$sym"} :
	    $type eq '@' ? \@{"${pkg}::$sym"} :
	    $type eq '%' ? \%{"${pkg}::$sym"} :
	    $type eq '*' ?  *{"${pkg}::$sym"} :
	    do { require Carp; Carp::croak("Can't export symbol: $type$sym") };
    }
}

sub export_to_level
{
      my $pkg = shift;
      my $level = shift;
      my $callpkg = caller($level);
      $pkg->export($callpkg, @_);
}

sub import {
    my $pkg = shift;
    my $callpkg = caller($ExportLevel);
    export $pkg, $callpkg, @_;
}


# Utility functions

sub _push_tags {
    my($pkg, $var, $syms) = @_;
    my $nontag;
    *export_tags = \%{"${pkg}::EXPORT_TAGS"};
    push(@{"${pkg}::$var"},
	map { $export_tags{$_} ? @{$export_tags{$_}} : scalar(++$nontag,$_) }
		(@$syms) ? @$syms : keys %export_tags);
    if ($nontag and $^W) {
	# This may change to a die one day
	require Carp;
	Carp::carp("Some names are not tags");
    }
}

sub export_tags    { _push_tags((caller)[0], "EXPORT",    \@_) }
sub export_ok_tags { _push_tags((caller)[0], "EXPORT_OK", \@_) }


# Default methods

sub export_fail {
    my $self = shift;
    @_;
}

sub require_version {
    my($self, $wanted) = @_;
    my $pkg = ref $self || $self;
    my $version = ${"${pkg}::VERSION"};
    if (!$version or $version < $wanted) {
	$version ||= "(undef)";
	my $file = $INC{"$pkg.pm"};
	$file &&= " ($file)";
	require Carp;
	Carp::croak("$pkg $wanted required--this is only version $version$file")
    }
    $version;
}

1;

# A simple self test harness. Change 'require Carp' to 'use Carp ()' for testing.
# package main; eval(join('',<DATA>)) or die $@ unless caller;
__END__
package Test;
$INC{'Exporter.pm'} = 1;
@ISA = qw(Exporter);
@EXPORT      = qw(A1 A2 A3 A4 A5);
@EXPORT_OK   = qw(B1 B2 B3 B4 B5);
%EXPORT_TAGS = (T1=>[qw(A1 A2 B1 B2)], T2=>[qw(A1 A2 B3 B4)], T3=>[qw(X3)]);
@EXPORT_FAIL = qw(B4);
Exporter::export_ok_tags('T3', 'unknown_tag');
sub export_fail {
    map { "Test::$_" } @_	# edit symbols just as an example
}

package main;
$Exporter::Verbose = 1;
#import Test;
#import Test qw(X3);		# export ok via export_ok_tags()
#import Test qw(:T1 !A2 /5/ !/3/ B5);
import Test qw(:T2 !B4);
import Test qw(:T2);		# should fail
1;

=head1 NAME

Exporter - Implements default import method for modules

=head1 SYNOPSIS

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

=head1 DESCRIPTION

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

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

=head2 Selecting What To Export

Do B<not> export method names!

Do B<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-E<gt>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 L<perlmod>.

=head2 Specialised Import Lists

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., C</^EXIT/> rather than C</EXIT/>.

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

=head2 Exporting without using Export's import method

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->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!


=head2 Module Version Checking

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

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.

=head2 Managing Unknown Symbols

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 C<@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).

=head2 Tag Handling Utility Functions

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 C<-w>) to avoid misspelt tags
names being silently added to @EXPORT or @EXPORT_OK. Future versions
may make this a fatal error.

=cut
Commit	Line	Data
8990e307 LW	1	package Exporter;
8990e307 LW	2
748a9306	3	require 5.001;
8990e307	4
68dc0745	5	#
	6	# We go to a lot of trouble not to 'require Carp' at file scope,
	7	# because Carp requires Exporter, and something has to give.
	8	#
	9
a0d0e21e	10	$ExportLevel = 0;
c07a80fd	11	$Verbose = 0 unless $Verbose;
748a9306	12
a0d0e21e	13	sub export {
748a9306 LW	14
	15	# First make import warnings look like they're coming from the "use".
	16	local $SIG{__WARN__} = sub {
	17	my $text = shift;
68dc0745	18	if ($text =~ s/ at \SExporter.pm line \d+.\n//) {
	19	require Carp;
	20	local $Carp::CarpLevel = 1; # ignore package calling us too.
	21	Carp::carp($text);
	22	}
	23	else {
	24	warn $text;
	25	}
748a9306	26	};
4633a7c4	27	local $SIG{__DIE__} = sub {
68dc0745	28	require Carp;
68dc0745	29	local $Carp::CarpLevel = 1; # ignore package calling us too.
4633a7c4 LW	30	Carp::croak("$_[0]Illegal null symbol in \@${1}::EXPORT")
	31	if $_[0] =~ /^Unable to create sub named "(.*?)::"/;
	32	};
748a9306	33
2b5b2650	34	my($pkg, $callpkg, @imports) = @_;
	35	my($type, $sym, $oops);
	36	exports = {"${pkg}::EXPORT"};
	37
8990e307	38	if (@imports) {
8990e307 LW	39	if (!%exports) {
8990e307 LW	40	grep(s/^&//, @exports);
2b5b2650	41	@exports{@exports} = (1) x @exports;
	42	my $ok = \@{"${pkg}::EXPORT_OK"};
	43	if (@$ok) {
	44	grep(s/^&//, @$ok);
	45	@exports{@$ok} = (1) x @$ok;
a0d0e21e	46	}
8990e307	47	}
748a9306 LW	48
748a9306 LW	49	if ($imports[0] =~ m#^[/!:]#){
748a9306 LW	50	my $tagsref = \%{"${pkg}::EXPORT_TAGS"};
	51	my $tagdata;
	52	my %imports;
2b5b2650	53	my($remove, $spec, @names, @allexports);
748a9306	54	# negated first item implies starting with default set:
2b5b2650	55	unshift @imports, ':DEFAULT' if $imports[0] =~ m/^!/;
	56	foreach $spec (@imports){
	57	$remove = $spec =~ s/^!//;
748a9306	58
2b5b2650	59	if ($spec =~ s/^://){
2b5b2650	60	if ($spec eq 'DEFAULT'){
748a9306 LW	61	@names = @exports;
748a9306 LW	62	}
2b5b2650	63	elsif ($tagdata = $tagsref->{$spec}) {
748a9306 LW	64	@names = @$tagdata;
748a9306 LW	65	}
2b5b2650	66	else {
	67	warn qq["$spec" is not defined in %${pkg}::EXPORT_TAGS];
	68	++$oops;
	69	next;
	70	}
	71	}
	72	elsif ($spec =~ m:^/(.*)/$:){
	73	my $patn = $1;
	74	@allexports = keys %exports unless @allexports; # only do keys once
	75	@names = grep(/$patn/, @allexports); # not anchored by default
748a9306	76	}
2b5b2650	77	else {
	78	@names = ($spec); # is a normal symbol name
	79	}
	80
	81	warn "Import ".($remove ? "del":"add").": @names "
	82	if $Verbose;
748a9306	83
2b5b2650	84	if ($remove) {
2b5b2650	85	foreach $sym (@names) { delete $imports{$sym} }
748a9306 LW	86	}
748a9306 LW	87	else {
2b5b2650	88	@imports{@names} = (1) x @names;
748a9306 LW	89	}
	90	}
	91	@imports = keys %imports;
	92	}
	93
8990e307 LW	94	foreach $sym (@imports) {
8990e307 LW	95	if (!$exports{$sym}) {
e50aee73 AD	96	if ($sym =~ m/^\d/) {
	97	$pkg->require_version($sym);
	98	# If the version number was the only thing specified
	99	# then we should act as if nothing was specified:
	100	if (@imports == 1) {
	101	@imports = @exports;
	102	last;
	103	}
3221d3b0	104	# We need a way to emulate 'use Foo ()' but still
	105	# allow an easy version check: "use Foo 1.23, ''";
	106	if (@imports == 2 and !$imports[1]) {
	107	@imports = ();
	108	last;
	109	}
e50aee73	110	} elsif ($sym !~ s/^&// \|\| !$exports{$sym}) {
3bb63ce6 MG	111	require Carp;
3bb63ce6 MG	112	Carp::carp(qq["$sym" is not exported by the $pkg module]);
8990e307	113	$oops++;
8990e307 LW	114	}
	115	}
	116	}
68dc0745	117	if ($oops) {
	118	require Carp;
	119	Carp::croak("Can't continue after import errors");
	120	}
8990e307 LW	121	}
	122	else {
	123	@imports = @exports;
	124	}
2b5b2650	125
	126	fail = {"${pkg}::EXPORT_FAIL"};
	127	if (@fail) {
	128	if (!%fail) {
	129	# Build cache of symbols. Optimise the lookup by adding
	130	# barewords twice... both with and without a leading &.
	131	# (Technique could be applied to %exports cache at cost of memory)
	132	my @expanded = map { /^\w/ ? ($_, '&'.$_) : $_ } @fail;
	133	warn "${pkg}::EXPORT_FAIL cached: @expanded" if $Verbose;
	134	@fail{@expanded} = (1) x @expanded;
	135	}
	136	my @failed;
	137	foreach $sym (@imports) { push(@failed, $sym) if $fail{$sym} }
	138	if (@failed) {
	139	@failed = $pkg->export_fail(@failed);
	140	foreach $sym (@failed) {
3bb63ce6 MG	141	require Carp;
	142	Carp::carp(qq["$sym" is not implemented by the $pkg module ],
	143	"on this architecture");
2b5b2650	144	}
68dc0745	145	if (@failed) {
	146	require Carp;
	147	Carp::croak("Can't continue after import errors");
	148	}
2b5b2650	149	}
	150	}
	151
c07a80fd	152	warn "Importing into $callpkg from $pkg: ",
2b5b2650	153	join(", ",sort @imports) if $Verbose;
2b5b2650	154
8990e307	155	foreach $sym (@imports) {
2b5b2650	156	# shortcut for the common case of no type character
	157	(*{"${callpkg}::$sym"} = \&{"${pkg}::$sym"}, next)
	158	unless $sym =~ s/^(\W)//;
	159	$type = $1;
748a9306 LW	160	*{"${callpkg}::$sym"} =
	161	$type eq '&' ? \&{"${pkg}::$sym"} :
	162	$type eq '$' ? \${"${pkg}::$sym"} :
	163	$type eq '@' ? \@{"${pkg}::$sym"} :
	164	$type eq '%' ? \%{"${pkg}::$sym"} :
	165	$type eq '' ? {"${pkg}::$sym"} :
68dc0745	166	do { require Carp; Carp::croak("Can't export symbol: $type$sym") };
8990e307	167	}
2b5b2650	168	}
8990e307	169
84902520 TB	170	sub export_to_level
	171	{
	172	my $pkg = shift;
e49da4e7	173	my $level = shift;
84902520 TB	174	my $callpkg = caller($level);
	175	$pkg->export($callpkg, @_);
	176	}
	177
a0d0e21e	178	sub import {
748a9306	179	my $pkg = shift;
2b5b2650	180	my $callpkg = caller($ExportLevel);
748a9306 LW	181	export $pkg, $callpkg, @_;
	182	}
	183
2b5b2650	184
84902520	185
2b5b2650	186	# Utility functions
	187
	188	sub _push_tags {
	189	my($pkg, $var, $syms) = @_;
	190	my $nontag;
c07a80fd	191	*export_tags = \%{"${pkg}::EXPORT_TAGS"};
2b5b2650	192	push(@{"${pkg}::$var"},
	193	map { $export_tags{$_} ? @{$export_tags{$_}} : scalar(++$nontag,$_) }
	194	(@$syms) ? @$syms : keys %export_tags);
68dc0745	195	if ($nontag and $^W) {
	196	# This may change to a die one day
	197	require Carp;
	198	Carp::carp("Some names are not tags");
	199	}
2b5b2650	200	}
	201
	202	sub export_tags { _push_tags((caller)[0], "EXPORT", \@_) }
	203	sub export_ok_tags { _push_tags((caller)[0], "EXPORT_OK", \@_) }
	204
	205
	206	# Default methods
	207
	208	sub export_fail {
a45ab7f6	209	my $self = shift;
2b5b2650	210	@_;
a0d0e21e LW	211	}
a0d0e21e LW	212
e50aee73 AD	213	sub require_version {
	214	my($self, $wanted) = @_;
	215	my $pkg = ref $self \|\| $self;
3221d3b0	216	my $version = ${"${pkg}::VERSION"};
	217	if (!$version or $version < $wanted) {
	218	$version \|\|= "(undef)";
	219	my $file = $INC{"$pkg.pm"};
	220	$file &&= " ($file)";
68dc0745	221	require Carp;
3221d3b0	222	Carp::croak("$pkg $wanted required--this is only version $version$file")
3221d3b0	223	}
e50aee73 AD	224	$version;
	225	}
	226
8990e307	227	1;
2b5b2650	228
	229	# A simple self test harness. Change 'require Carp' to 'use Carp ()' for testing.
	230	# package main; eval(join('',<DATA>)) or die $@ unless caller;
	231	__END__
	232	package Test;
	233	$INC{'Exporter.pm'} = 1;
	234	@ISA = qw(Exporter);
	235	@EXPORT = qw(A1 A2 A3 A4 A5);
	236	@EXPORT_OK = qw(B1 B2 B3 B4 B5);
	237	%EXPORT_TAGS = (T1=>[qw(A1 A2 B1 B2)], T2=>[qw(A1 A2 B3 B4)], T3=>[qw(X3)]);
	238	@EXPORT_FAIL = qw(B4);
	239	Exporter::export_ok_tags('T3', 'unknown_tag');
	240	sub export_fail {
	241	map { "Test::$_" } @_ # edit symbols just as an example
	242	}
	243
	244	package main;
	245	$Exporter::Verbose = 1;
	246	#import Test;
	247	#import Test qw(X3); # export ok via export_ok_tags()
	248	#import Test qw(:T1 !A2 /5/ !/3/ B5);
	249	import Test qw(:T2 !B4);
	250	import Test qw(:T2); # should fail
	251	1;
	252
	253	=head1 NAME
	254
	255	Exporter - Implements default import method for modules
	256
	257	=head1 SYNOPSIS
	258
	259	In module ModuleName.pm:
	260
	261	package ModuleName;
	262	require Exporter;
	263	@ISA = qw(Exporter);
	264
	265	@EXPORT = qw(...); # symbols to export by default
	266	@EXPORT_OK = qw(...); # symbols to export on request
	267	%EXPORT_TAGS = tag => [...]; # define names for sets of symbols
	268
	269	In other files which wish to use ModuleName:
	270
	271	use ModuleName; # import default symbols into my package
	272
	273	use ModuleName qw(...); # import listed symbols into my package
	274
	275	use ModuleName (); # do not import any symbols
	276
	277	=head1 DESCRIPTION
	278
	279	The Exporter module implements a default C<import> method which
68dc0745	280	many modules choose to inherit rather than implement their own.
2b5b2650	281
	282	Perl automatically calls the C<import> method when processing a
	283	C<use> statement for a module. Modules and C<use> are documented
	284	in L<perlfunc> and L<perlmod>. Understanding the concept of
	285	modules and how the C<use> statement operates is important to
	286	understanding the Exporter.
	287
	288	=head2 Selecting What To Export
	289
	290	Do B<not> export method names!
	291
	292	Do B<not> export anything else by default without a good reason!
	293
	294	Exports pollute the namespace of the module user. If you must export
	295	try to use @EXPORT_OK in preference to @EXPORT and avoid short or
	296	common symbol names to reduce the risk of name clashes.
	297
	298	Generally anything not exported is still accessible from outside the
1fef88e7	299	module using the ModuleName::item_name (or $blessed_ref-E<gt>method)
2b5b2650	300	syntax. By convention you can use a leading underscore on names to
	301	informally indicate that they are 'internal' and not for public use.
	302
	303	(It is actually possible to get private functions by saying:
	304
	305	my $subref = sub { ... };
	306	&$subref;
	307
	308	But there's no way to call that directly as a method, since a method
	309	must have a name in the symbol table.)
	310
	311	As a general rule, if the module is trying to be object oriented
	312	then export nothing. If it's just a collection of functions then
	313	@EXPORT_OK anything but use @EXPORT with caution.
	314
	315	Other module design guidelines can be found in L<perlmod>.
	316
	317	=head2 Specialised Import Lists
	318
	319	If the first entry in an import list begins with !, : or / then the
	320	list is treated as a series of specifications which either add to or
	321	delete from the list of names to import. They are processed left to
	322	right. Specifications are in the form:
	323
	324	[!]name This name only
	325	[!]:DEFAULT All names in @EXPORT
	326	[!]:tag All names in $EXPORT_TAGS{tag} anonymous list
	327	[!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
	328
	329	A leading ! indicates that matching names should be deleted from the
	330	list of names to import. If the first specification is a deletion it
	331	is treated as though preceded by :DEFAULT. If you just want to import
	332	extra names in addition to the default set you will still need to
	333	include :DEFAULT explicitly.
	334
	335	e.g., Module.pm defines:
	336
	337	@EXPORT = qw(A1 A2 A3 A4 A5);
	338	@EXPORT_OK = qw(B1 B2 B3 B4 B5);
	339	%EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
	340
	341	Note that you cannot use tags in @EXPORT or @EXPORT_OK.
	342	Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
	343
	344	An application using Module can say something like:
	345
	346	use Module qw(:DEFAULT :T2 !B3 A3);
	347
	348	Other examples include:
	349
	350	use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
	351	use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
	352
	353	Remember that most patterns (using //) will need to be anchored
	354	with a leading ^, e.g., C</^EXIT/> rather than C</EXIT/>.
	355
	356	You can say C<BEGIN { $Exporter::Verbose=1 }> to see how the
	357	specifications are being processed and what is actually being imported
	358	into modules.
	359
84902520 TB	360	=head2 Exporting without using Export's import method
	361
	362	Exporter has a special method, 'export_to_level' which is used in situations
	363	where you can't directly call Export's import method. The export_to_level
	364	method looks like:
	365
	366	MyPackage->export_to_level($where_to_export, @what_to_export);
	367
	368	where $where_to_export is an integer telling how far up the calling stack
	369	to export your symbols, and @what_to_export is an array telling what
	370	symbols to export (usually this is @_).
	371
	372	For example, suppose that you have a module, A, which already has an
	373	import function:
	374
	375	package A;
	376
	377	@ISA = qw(Exporter);
	378	@EXPORT_OK = qw ($b);
	379
	380	sub import
	381	{
	382	$A::b = 1; # not a very useful import method
	383	}
	384
	385	and you want to Export symbol $A::b back to the module that called
	386	package A. Since Exporter relies on the import method to work, via
	387	inheritance, as it stands Exporter::import() will never get called.
	388	Instead, say the following:
	389
	390	package A;
	391	@ISA = qw(Exporter);
	392	@EXPORT_OK = qw ($b);
	393
	394	sub import
	395	{
	396	$A::b = 1;
	397	A->export_to_level(1, @_);
	398	}
	399
	400	This will export the symbols one level 'above' the current package - ie: to
	401	the program or module that used package A.
	402
	403	Note: Be careful not to modify '@_' at all before you call export_to_level
	404	- or people using your package will get very unexplained results!
	405
	406
2b5b2650	407	=head2 Module Version Checking
	408
	409	The Exporter module will convert an attempt to import a number from a
1fef88e7	410	module into a call to $module_name-E<gt>require_version($value). This can
2b5b2650	411	be used to validate that the version of the module being used is
	412	greater than or equal to the required version.
	413
	414	The Exporter module supplies a default require_version method which
	415	checks the value of $VERSION in the exporting module.
	416
	417	Since the default require_version method treats the $VERSION number as
d5e40bcc	418	a simple numeric value it will regard version 1.10 as lower than
	419	1.9. For this reason it is strongly recommended that you use numbers
	420	with at least two decimal places, e.g., 1.09.
2b5b2650	421
	422	=head2 Managing Unknown Symbols
	423
	424	In some situations you may want to prevent certain symbols from being
	425	exported. Typically this applies to extensions which have functions
	426	or constants that may not exist on some systems.
	427
	428	The names of any symbols that cannot be exported should be listed
	429	in the C<@EXPORT_FAIL> array.
	430
7a2e2cd6	431	If a module attempts to import any of these symbols the Exporter
2b5b2650	432	will give the module an opportunity to handle the situation before
	433	generating an error. The Exporter will call an export_fail method
	434	with a list of the failed symbols:
	435
	436	@failed_symbols = $module_name->export_fail(@failed_symbols);
	437
	438	If the export_fail method returns an empty list then no error is
	439	recorded and all the requested symbols are exported. If the returned
	440	list is not empty then an error is generated for each symbol and the
	441	export fails. The Exporter provides a default export_fail method which
	442	simply returns the list unchanged.
	443
	444	Uses for the export_fail method include giving better error messages
	445	for some symbols and performing lazy architectural checks (put more
	446	symbols into @EXPORT_FAIL by default and then take them out if someone
	447	actually tries to use them and an expensive check shows that they are
	448	usable on that platform).
	449
	450	=head2 Tag Handling Utility Functions
	451
	452	Since the symbols listed within %EXPORT_TAGS must also appear in either
	453	@EXPORT or @EXPORT_OK, two utility functions are provided which allow
	454	you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:
	455
	456	%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
	457
	458	Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
	459	Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
	460
	461	Any names which are not tags are added to @EXPORT or @EXPORT_OK
d5e40bcc	462	unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
2b5b2650	463	names being silently added to @EXPORT or @EXPORT_OK. Future versions
	464	may make this a fatal error.
	465
	466	=cut