[p5sagit/p5-mst-13.2.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}) {
		    warn 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 $sym (@failed) {
		warn qq["$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 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 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	1	package Exporter;
8990e307	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	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 $Carp::CarpLevel = 1; # ignore package calling us too.
4633a7c4	30	Carp::croak("$_[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	39	if (!%exports) {
8990e307	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	48
748a9306	49	if ($imports[0] =~ m#^[/!:]#){
748a9306	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 ($spec eq 'DEFAULT'){
748a9306	61	@names = @exports;
748a9306	62	}
2b5b2650	63	elsif ($tagdata = $tagsref->{$spec}) {
748a9306	64	@names = @$tagdata;
748a9306	65	}
2b5b2650	66	else {
	67	warn qq["$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	86	}
748a9306	87	else {
2b5b2650	88	@imports{@names} = (1) x @names;
748a9306	89	}
	90	}
	91	@imports = keys %imports;
	92	}
	93
8990e307	94	foreach $sym (@imports) {
8990e307	95	if (!$exports{$sym}) {
e50aee73	96	if ($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}) {
2b5b2650	111	warn qq["$sym" is not exported by the $pkg module];
8990e307	112	$oops++;
8990e307	113	}
	114	}
	115	}
68dc0745	116	if ($oops) {
	117	require Carp;
	118	Carp::croak("Can't continue after import errors");
	119	}
8990e307	120	}
	121	else {
	122	@imports = @exports;
	123	}
2b5b2650	124
	125	fail = {"${pkg}::EXPORT_FAIL"};
	126	if (@fail) {
	127	if (!%fail) {
	128	# Build cache of symbols. Optimise the lookup by adding
	129	# barewords twice... both with and without a leading &.
	130	# (Technique could be applied to %exports cache at cost of memory)
	131	my @expanded = map { /^\w/ ? ($_, '&'.$_) : $_ } @fail;
	132	warn "${pkg}::EXPORT_FAIL cached: @expanded" if $Verbose;
	133	@fail{@expanded} = (1) x @expanded;
	134	}
	135	my @failed;
	136	foreach $sym (@imports) { push(@failed, $sym) if $fail{$sym} }
	137	if (@failed) {
	138	@failed = $pkg->export_fail(@failed);
	139	foreach $sym (@failed) {
	140	warn qq["$sym" is not implemented by the $pkg module ],
	141	"on this architecture";
	142	}
68dc0745	143	if (@failed) {
	144	require Carp;
	145	Carp::croak("Can't continue after import errors");
	146	}
2b5b2650	147	}
	148	}
	149
c07a80fd	150	warn "Importing into $callpkg from $pkg: ",
2b5b2650	151	join(", ",sort @imports) if $Verbose;
2b5b2650	152
8990e307	153	foreach $sym (@imports) {
2b5b2650	154	# shortcut for the common case of no type character
	155	(*{"${callpkg}::$sym"} = \&{"${pkg}::$sym"}, next)
	156	unless $sym =~ s/^(\W)//;
	157	$type = $1;
748a9306	158	*{"${callpkg}::$sym"} =
	159	$type eq '&' ? \&{"${pkg}::$sym"} :
	160	$type eq '$' ? \${"${pkg}::$sym"} :
	161	$type eq '@' ? \@{"${pkg}::$sym"} :
	162	$type eq '%' ? \%{"${pkg}::$sym"} :
	163	$type eq '' ? {"${pkg}::$sym"} :
68dc0745	164	do { require Carp; Carp::croak("Can't export symbol: $type$sym") };
8990e307	165	}
2b5b2650	166	}
8990e307	167
a0d0e21e	168	sub import {
748a9306	169	my $pkg = shift;
2b5b2650	170	my $callpkg = caller($ExportLevel);
748a9306	171	export $pkg, $callpkg, @_;
	172	}
	173
2b5b2650	174
	175	# Utility functions
	176
	177	sub _push_tags {
	178	my($pkg, $var, $syms) = @_;
	179	my $nontag;
c07a80fd	180	*export_tags = \%{"${pkg}::EXPORT_TAGS"};
2b5b2650	181	push(@{"${pkg}::$var"},
	182	map { $export_tags{$_} ? @{$export_tags{$_}} : scalar(++$nontag,$_) }
	183	(@$syms) ? @$syms : keys %export_tags);
68dc0745	184	if ($nontag and $^W) {
	185	# This may change to a die one day
	186	require Carp;
	187	Carp::carp("Some names are not tags");
	188	}
2b5b2650	189	}
	190
	191	sub export_tags { _push_tags((caller)[0], "EXPORT", \@_) }
	192	sub export_ok_tags { _push_tags((caller)[0], "EXPORT_OK", \@_) }
	193
	194
	195	# Default methods
	196
	197	sub export_fail {
a45ab7f6	198	my $self = shift;
2b5b2650	199	@_;
a0d0e21e	200	}
a0d0e21e	201
e50aee73	202	sub require_version {
	203	my($self, $wanted) = @_;
	204	my $pkg = ref $self \|\| $self;
3221d3b0	205	my $version = ${"${pkg}::VERSION"};
	206	if (!$version or $version < $wanted) {
	207	$version \|\|= "(undef)";
	208	my $file = $INC{"$pkg.pm"};
	209	$file &&= " ($file)";
68dc0745	210	require Carp;
3221d3b0	211	Carp::croak("$pkg $wanted required--this is only version $version$file")
3221d3b0	212	}
e50aee73	213	$version;
	214	}
	215
8990e307	216	1;
2b5b2650	217
	218	# A simple self test harness. Change 'require Carp' to 'use Carp ()' for testing.
	219	# package main; eval(join('',<DATA>)) or die $@ unless caller;
	220	__END__
	221	package Test;
	222	$INC{'Exporter.pm'} = 1;
	223	@ISA = qw(Exporter);
	224	@EXPORT = qw(A1 A2 A3 A4 A5);
	225	@EXPORT_OK = qw(B1 B2 B3 B4 B5);
	226	%EXPORT_TAGS = (T1=>[qw(A1 A2 B1 B2)], T2=>[qw(A1 A2 B3 B4)], T3=>[qw(X3)]);
	227	@EXPORT_FAIL = qw(B4);
	228	Exporter::export_ok_tags('T3', 'unknown_tag');
	229	sub export_fail {
	230	map { "Test::$_" } @_ # edit symbols just as an example
	231	}
	232
	233	package main;
	234	$Exporter::Verbose = 1;
	235	#import Test;
	236	#import Test qw(X3); # export ok via export_ok_tags()
	237	#import Test qw(:T1 !A2 /5/ !/3/ B5);
	238	import Test qw(:T2 !B4);
	239	import Test qw(:T2); # should fail
	240	1;
	241
	242	=head1 NAME
	243
	244	Exporter - Implements default import method for modules
	245
	246	=head1 SYNOPSIS
	247
	248	In module ModuleName.pm:
	249
	250	package ModuleName;
	251	require Exporter;
	252	@ISA = qw(Exporter);
	253
	254	@EXPORT = qw(...); # symbols to export by default
	255	@EXPORT_OK = qw(...); # symbols to export on request
	256	%EXPORT_TAGS = tag => [...]; # define names for sets of symbols
	257
	258	In other files which wish to use ModuleName:
	259
	260	use ModuleName; # import default symbols into my package
	261
	262	use ModuleName qw(...); # import listed symbols into my package
	263
	264	use ModuleName (); # do not import any symbols
	265
	266	=head1 DESCRIPTION
	267
	268	The Exporter module implements a default C<import> method which
68dc0745	269	many modules choose to inherit rather than implement their own.
2b5b2650	270
	271	Perl automatically calls the C<import> method when processing a
	272	C<use> statement for a module. Modules and C<use> are documented
	273	in L<perlfunc> and L<perlmod>. Understanding the concept of
	274	modules and how the C<use> statement operates is important to
	275	understanding the Exporter.
	276
	277	=head2 Selecting What To Export
	278
	279	Do B<not> export method names!
	280
	281	Do B<not> export anything else by default without a good reason!
	282
	283	Exports pollute the namespace of the module user. If you must export
	284	try to use @EXPORT_OK in preference to @EXPORT and avoid short or
	285	common symbol names to reduce the risk of name clashes.
	286
	287	Generally anything not exported is still accessible from outside the
1fef88e7	288	module using the ModuleName::item_name (or $blessed_ref-E<gt>method)
2b5b2650	289	syntax. By convention you can use a leading underscore on names to
	290	informally indicate that they are 'internal' and not for public use.
	291
	292	(It is actually possible to get private functions by saying:
	293
	294	my $subref = sub { ... };
	295	&$subref;
	296
	297	But there's no way to call that directly as a method, since a method
	298	must have a name in the symbol table.)
	299
	300	As a general rule, if the module is trying to be object oriented
	301	then export nothing. If it's just a collection of functions then
	302	@EXPORT_OK anything but use @EXPORT with caution.
	303
	304	Other module design guidelines can be found in L<perlmod>.
	305
	306	=head2 Specialised Import Lists
	307
	308	If the first entry in an import list begins with !, : or / then the
	309	list is treated as a series of specifications which either add to or
	310	delete from the list of names to import. They are processed left to
	311	right. Specifications are in the form:
	312
	313	[!]name This name only
	314	[!]:DEFAULT All names in @EXPORT
	315	[!]:tag All names in $EXPORT_TAGS{tag} anonymous list
	316	[!]/pattern/ All names in @EXPORT and @EXPORT_OK which match
	317
	318	A leading ! indicates that matching names should be deleted from the
	319	list of names to import. If the first specification is a deletion it
	320	is treated as though preceded by :DEFAULT. If you just want to import
	321	extra names in addition to the default set you will still need to
	322	include :DEFAULT explicitly.
	323
	324	e.g., Module.pm defines:
	325
	326	@EXPORT = qw(A1 A2 A3 A4 A5);
	327	@EXPORT_OK = qw(B1 B2 B3 B4 B5);
	328	%EXPORT_TAGS = (T1 => [qw(A1 A2 B1 B2)], T2 => [qw(A1 A2 B3 B4)]);
	329
	330	Note that you cannot use tags in @EXPORT or @EXPORT_OK.
	331	Names in EXPORT_TAGS must also appear in @EXPORT or @EXPORT_OK.
	332
	333	An application using Module can say something like:
	334
	335	use Module qw(:DEFAULT :T2 !B3 A3);
	336
	337	Other examples include:
	338
	339	use Socket qw(!/^[AP]F_/ !SOMAXCONN !SOL_SOCKET);
	340	use POSIX qw(:errno_h :termios_h !TCSADRAIN !/^EXIT/);
	341
	342	Remember that most patterns (using //) will need to be anchored
	343	with a leading ^, e.g., C</^EXIT/> rather than C</EXIT/>.
	344
	345	You can say C<BEGIN { $Exporter::Verbose=1 }> to see how the
	346	specifications are being processed and what is actually being imported
	347	into modules.
	348
	349	=head2 Module Version Checking
	350
	351	The Exporter module will convert an attempt to import a number from a
1fef88e7	352	module into a call to $module_name-E<gt>require_version($value). This can
2b5b2650	353	be used to validate that the version of the module being used is
	354	greater than or equal to the required version.
	355
	356	The Exporter module supplies a default require_version method which
	357	checks the value of $VERSION in the exporting module.
	358
	359	Since the default require_version method treats the $VERSION number as
d5e40bcc	360	a simple numeric value it will regard version 1.10 as lower than
	361	1.9. For this reason it is strongly recommended that you use numbers
	362	with at least two decimal places, e.g., 1.09.
2b5b2650	363
	364	=head2 Managing Unknown Symbols
	365
	366	In some situations you may want to prevent certain symbols from being
	367	exported. Typically this applies to extensions which have functions
	368	or constants that may not exist on some systems.
	369
	370	The names of any symbols that cannot be exported should be listed
	371	in the C<@EXPORT_FAIL> array.
	372
7a2e2cd6	373	If a module attempts to import any of these symbols the Exporter
2b5b2650	374	will give the module an opportunity to handle the situation before
	375	generating an error. The Exporter will call an export_fail method
	376	with a list of the failed symbols:
	377
	378	@failed_symbols = $module_name->export_fail(@failed_symbols);
	379
	380	If the export_fail method returns an empty list then no error is
	381	recorded and all the requested symbols are exported. If the returned
	382	list is not empty then an error is generated for each symbol and the
	383	export fails. The Exporter provides a default export_fail method which
	384	simply returns the list unchanged.
	385
	386	Uses for the export_fail method include giving better error messages
	387	for some symbols and performing lazy architectural checks (put more
	388	symbols into @EXPORT_FAIL by default and then take them out if someone
	389	actually tries to use them and an expensive check shows that they are
	390	usable on that platform).
	391
	392	=head2 Tag Handling Utility Functions
	393
	394	Since the symbols listed within %EXPORT_TAGS must also appear in either
	395	@EXPORT or @EXPORT_OK, two utility functions are provided which allow
	396	you to easily add tagged sets of symbols to @EXPORT or @EXPORT_OK:
	397
	398	%EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
	399
	400	Exporter::export_tags('foo'); # add aa, bb and cc to @EXPORT
	401	Exporter::export_ok_tags('bar'); # add aa, cc and dd to @EXPORT_OK
	402
	403	Any names which are not tags are added to @EXPORT or @EXPORT_OK
d5e40bcc	404	unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
2b5b2650	405	names being silently added to @EXPORT or @EXPORT_OK. Future versions
	406	may make this a fatal error.
	407
	408	=cut