New tlperl part VIII

git-svn-id: svn://tug.org/texlive/trunk@21430 c570f23f-e606-0410-a88d-b1316a301751

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+package Exporter;
+
+require 5.006;
+
+# Be lean.
+#use strict;
+#no strict 'refs';
+
+our $Debug = 0;
+our $ExportLevel = 0;
+our $Verbose ||= 0;
+our $VERSION = '5.64_01';
+our (%Cache);
+
+sub as_heavy {
+  require Exporter::Heavy;
+  # Unfortunately, this does not work if the caller is aliased as *name = \&foo
+  # Thus the need to create a lot of identical subroutines
+  my $c = (caller(1))[3];
+  $c =~ s/.*:://;
+  \&{"Exporter::Heavy::heavy_$c"};
+}
+
+sub export {
+  goto &{as_heavy()};
+}
+
+sub import {
+  my $pkg = shift;
+  my $callpkg = caller($ExportLevel);
+
+  if ($pkg eq "Exporter" and @_ and $_[0] eq "import") {
+    *{$callpkg."::import"} = \&import;
+    return;
+  }
+
+  # We *need* to treat @{"$pkg\::EXPORT_FAIL"} since Carp uses it :-(
+  my($exports, $fail) = (\@{"$pkg\::EXPORT"}, \@{"$pkg\::EXPORT_FAIL"});
+  return export $pkg, $callpkg, @_
+    if $Verbose or $Debug or @$fail > 1;
+  my $export_cache = ($Cache{$pkg} ||= {});
+  my $args = @_ or @_ = @$exports;
+
+  local $_;
+  if ($args and not %$export_cache) {
+    s/^&//, $export_cache->{$_} = 1
+      foreach (@$exports, @{"$pkg\::EXPORT_OK"});
+  }
+  my $heavy;
+  # Try very hard not to use {} and hence have to  enter scope on the foreach
+  # We bomb out of the loop with last as soon as heavy is set.
+  if ($args or $fail) {
+    ($heavy = (/\W/ or $args and not exists $export_cache->{$_}
+               or @$fail and $_ eq $fail->[0])) and last
+                 foreach (@_);
+  } else {
+    ($heavy = /\W/) and last
+      foreach (@_);
+  }
+  return export $pkg, $callpkg, ($args ? @_ : ()) if $heavy;
+  local $SIG{__WARN__} = 
+	sub {require Carp; &Carp::carp};
+  # shortcut for the common case of no type character
+  *{"$callpkg\::$_"} = \&{"$pkg\::$_"} foreach @_;
+}
+
+# Default methods
+
+sub export_fail {
+    my $self = shift;
+    @_;
+}
+
+# Unfortunately, caller(1)[3] "does not work" if the caller is aliased as
+# *name = \&foo.  Thus the need to create a lot of identical subroutines
+# Otherwise we could have aliased them to export().
+
+sub export_to_level {
+  goto &{as_heavy()};
+}
+
+sub export_tags {
+  goto &{as_heavy()};
+}
+
+sub export_ok_tags {
+  goto &{as_heavy()};
+}
+
+sub require_version {
+  goto &{as_heavy()};
+}
+
+1;
+__END__
+
+=head1 NAME
+
+Exporter - Implements default import method for modules
+
+=head1 SYNOPSIS
+
+In module F<YourModule.pm>:
+
+  package YourModule;
+  require Exporter;
+  @ISA = qw(Exporter);
+  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
+
+or
+
+  package YourModule;
+  use Exporter 'import'; # gives you Exporter's import() method directly
+  @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
+
+In other files which wish to use C<YourModule>:
+
+  use YourModule qw(frobnicate);      # import listed symbols
+  frobnicate ($left, $right)          # calls YourModule::frobnicate
+
+Take a look at L</Good Practices> for some variants
+you will like to use in modern Perl code.
+
+=head1 DESCRIPTION
+
+The Exporter module implements an C<import> method which allows a module
+to export functions and variables to its users' namespaces. Many modules
+use Exporter rather than implementing their own C<import> method because
+Exporter provides a highly flexible interface, with an implementation optimised
+for the common case.
+
+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 How to Export
+
+The arrays C<@EXPORT> and C<@EXPORT_OK> in a module hold lists of
+symbols that are going to be exported into the users name space by
+default, or which they can request to be exported, respectively.  The
+symbols can represent functions, scalars, arrays, hashes, or typeglobs.
+The symbols must be given by full name with the exception that the
+ampersand in front of a function is optional, e.g.
+
+    @EXPORT    = qw(afunc $scalar @array);   # afunc is a function
+    @EXPORT_OK = qw(&bfunc %hash *typeglob); # explicit prefix on &bfunc
+
+If you are only exporting function names it is recommended to omit the
+ampersand, as the implementation is faster this way.
+
+=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 C<@EXPORT_OK> in preference to C<@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 C<YourModule::item_name> (or C<< $blessed_ref->method >>)
+syntax.  By convention you can use a leading underscore on names to
+informally indicate that they are 'internal' and not for public use.
+
+(It is actually possible to get private functions by saying:
+
+  my $subref = sub { ... };
+  $subref->(@args);            # Call it as a function
+  $obj->$subref(@args);        # Use it as a method
+
+However if you use them for methods it is up to you to figure out
+how to make inheritance work.)
+
+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
+C<@EXPORT_OK> anything but use C<@EXPORT> with caution. For function and
+method names use barewords in preference to names prefixed with
+ampersands for the export lists.
+
+Other module design guidelines can be found in L<perlmod>.
+
+=head2 How to Import
+
+In other files which wish to use your module there are three basic ways for
+them to load your module and import its symbols:
+
+=over 4
+
+=item C<use YourModule;>
+
+This imports all the symbols from YourModule's C<@EXPORT> into the namespace
+of the C<use> statement.
+
+=item C<use YourModule ();>
+
+This causes perl to load your module but does not import any symbols.
+
+=item C<use YourModule qw(...);>
+
+This imports only the symbols listed by the caller into their namespace.
+All listed symbols must be in your C<@EXPORT> or C<@EXPORT_OK>, else an error
+occurs. The advanced export features of Exporter are accessed like this,
+but with list entries that are syntactically distinct from symbol names.
+
+=back
+
+Unless you want to use its advanced features, this is probably all you
+need to know to use Exporter.
+
+=head1 Advanced features
+
+=head2 Specialised Import Lists
+
+If any of the entries in an import list begins with !, : or / then
+the list is treated as a series of specifications which either add to
+or delete from the list of names to import. They are processed left to
+right. Specifications are in the form:
+
+    [!]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., F<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 { $Exporter::Verbose=1 }> to see how the
+specifications are being processed and what is actually being imported
+into modules.
+
+=head2 Exporting without using Exporter's import method
+
+Exporter has a special method, 'export_to_level' which is used in situations
+where you can't directly call Exporter's import method. The export_to_level
+method looks like:
+
+    MyPackage->export_to_level($where_to_export, $package, @what_to_export);
+
+where C<$where_to_export> is an integer telling how far up the calling stack
+to export your symbols, and C<@what_to_export> is an array telling what
+symbols *to* export (usually this is C<@_>).  The C<$package> argument is
+currently unused.
+
+For example, suppose that you have a module, A, which already has an
+import function:
+
+    package A;
+
+    @ISA = qw(Exporter);
+    @EXPORT_OK = qw ($b);
+
+    sub import
+    {
+	$A::b = 1;     # not a very useful import method
+    }
+
+and you want to Export symbol C<$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 C<@_> at all before you call export_to_level
+- or people using your package will get very unexplained results!
+
+=head2 Exporting without inheriting from Exporter
+
+By including Exporter in your C<@ISA> you inherit an Exporter's import() method
+but you also inherit several other helper methods which you probably don't
+want. To avoid this you can do
+
+  package YourModule;
+  use Exporter qw( import );
+
+which will export Exporter's own import() method into YourModule.
+Everything will work as before but you won't need to include Exporter in
+C<@YourModule::ISA>.
+
+Note: This feature was introduced in version 5.57
+of Exporter, released with perl 5.8.3.
+
+=head2 Module Version Checking
+
+The Exporter module will convert an attempt to import a number from a
+module into a call to C<< $module_name->require_version($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 C<require_version> method which
+checks the value of C<$VERSION> in the exporting module.
+
+Since the default C<require_version> method treats the C<$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 = $module_name->export_fail(@failed_symbols);
+
+If the C<export_fail> method returns an empty list then no error is
+recorded and all the requested symbols are exported. If the returned
+list is not empty then an error is generated for each symbol and the
+export fails. The Exporter provides a default C<export_fail> method which
+simply returns the list unchanged.
+
+Uses for the C<export_fail> method include giving better error messages
+for some symbols and performing lazy architectural checks (put more
+symbols into C<@EXPORT_FAIL> by default and then take them out if someone
+actually tries to use them and an expensive check shows that they are
+usable on that platform).
+
+=head2 Tag Handling Utility Functions
+
+Since the symbols listed within C<%EXPORT_TAGS> must also appear in either
+C<@EXPORT> or C<@EXPORT_OK>, two utility functions are provided which allow
+you to easily add tagged sets of symbols to C<@EXPORT> or C<@EXPORT_OK>:
+
+  %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
+
+  Exporter::export_tags('foo');     # add aa, bb and cc to @EXPORT
+  Exporter::export_ok_tags('bar');  # add aa, cc and dd to @EXPORT_OK
+
+Any names which are not tags are added to C<@EXPORT> or C<@EXPORT_OK>
+unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
+names being silently added to C<@EXPORT> or C<@EXPORT_OK>. Future versions
+may make this a fatal error.
+
+=head2 Generating combined tags
+
+If several symbol categories exist in C<%EXPORT_TAGS>, it's usually
+useful to create the utility ":all" to simplify "use" statements.
+
+The simplest way to do this is:
+
+  %EXPORT_TAGS = (foo => [qw(aa bb cc)], bar => [qw(aa cc dd)]);
+
+  # add all the other ":class" tags to the ":all" class,
+  # deleting duplicates
+  {
+    my %seen;
+
+    push @{$EXPORT_TAGS{all}},
+      grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}} foreach keys %EXPORT_TAGS;
+  }
+
+F<CGI.pm> creates an ":all" tag which contains some (but not really
+all) of its categories.  That could be done with one small
+change:
+
+  # add some of the other ":class" tags to the ":all" class,
+  # deleting duplicates
+  {
+    my %seen;
+
+    push @{$EXPORT_TAGS{all}},
+      grep {!$seen{$_}++} @{$EXPORT_TAGS{$_}}
+        foreach qw/html2 html3 netscape form cgi internal/;
+  }
+
+Note that the tag names in C<%EXPORT_TAGS> don't have the leading ':'.
+
+=head2 C<AUTOLOAD>ed Constants
+
+Many modules make use of C<AUTOLOAD>ing for constant subroutines to
+avoid having to compile and waste memory on rarely used values (see
+L<perlsub> for details on constant subroutines).  Calls to such
+constant subroutines are not optimized away at compile time because
+they can't be checked at compile time for constancy.
+
+Even if a prototype is available at compile time, the body of the
+subroutine is not (it hasn't been C<AUTOLOAD>ed yet). perl needs to
+examine both the C<()> prototype and the body of a subroutine at
+compile time to detect that it can safely replace calls to that
+subroutine with the constant value.
+
+A workaround for this is to call the constants once in a C<BEGIN> block:
+
+   package My ;
+
+   use Socket ;
+
+   foo( SO_LINGER );     ## SO_LINGER NOT optimized away; called at runtime
+   BEGIN { SO_LINGER }
+   foo( SO_LINGER );     ## SO_LINGER optimized away at compile time.
+
+This forces the C<AUTOLOAD> for C<SO_LINGER> to take place before
+SO_LINGER is encountered later in C<My> package.
+
+If you are writing a package that C<AUTOLOAD>s, consider forcing
+an C<AUTOLOAD> for any constants explicitly imported by other packages
+or which are usually used when your package is C<use>d.
+
+=head1 Good Practices
+
+=head2 Declaring C<@EXPORT_OK> and Friends
+
+When using C<Exporter> with the standard C<strict> and C<warnings>
+pragmas, the C<our> keyword is needed to declare the package
+variables C<@EXPORT_OK>, C<@EXPORT>, C<@ISA>, etc.
+
+  our @ISA = qw(Exporter);
+  our @EXPORT_OK = qw(munge frobnicate);
+
+If backward compatibility for Perls under 5.6 is important,
+one must write instead a C<use vars> statement.
+
+  use vars qw(@ISA @EXPORT_OK);
+  @ISA = qw(Exporter);
+  @EXPORT_OK = qw(munge frobnicate);
+
+=head2 Playing Safe
+
+There are some caveats with the use of runtime statements
+like C<require Exporter> and the assignment to package
+variables, which can very subtle for the unaware programmer.
+This may happen for instance with mutually recursive
+modules, which are affected by the time the relevant
+constructions are executed.
+
+The ideal (but a bit ugly) way to never have to think
+about that is to use C<BEGIN> blocks. So the first part
+of the L</SYNOPSIS> code could be rewritten as:
+
+  package YourModule;
+
+  use strict;
+  use warnings;
+
+  our (@ISA, @EXPORT_OK);
+  BEGIN {
+     require Exporter;
+     @ISA = qw(Exporter);
+     @EXPORT_OK = qw(munge frobnicate);  # symbols to export on request
+  }
+
+The C<BEGIN> will assure that the loading of F<Exporter.pm>
+and the assignments to C<@ISA> and C<@EXPORT_OK> happen
+immediately, leaving no room for something to get awry
+or just plain wrong.
+
+With respect to loading C<Exporter> and inheriting, there
+are alternatives with the use of modules like C<base> and C<parent>.
+
+  use base qw( Exporter );
+  # or
+  use parent qw( Exporter );
+
+Any of these statements are nice replacements for
+C<BEGIN { require Exporter; @ISA = qw(Exporter); }>
+with the same compile-time effect. The basic difference
+is that C<base> code interacts with declared C<fields>
+while C<parent> is a streamlined version of the older
+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
+
+
+