Refactoring the /Can't return (?:array|hash) to scalar context/ croak

[perl5.git] / lib / Exporter.pm

package Exporter;

require 5.006;

# Be lean.
#use strict;
#no strict 'refs';

our $Debug = 0;
our $ExportLevel = 0;
our $Verbose ||= 0;
our $VERSION = '5.62';
our (%Cache);
# Carp does this now for us, so we can finally live w/o Carp
#$Carp::Internal{Exporter} = 1;

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 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 YourModule:

  use ModuleName 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 @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->(@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
@EXPORT_OK anything but use @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 ModuleName;>

This imports all the symbols from ModuleName's @EXPORT into the namespace
of the C<use> statement.

=item C<use ModuleName ();>

This causes perl to load your module but does not import any symbols.

=item C<use ModuleName qw(...);>

This imports only the symbols listed by the caller into their namespace.
All listed symbols must be in your @EXPORT or @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., 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 $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 @_).  The $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 $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 @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
@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 $module_name-E<gt>require_version($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.

=head2 Generating combined tags

If several symbol categories exist in %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;
  }

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 %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