perl/lib/attributes.pm

package attributes;

our $VERSION = 0.06;

@EXPORT_OK = qw(get reftype);
@EXPORT = ();
%EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]);

use strict;

sub croak {
    require Carp;
    goto &Carp::croak;
}

sub carp {
    require Carp;
    goto &Carp::carp;
}

## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{}
#sub reftype ($) ;
#sub _fetch_attrs ($) ;
#sub _guess_stash ($) ;
#sub _modify_attrs ;
#sub _warn_reserved () ;
#
# The extra trips through newATTRSUB in the interpreter wipe out any savings
# from avoiding the BEGIN block.  Just do the bootstrap now.
BEGIN { bootstrap attributes }

sub import {
    @_ > 2 && ref $_[2] or do {
        require Exporter;
        goto &Exporter::import;
    };
    my (undef,$home_stash,$svref,@attrs) = @_;

    my $svtype = uc reftype($svref);
    my $pkgmeth;
    $pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES")
        if defined $home_stash && $home_stash ne '';
    my @badattrs;
    if ($pkgmeth) {
        my @pkgattrs = _modify_attrs($svref, @attrs);
        @badattrs = $pkgmeth->($home_stash, $svref, @attrs);
        if (!@badattrs && @pkgattrs) {
            return unless _warn_reserved;
            @pkgattrs = grep { m/\A[[:lower:]]+(?:\z|\()/ } @pkgattrs;
            if (@pkgattrs) {
                for my $attr (@pkgattrs) {
                    $attr =~ s/\(.+\z//s;
                }
                my $s = ((@pkgattrs == 1) ? '' : 's');
                carp "$svtype package attribute$s " .
                    "may clash with future reserved word$s: " .
                    join(' : ' , @pkgattrs);
            }
        }
    }
    else {
        @badattrs = _modify_attrs($svref, @attrs);
    }
    if (@badattrs) {
        croak "Invalid $svtype attribute" .
            (( @badattrs == 1 ) ? '' : 's') .
            ": " .
            join(' : ', @badattrs);
    }
}

sub get ($) {
    @_ == 1  && ref $_[0] or
        croak 'Usage: '.__PACKAGE__.'::get $ref';
    my $svref = shift;
    my $svtype = uc reftype $svref;
    my $stash = _guess_stash $svref;
    $stash = caller unless defined $stash;
    my $pkgmeth;
    $pkgmeth = UNIVERSAL::can($stash, "FETCH_${svtype}_ATTRIBUTES")
        if defined $stash && $stash ne '';
    return $pkgmeth ?
                (_fetch_attrs($svref), $pkgmeth->($stash, $svref)) :
                (_fetch_attrs($svref))
        ;
}

sub require_version { goto &UNIVERSAL::VERSION }

1;
__END__
#The POD goes here

=head1 NAME

attributes - get/set subroutine or variable attributes

=head1 SYNOPSIS

  sub foo : method ;
  my ($x,@y,%z) : Bent = 1;
  my $s = sub : method { ... };

  use attributes ();    # optional, to get subroutine declarations
  my @attrlist = attributes::get(\&foo);

  use attributes 'get'; # import the attributes::get subroutine
  my @attrlist = get \&foo;

=head1 DESCRIPTION

Subroutine declarations and definitions may optionally have attribute lists
associated with them.  (Variable C<my> declarations also may, but see the
warning below.)  Perl handles these declarations by passing some information
about the call site and the thing being declared along with the attribute
list to this module.  In particular, the first example above is equivalent to
the following:

    use attributes __PACKAGE__, \&foo, 'method';

The second example in the synopsis does something equivalent to this:

    use attributes ();
    my ($x,@y,%z);
    attributes::->import(__PACKAGE__, \$x, 'Bent');
    attributes::->import(__PACKAGE__, \@y, 'Bent');
    attributes::->import(__PACKAGE__, \%z, 'Bent');
    ($x,@y,%z) = 1;

Yes, that's a lot of expansion.

B<WARNING>: attribute declarations for variables are still evolving.
The semantics and interfaces of such declarations could change in
future versions.  They are present for purposes of experimentation
with what the semantics ought to be.  Do not rely on the current
implementation of this feature.

There are only a few attributes currently handled by Perl itself (or
directly by this module, depending on how you look at it.)  However,
package-specific attributes are allowed by an extension mechanism.
(See L<"Package-specific Attribute Handling"> below.)

The setting of subroutine attributes happens at compile time.
Variable attributes in C<our> declarations are also applied at compile time.
However, C<my> variables get their attributes applied at run-time.
This means that you have to I<reach> the run-time component of the C<my>
before those attributes will get applied.  For example:

    my $x : Bent = 42 if 0;

will neither assign 42 to $x I<nor> will it apply the C<Bent> attribute
to the variable.

An attempt to set an unrecognized attribute is a fatal error.  (The
error is trappable, but it still stops the compilation within that
C<eval>.)  Setting an attribute with a name that's all lowercase
letters that's not a built-in attribute (such as "foo") will result in
a warning with B<-w> or C<use warnings 'reserved'>.

=head2 Built-in Attributes

The following are the built-in attributes for subroutines:

=over 4

=item locked

B<5.005 threads only!  The use of the "locked" attribute currently
only makes sense if you are using the deprecated "Perl 5.005 threads"
implementation of threads.>

Setting this attribute is only meaningful when the subroutine or
method is to be called by multiple threads.  When set on a method
subroutine (i.e., one marked with the B<method> attribute below),
Perl ensures that any invocation of it implicitly locks its first
argument before execution.  When set on a non-method subroutine,
Perl ensures that a lock is taken on the subroutine itself before
execution.  The semantics of the lock are exactly those of one
explicitly taken with the C<lock> operator immediately after the
subroutine is entered.

=item method

Indicates that the referenced subroutine is a method.
This has a meaning when taken together with the B<locked> attribute,
as described there.  It also means that a subroutine so marked
will not trigger the "Ambiguous call resolved as CORE::%s" warning.

=item lvalue

Indicates that the referenced subroutine is a valid lvalue and can
be assigned to. The subroutine must return a modifiable value such
as a scalar variable, as described in L<perlsub>.

=back

For global variables there is C<unique> attribute: see L<perlfunc/our>.

=head2 Available Subroutines

The following subroutines are available for general use once this module
has been loaded:

=over 4

=item get

This routine expects a single parameter--a reference to a
subroutine or variable.  It returns a list of attributes, which may be
empty.  If passed invalid arguments, it uses die() (via L<Carp::croak|Carp>)
to raise a fatal exception.  If it can find an appropriate package name
for a class method lookup, it will include the results from a
C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in
L<"Package-specific Attribute Handling"> below.
Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned.

=item reftype

This routine expects a single parameter--a reference to a subroutine or
variable.  It returns the built-in type of the referenced variable,
ignoring any package into which it might have been blessed.
This can be useful for determining the I<type> value which forms part of
the method names described in L<"Package-specific Attribute Handling"> below.

=back

Note that these routines are I<not> exported by default.

=head2 Package-specific Attribute Handling

B<WARNING>: the mechanisms described here are still experimental.  Do not
rely on the current implementation.  In particular, there is no provision
for applying package attributes to 'cloned' copies of subroutines used as
closures.  (See L<perlref/"Making References"> for information on closures.)
Package-specific attribute handling may change incompatibly in a future
release.

When an attribute list is present in a declaration, a check is made to see
whether an attribute 'modify' handler is present in the appropriate package
(or its @ISA inheritance tree).  Similarly, when C<attributes::get> is
called on a valid reference, a check is made for an appropriate attribute
'fetch' handler.  See L<"EXAMPLES"> to see how the "appropriate package"
determination works.

The handler names are based on the underlying type of the variable being
declared or of the reference passed.  Because these attributes are
associated with subroutine or variable declarations, this deliberately
ignores any possibility of being blessed into some package.  Thus, a
subroutine declaration uses "CODE" as its I<type>, and even a blessed
hash reference uses "HASH" as its I<type>.

The class methods invoked for modifying and fetching are these:

=over 4

=item FETCH_I<type>_ATTRIBUTES

This method receives a single argument, which is a reference to the
variable or subroutine for which package-defined attributes are desired.
The expected return value is a list of associated attributes.
This list may be empty.

=item MODIFY_I<type>_ATTRIBUTES

This method is called with two fixed arguments, followed by the list of
attributes from the relevant declaration.  The two fixed arguments are
the relevant package name and a reference to the declared subroutine or
variable.  The expected return value is a list of attributes which were
not recognized by this handler.  Note that this allows for a derived class
to delegate a call to its base class, and then only examine the attributes
which the base class didn't already handle for it.

The call to this method is currently made I<during> the processing of the
declaration.  In particular, this means that a subroutine reference will
probably be for an undefined subroutine, even if this declaration is
actually part of the definition.

=back

Calling C<attributes::get()> from within the scope of a null package
declaration C<package ;> for an unblessed variable reference will
not provide any starting package name for the 'fetch' method lookup.
Thus, this circumstance will not result in a method call for package-defined
attributes.  A named subroutine knows to which symbol table entry it belongs
(or originally belonged), and it will use the corresponding package.
An anonymous subroutine knows the package name into which it was compiled
(unless it was also compiled with a null package declaration), and so it
will use that package name.

=head2 Syntax of Attribute Lists

An attribute list is a sequence of attribute specifications, separated by
whitespace or a colon (with optional whitespace).
Each attribute specification is a simple
name, optionally followed by a parenthesised parameter list.
If such a parameter list is present, it is scanned past as for the rules
for the C<q()> operator.  (See L<perlop/"Quote and Quote-like Operators">.)
The parameter list is passed as it was found, however, and not as per C<q()>.

Some examples of syntactically valid attribute lists:

    switch(10,foo(7,3))  :  expensive
    Ugly('\(") :Bad
    _5x5
    locked method

Some examples of syntactically invalid attribute lists (with annotation):

    switch(10,foo()             # ()-string not balanced
    Ugly('(')                   # ()-string not balanced
    5x5                         # "5x5" not a valid identifier
    Y2::north                   # "Y2::north" not a simple identifier
    foo + bar                   # "+" neither a colon nor whitespace

=head1 EXPORTS

=head2 Default exports

None.

=head2 Available exports

The routines C<get> and C<reftype> are exportable.

=head2 Export tags defined

The C<:ALL> tag will get all of the above exports.

=head1 EXAMPLES

Here are some samples of syntactically valid declarations, with annotation
as to how they resolve internally into C<use attributes> invocations by
perl.  These examples are primarily useful to see how the "appropriate
package" is found for the possible method lookups for package-defined
attributes.

=over 4

=item 1.

Code:

    package Canine;
    package Dog;
    my Canine $spot : Watchful ;

Effect:

    use attributes ();
    attributes::->import(Canine => \$spot, "Watchful");

=item 2.

Code:

    package Felis;
    my $cat : Nervous;

Effect:

    use attributes ();
    attributes::->import(Felis => \$cat, "Nervous");

=item 3.

Code:

    package X;
    sub foo : locked ;

Effect:

    use attributes X => \&foo, "locked";

=item 4.

Code:

    package X;
    sub Y::x : locked { 1 }

Effect:

    use attributes Y => \&Y::x, "locked";

=item 5.

Code:

    package X;
    sub foo { 1 }

    package Y;
    BEGIN { *bar = \&X::foo; }

    package Z;
    sub Y::bar : locked ;

Effect:

    use attributes X => \&X::foo, "locked";

=back

This last example is purely for purposes of completeness.  You should not
be trying to mess with the attributes of something in a package that's
not your own.

=head1 SEE ALSO

L<perlsub/"Private Variables via my()"> and
L<perlsub/"Subroutine Attributes"> for details on the basic declarations;
L<attrs> for the obsolescent form of subroutine attribute specification
which this module replaces;
L<perlfunc/use> for details on the normal invocation mechanism.

=cut

Revision:	1.1
Committed:	Thu Jun 30 14:27:08 2005 UTC (21 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	package attributes;
2
3			our $VERSION = 0.06;
4
5			@EXPORT_OK = qw(get reftype);
6			@EXPORT = ();
7			%EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]);
8
9			use strict;
10
11			sub croak {
12			require Carp;
13			goto &Carp::croak;
14			}
15
16			sub carp {
17			require Carp;
18			goto &Carp::carp;
19			}
20
21			## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{}
22			#sub reftype ($) ;
23			#sub _fetch_attrs ($) ;
24			#sub _guess_stash ($) ;
25			#sub _modify_attrs ;
26			#sub _warn_reserved () ;
27			#
28			# The extra trips through newATTRSUB in the interpreter wipe out any savings
29			# from avoiding the BEGIN block. Just do the bootstrap now.
30			BEGIN { bootstrap attributes }
31
32			sub import {
33			@_ > 2 && ref $_[2] or do {
34			require Exporter;
35			goto &Exporter::import;
36			};
37			my (undef,$home_stash,$svref,@attrs) = @_;
38
39			my $svtype = uc reftype($svref);
40			my $pkgmeth;
41			$pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES")
42			if defined $home_stash && $home_stash ne '';
43			my @badattrs;
44			if ($pkgmeth) {
45			my @pkgattrs = _modify_attrs($svref, @attrs);
46			@badattrs = $pkgmeth->($home_stash, $svref, @attrs);
47			if (!@badattrs && @pkgattrs) {
48			return unless _warn_reserved;
49			@pkgattrs = grep { m/\A[[:lower:]]+(?:\z\|\()/ } @pkgattrs;
50			if (@pkgattrs) {
51			for my $attr (@pkgattrs) {
52			$attr =~ s/\(.+\z//s;
53			}
54			my $s = ((@pkgattrs == 1) ? '' : 's');
55			carp "$svtype package attribute$s " .
56			"may clash with future reserved word$s: " .
57			join(' : ' , @pkgattrs);
58			}
59			}
60			}
61			else {
62			@badattrs = _modify_attrs($svref, @attrs);
63			}
64			if (@badattrs) {
65			croak "Invalid $svtype attribute" .
66			(( @badattrs == 1 ) ? '' : 's') .
67			": " .
68			join(' : ', @badattrs);
69			}
70			}
71
72			sub get ($) {
73			@_ == 1 && ref $_[0] or
74			croak 'Usage: '.__PACKAGE__.'::get $ref';
75			my $svref = shift;
76			my $svtype = uc reftype $svref;
77			my $stash = _guess_stash $svref;
78			$stash = caller unless defined $stash;
79			my $pkgmeth;
80			$pkgmeth = UNIVERSAL::can($stash, "FETCH_${svtype}_ATTRIBUTES")
81			if defined $stash && $stash ne '';
82			return $pkgmeth ?
83			(_fetch_attrs($svref), $pkgmeth->($stash, $svref)) :
84			(_fetch_attrs($svref))
85			;
86			}
87
88			sub require_version { goto &UNIVERSAL::VERSION }
89
90			1;
91			__END__
92			#The POD goes here
93
94			=head1 NAME
95
96			attributes - get/set subroutine or variable attributes
97
98			=head1 SYNOPSIS
99
100			sub foo : method ;
101			my ($x,@y,%z) : Bent = 1;
102			my $s = sub : method { ... };
103
104			use attributes (); # optional, to get subroutine declarations
105			my @attrlist = attributes::get(\&foo);
106
107			use attributes 'get'; # import the attributes::get subroutine
108			my @attrlist = get \&foo;
109
110			=head1 DESCRIPTION
111
112			Subroutine declarations and definitions may optionally have attribute lists
113			associated with them. (Variable C<my> declarations also may, but see the
114			warning below.) Perl handles these declarations by passing some information
115			about the call site and the thing being declared along with the attribute
116			list to this module. In particular, the first example above is equivalent to
117			the following:
118
119			use attributes __PACKAGE__, \&foo, 'method';
120
121			The second example in the synopsis does something equivalent to this:
122
123			use attributes ();
124			my ($x,@y,%z);
125			attributes::->import(__PACKAGE__, \$x, 'Bent');
126			attributes::->import(__PACKAGE__, \@y, 'Bent');
127			attributes::->import(__PACKAGE__, \%z, 'Bent');
128			($x,@y,%z) = 1;
129
130			Yes, that's a lot of expansion.
131
132			B<WARNING>: attribute declarations for variables are still evolving.
133			The semantics and interfaces of such declarations could change in
134			future versions. They are present for purposes of experimentation
135			with what the semantics ought to be. Do not rely on the current
136			implementation of this feature.
137
138			There are only a few attributes currently handled by Perl itself (or
139			directly by this module, depending on how you look at it.) However,
140			package-specific attributes are allowed by an extension mechanism.
141			(See L<"Package-specific Attribute Handling"> below.)
142
143			The setting of subroutine attributes happens at compile time.
144			Variable attributes in C<our> declarations are also applied at compile time.
145			However, C<my> variables get their attributes applied at run-time.
146			This means that you have to I<reach> the run-time component of the C<my>
147			before those attributes will get applied. For example:
148
149			my $x : Bent = 42 if 0;
150
151			will neither assign 42 to $x I<nor> will it apply the C<Bent> attribute
152			to the variable.
153
154			An attempt to set an unrecognized attribute is a fatal error. (The
155			error is trappable, but it still stops the compilation within that
156			C<eval>.) Setting an attribute with a name that's all lowercase
157			letters that's not a built-in attribute (such as "foo") will result in
158			a warning with B<-w> or C<use warnings 'reserved'>.
159
160			=head2 Built-in Attributes
161
162			The following are the built-in attributes for subroutines:
163
164			=over 4
165
166			=item locked
167
168			B<5.005 threads only! The use of the "locked" attribute currently
169			only makes sense if you are using the deprecated "Perl 5.005 threads"
170			implementation of threads.>
171
172			Setting this attribute is only meaningful when the subroutine or
173			method is to be called by multiple threads. When set on a method
174			subroutine (i.e., one marked with the B<method> attribute below),
175			Perl ensures that any invocation of it implicitly locks its first
176			argument before execution. When set on a non-method subroutine,
177			Perl ensures that a lock is taken on the subroutine itself before
178			execution. The semantics of the lock are exactly those of one
179			explicitly taken with the C<lock> operator immediately after the
180			subroutine is entered.
181
182			=item method
183
184			Indicates that the referenced subroutine is a method.
185			This has a meaning when taken together with the B<locked> attribute,
186			as described there. It also means that a subroutine so marked
187			will not trigger the "Ambiguous call resolved as CORE::%s" warning.
188
189			=item lvalue
190
191			Indicates that the referenced subroutine is a valid lvalue and can
192			be assigned to. The subroutine must return a modifiable value such
193			as a scalar variable, as described in L<perlsub>.
194
195			=back
196
197			For global variables there is C<unique> attribute: see L<perlfunc/our>.
198
199			=head2 Available Subroutines
200
201			The following subroutines are available for general use once this module
202			has been loaded:
203
204			=over 4
205
206			=item get
207
208			This routine expects a single parameter--a reference to a
209			subroutine or variable. It returns a list of attributes, which may be
210			empty. If passed invalid arguments, it uses die() (via L<Carp::croak\|Carp>)
211			to raise a fatal exception. If it can find an appropriate package name
212			for a class method lookup, it will include the results from a
213			C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in
214			L<"Package-specific Attribute Handling"> below.
215			Otherwise, only L<built-in attributes\|"Built-in Attributes"> will be returned.
216
217			=item reftype
218
219			This routine expects a single parameter--a reference to a subroutine or
220			variable. It returns the built-in type of the referenced variable,
221			ignoring any package into which it might have been blessed.
222			This can be useful for determining the I<type> value which forms part of
223			the method names described in L<"Package-specific Attribute Handling"> below.
224
225			=back
226
227			Note that these routines are I<not> exported by default.
228
229			=head2 Package-specific Attribute Handling
230
231			B<WARNING>: the mechanisms described here are still experimental. Do not
232			rely on the current implementation. In particular, there is no provision
233			for applying package attributes to 'cloned' copies of subroutines used as
234			closures. (See L<perlref/"Making References"> for information on closures.)
235			Package-specific attribute handling may change incompatibly in a future
236			release.
237
238			When an attribute list is present in a declaration, a check is made to see
239			whether an attribute 'modify' handler is present in the appropriate package
240			(or its @ISA inheritance tree). Similarly, when C<attributes::get> is
241			called on a valid reference, a check is made for an appropriate attribute
242			'fetch' handler. See L<"EXAMPLES"> to see how the "appropriate package"
243			determination works.
244
245			The handler names are based on the underlying type of the variable being
246			declared or of the reference passed. Because these attributes are
247			associated with subroutine or variable declarations, this deliberately
248			ignores any possibility of being blessed into some package. Thus, a
249			subroutine declaration uses "CODE" as its I<type>, and even a blessed
250			hash reference uses "HASH" as its I<type>.
251
252			The class methods invoked for modifying and fetching are these:
253
254			=over 4
255
256			=item FETCH_I<type>_ATTRIBUTES
257
258			This method receives a single argument, which is a reference to the
259			variable or subroutine for which package-defined attributes are desired.
260			The expected return value is a list of associated attributes.
261			This list may be empty.
262
263			=item MODIFY_I<type>_ATTRIBUTES
264
265			This method is called with two fixed arguments, followed by the list of
266			attributes from the relevant declaration. The two fixed arguments are
267			the relevant package name and a reference to the declared subroutine or
268			variable. The expected return value is a list of attributes which were
269			not recognized by this handler. Note that this allows for a derived class
270			to delegate a call to its base class, and then only examine the attributes
271			which the base class didn't already handle for it.
272
273			The call to this method is currently made I<during> the processing of the
274			declaration. In particular, this means that a subroutine reference will
275			probably be for an undefined subroutine, even if this declaration is
276			actually part of the definition.
277
278			=back
279
280			Calling C<attributes::get()> from within the scope of a null package
281			declaration C<package ;> for an unblessed variable reference will
282			not provide any starting package name for the 'fetch' method lookup.
283			Thus, this circumstance will not result in a method call for package-defined
284			attributes. A named subroutine knows to which symbol table entry it belongs
285			(or originally belonged), and it will use the corresponding package.
286			An anonymous subroutine knows the package name into which it was compiled
287			(unless it was also compiled with a null package declaration), and so it
288			will use that package name.
289
290			=head2 Syntax of Attribute Lists
291
292			An attribute list is a sequence of attribute specifications, separated by
293			whitespace or a colon (with optional whitespace).
294			Each attribute specification is a simple
295			name, optionally followed by a parenthesised parameter list.
296			If such a parameter list is present, it is scanned past as for the rules
297			for the C<q()> operator. (See L<perlop/"Quote and Quote-like Operators">.)
298			The parameter list is passed as it was found, however, and not as per C<q()>.
299
300			Some examples of syntactically valid attribute lists:
301
302			switch(10,foo(7,3)) : expensive
303			Ugly('\(") :Bad
304			_5x5
305			locked method
306
307			Some examples of syntactically invalid attribute lists (with annotation):
308
309			switch(10,foo() # ()-string not balanced
310			Ugly('(') # ()-string not balanced
311			5x5 # "5x5" not a valid identifier
312			Y2::north # "Y2::north" not a simple identifier
313			foo + bar # "+" neither a colon nor whitespace
314
315			=head1 EXPORTS
316
317			=head2 Default exports
318
319			None.
320
321			=head2 Available exports
322
323			The routines C<get> and C<reftype> are exportable.
324
325			=head2 Export tags defined
326
327			The C<:ALL> tag will get all of the above exports.
328
329			=head1 EXAMPLES
330
331			Here are some samples of syntactically valid declarations, with annotation
332			as to how they resolve internally into C<use attributes> invocations by
333			perl. These examples are primarily useful to see how the "appropriate
334			package" is found for the possible method lookups for package-defined
335			attributes.
336
337			=over 4
338
339			=item 1.
340
341			Code:
342
343			package Canine;
344			package Dog;
345			my Canine $spot : Watchful ;
346
347			Effect:
348
349			use attributes ();
350			attributes::->import(Canine => \$spot, "Watchful");
351
352			=item 2.
353
354			Code:
355
356			package Felis;
357			my $cat : Nervous;
358
359			Effect:
360
361			use attributes ();
362			attributes::->import(Felis => \$cat, "Nervous");
363
364			=item 3.
365
366			Code:
367
368			package X;
369			sub foo : locked ;
370
371			Effect:
372
373			use attributes X => \&foo, "locked";
374
375			=item 4.
376
377			Code:
378
379			package X;
380			sub Y::x : locked { 1 }
381
382			Effect:
383
384			use attributes Y => \&Y::x, "locked";
385
386			=item 5.
387
388			Code:
389
390			package X;
391			sub foo { 1 }
392
393			package Y;
394			BEGIN { *bar = \&X::foo; }
395
396			package Z;
397			sub Y::bar : locked ;
398
399			Effect:
400
401			use attributes X => \&X::foo, "locked";
402
403			=back
404
405			This last example is purely for purposes of completeness. You should not
406			be trying to mess with the attributes of something in a package that's
407			not your own.
408
409			=head1 SEE ALSO
410
411			L<perlsub/"Private Variables via my()"> and
412			L<perlsub/"Subroutine Attributes"> for details on the basic declarations;
413			L<attrs> for the obsolescent form of subroutine attribute specification
414			which this module replaces;
415			L<perlfunc/use> for details on the normal invocation mechanism.
416
417			=cut
418