5 @EXPORT_OK = qw(get reftype);
7 %EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]);
21 ## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{}
23 #sub _fetch_attrs ($) ;
24 #sub _guess_stash ($) ;
26 #sub _warn_reserved () ;
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 }
33 @_ > 2 && ref $_[2] or do {
35 goto &Exporter::import;
37 my (undef,$home_stash,$svref,@attrs) = @_;
39 my $svtype = uc reftype($svref);
41 $pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES")
42 if defined $home_stash && $home_stash ne '';
45 my @pkgattrs = _modify_attrs($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;
51 for my $attr (@pkgattrs) {
54 my $s = ((@pkgattrs == 1) ? '' : 's');
55 carp "$svtype package attribute$s " .
56 "may clash with future reserved word$s: " .
57 join(' : ' , @pkgattrs);
62 @badattrs = _modify_attrs($svref, @attrs);
65 croak "Invalid $svtype attribute" .
66 (( @badattrs == 1 ) ? '' : 's') .
68 join(' : ', @badattrs);
73 @_ == 1 && ref $_[0] or
74 croak 'Usage: '.__PACKAGE__.'::get $ref';
76 my $svtype = uc reftype $svref;
77 my $stash = _guess_stash $svref;
78 $stash = caller unless defined $stash;
80 $pkgmeth = UNIVERSAL::can($stash, "FETCH_${svtype}_ATTRIBUTES")
81 if defined $stash && $stash ne '';
83 (_fetch_attrs($svref), $pkgmeth->($stash, $svref)) :
84 (_fetch_attrs($svref))
88 sub require_version { goto &UNIVERSAL::VERSION }
96 attributes - get/set subroutine or variable attributes
101 my ($x,@y,%z) : Bent ;
102 my $s = sub : method { ... };
104 use attributes (); # optional, to get subroutine declarations
105 my @attrlist = attributes::get(\&foo);
107 use attributes 'get'; # import the attributes::get subroutine
108 my @attrlist = get \&foo;
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
119 use attributes __PACKAGE__, \&foo, 'method';
121 The second example in the synopsis does something equivalent to this:
123 use attributes __PACKAGE__, \$x, 'Bent';
124 use attributes __PACKAGE__, \@y, 'Bent';
125 use attributes __PACKAGE__, \%z, 'Bent';
127 Yes, that's three invocations.
129 B<WARNING>: attribute declarations for variables are an I<experimental>
130 feature. The semantics of such declarations could change or be removed
131 in future versions. They are present for purposes of experimentation
132 with what the semantics ought to be. Do not rely on the current
133 implementation of this feature. Variable attributes are currently
134 not usable for tieing.
136 There are only a few attributes currently handled by Perl itself (or
137 directly by this module, depending on how you look at it.) However,
138 package-specific attributes are allowed by an extension mechanism.
139 (See L<"Package-specific Attribute Handling"> below.)
141 The setting of attributes happens at compile time. An attempt to set
142 an unrecognized attribute is a fatal error. (The error is trappable, but
143 it still stops the compilation within that C<eval>.) Setting an attribute
144 with a name that's all lowercase letters that's not a built-in attribute
146 will result in a warning with B<-w> or C<use warnings 'reserved'>.
148 =head2 Built-in Attributes
150 The following are the built-in attributes for subroutines:
156 Setting this attribute is only meaningful when the subroutine or
157 method is to be called by multiple threads. When set on a method
158 subroutine (i.e., one marked with the B<method> attribute below),
159 Perl ensures that any invocation of it implicitly locks its first
160 argument before execution. When set on a non-method subroutine,
161 Perl ensures that a lock is taken on the subroutine itself before
162 execution. The semantics of the lock are exactly those of one
163 explicitly taken with the C<lock> operator immediately after the
164 subroutine is entered.
168 Indicates that the referenced subroutine is a method.
169 This has a meaning when taken together with the B<locked> attribute,
170 as described there. It also means that a subroutine so marked
171 will not trigger the "Ambiguous call resolved as CORE::%s" warning.
175 Indicates that the referenced subroutine is a valid lvalue and can
176 be assigned to. The subroutine must return a modifiable value such
177 as a scalar variable, as described in L<perlsub>.
181 There are no built-in attributes for anything other than subroutines.
183 =head2 Available Subroutines
185 The following subroutines are available for general use once this module
192 This routine expects a single parameter--a reference to a
193 subroutine or variable. It returns a list of attributes, which may be
194 empty. If passed invalid arguments, it uses die() (via L<Carp::croak|Carp>)
195 to raise a fatal exception. If it can find an appropriate package name
196 for a class method lookup, it will include the results from a
197 C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in
198 L<"Package-specific Attribute Handling"> below.
199 Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned.
203 This routine expects a single parameter--a reference to a subroutine or
204 variable. It returns the built-in type of the referenced variable,
205 ignoring any package into which it might have been blessed.
206 This can be useful for determining the I<type> value which forms part of
207 the method names described in L<"Package-specific Attribute Handling"> below.
211 Note that these routines are I<not> exported by default.
213 =head2 Package-specific Attribute Handling
215 B<WARNING>: the mechanisms described here are still experimental. Do not
216 rely on the current implementation. In particular, there is no provision
217 for applying package attributes to 'cloned' copies of subroutines used as
218 closures. (See L<perlref/"Making References"> for information on closures.)
219 Package-specific attribute handling may change incompatibly in a future
222 When an attribute list is present in a declaration, a check is made to see
223 whether an attribute 'modify' handler is present in the appropriate package
224 (or its @ISA inheritance tree). Similarly, when C<attributes::get> is
225 called on a valid reference, a check is made for an appropriate attribute
226 'fetch' handler. See L<"EXAMPLES"> to see how the "appropriate package"
229 The handler names are based on the underlying type of the variable being
230 declared or of the reference passed. Because these attributes are
231 associated with subroutine or variable declarations, this deliberately
232 ignores any possibility of being blessed into some package. Thus, a
233 subroutine declaration uses "CODE" as its I<type>, and even a blessed
234 hash reference uses "HASH" as its I<type>.
236 The class methods invoked for modifying and fetching are these:
240 =item FETCH_I<type>_ATTRIBUTES
242 This method receives a single argument, which is a reference to the
243 variable or subroutine for which package-defined attributes are desired.
244 The expected return value is a list of associated attributes.
245 This list may be empty.
247 =item MODIFY_I<type>_ATTRIBUTES
249 This method is called with two fixed arguments, followed by the list of
250 attributes from the relevant declaration. The two fixed arguments are
251 the relevant package name and a reference to the declared subroutine or
252 variable. The expected return value as a list of attributes which were
253 not recognized by this handler. Note that this allows for a derived class
254 to delegate a call to its base class, and then only examine the attributes
255 which the base class didn't already handle for it.
257 The call to this method is currently made I<during> the processing of the
258 declaration. In particular, this means that a subroutine reference will
259 probably be for an undefined subroutine, even if this declaration is
260 actually part of the definition.
264 Calling C<attributes::get()> from within the scope of a null package
265 declaration C<package ;> for an unblessed variable reference will
266 not provide any starting package name for the 'fetch' method lookup.
267 Thus, this circumstance will not result in a method call for package-defined
268 attributes. A named subroutine knows to which symbol table entry it belongs
269 (or originally belonged), and it will use the corresponding package.
270 An anonymous subroutine knows the package name into which it was compiled
271 (unless it was also compiled with a null package declaration), and so it
272 will use that package name.
274 =head2 Syntax of Attribute Lists
276 An attribute list is a sequence of attribute specifications, separated by
277 whitespace or a colon (with optional whitespace).
278 Each attribute specification is a simple
279 name, optionally followed by a parenthesised parameter list.
280 If such a parameter list is present, it is scanned past as for the rules
281 for the C<q()> operator. (See L<perlop/"Quote and Quote-like Operators">.)
282 The parameter list is passed as it was found, however, and not as per C<q()>.
284 Some examples of syntactically valid attribute lists:
286 switch(10,foo(7,3)) : expensive
291 Some examples of syntactically invalid attribute lists (with annotation):
293 switch(10,foo() # ()-string not balanced
294 Ugly('(') # ()-string not balanced
295 5x5 # "5x5" not a valid identifier
296 Y2::north # "Y2::north" not a simple identifier
297 foo + bar # "+" neither a colon nor whitespace
301 =head2 Default exports
305 =head2 Available exports
307 The routines C<get> and C<reftype> are exportable.
309 =head2 Export tags defined
311 The C<:ALL> tag will get all of the above exports.
315 Here are some samples of syntactically valid declarations, with annotation
316 as to how they resolve internally into C<use attributes> invocations by
317 perl. These examples are primarily useful to see how the "appropriate
318 package" is found for the possible method lookups for package-defined
329 my Canine $spot : Watchful ;
333 use attributes Canine => \$spot, "Watchful";
344 use attributes Felis => \$cat, "Nervous";
355 use attributes X => \&foo, "locked";
362 sub Y::x : locked { 1 }
366 use attributes Y => \&Y::x, "locked";
376 BEGIN { *bar = \&X::foo; }
379 sub Y::bar : locked ;
383 use attributes X => \&X::foo, "locked";
387 This last example is purely for purposes of completeness. You should not
388 be trying to mess with the attributes of something in a package that's
393 L<perlsub/"Private Variables via my()"> and
394 L<perlsub/"Subroutine Attributes"> for details on the basic declarations;
395 L<attrs> for the obsolescent form of subroutine attribute specification
396 which this module replaces;
397 L<perlfunc/use> for details on the normal invocation mechanism.