1attributes(3pm) Perl Programmers Reference Guide attributes(3pm)
2
6 attributes - get/set subroutine or variable attributes
7
9 sub foo : method ;
10 my ($x,@y,%z) : Bent = 1;
11 my $s = sub : method { ... };
12 use attributes (); # optional, to get subroutine declarations
14 my @attrlist = attributes::get(\&foo);
15 use attributes 'get'; # import the attributes::get subroutine
17 my @attrlist = get \&foo;
18
20 Subroutine declarations and definitions may optionally have attribute
21 lists associated with them. (Variable "my" declarations also may, but
22 see the warning below.) Perl handles these declarations by passing
23 some information about the call site and the thing being declared along
24 with the attribute list to this module. In particular, the first
25 example above is equivalent to the following:
26 use attributes __PACKAGE__, \&foo, 'method';
28 The second example in the synopsis does something equivalent to this:
30 use attributes ();
32 my ($x,@y,%z);
33 attributes::->import(__PACKAGE__, \$x, 'Bent');
34 attributes::->import(__PACKAGE__, \@y, 'Bent');
35 attributes::->import(__PACKAGE__, \%z, 'Bent');
36 ($x,@y,%z) = 1;
37 Yes, that's a lot of expansion.
39 WARNING: attribute declarations for variables are still evolving. The
41 semantics and interfaces of such declarations could change in future
42 versions. They are present for purposes of experimentation with what
43 the semantics ought to be. Do not rely on the current implementation
44 of this feature.
45 There are only a few attributes currently handled by Perl itself (or
47 directly by this module, depending on how you look at it.) However,
48 package-specific attributes are allowed by an extension mechanism.
49 (See "Package-specific Attribute Handling" below.)
50 The setting of subroutine attributes happens at compile time. Variable
52 attributes in "our" declarations are also applied at compile time.
53 However, "my" variables get their attributes applied at run-time. This
54 means that you have to reach the run-time component of the "my" before
55 those attributes will get applied. For example:
56 my $x : Bent = 42 if 0;
58 will neither assign 42 to $x nor will it apply the "Bent" attribute to
60 the variable.
61 An attempt to set an unrecognized attribute is a fatal error. (The
63 error is trappable, but it still stops the compilation within that
64 "eval".) Setting an attribute with a name that's all lowercase letters
65 that's not a built-in attribute (such as "foo") will result in a
66 warning with -w or "use warnings 'reserved'".
67 What "import" does
69 In the description it is mentioned that
70 sub foo : method;
72 is equivalent to
74 use attributes __PACKAGE__, \&foo, 'method';
76 As you might know this calls the "import" function of "attributes" at
78 compile time with these parameters: 'attributes', the caller's package
79 name, the reference to the code and 'method'.
80 attributes->import( __PACKAGE__, \&foo, 'method' );
82 So you want to know what "import" actually does?
84 First of all "import" gets the type of the third parameter ('CODE' in
86 this case). "attributes.pm" checks if there is a subroutine called
87 "MODIFY_<reftype>_ATTRIBUTES" in the caller's namespace (here: 'main').
88 In this case a subroutine "MODIFY_CODE_ATTRIBUTES" is required. Then
89 this method is called to check if you have used a "bad attribute". The
90 subroutine call in this example would look like
91 MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' );
93 "MODIFY_<reftype>_ATTRIBUTES" has to return a list of all "bad
95 attributes". If there are any bad attributes "import" croaks.
96 (See "Package-specific Attribute Handling" below.)
98 Built-in Attributes
100 The following are the built-in attributes for subroutines:
101 lvalue
103 Indicates that the referenced subroutine is a valid lvalue and can
104 be assigned to. The subroutine must return a modifiable value such
105 as a scalar variable, as described in perlsub.
106 This module allows one to set this attribute on a subroutine that
108 is already defined. For Perl subroutines (XSUBs are fine), it may
109 or may not do what you want, depending on the code inside the
110 subroutine, with details subject to change in future Perl versions.
111 You may run into problems with lvalue context not being propagated
112 properly into the subroutine, or maybe even assertion failures.
113 For this reason, a warning is emitted if warnings are enabled. In
114 other words, you should only do this if you really know what you
115 are doing. You have been warned.
116 method
118 Indicates that the referenced subroutine is a method. A subroutine
119 so marked will not trigger the "Ambiguous call resolved as
120 CORE::%s" warning.
121 prototype(..)
123 The "prototype" attribute is an alternate means of specifying a
124 prototype on a sub. The desired prototype is within the parens.
125 The prototype from the attribute is assigned to the sub immediately
127 after the prototype from the sub, which means that if both are
128 declared at the same time, the traditionally defined prototype is
129 ignored. In other words, "sub foo($$) : prototype(@) {}" is
130 indistinguishable from "sub foo(@){}".
131 If illegalproto warnings are enabled, the prototype declared inside
133 this attribute will be sanity checked at compile time.
134 locked
136 The "locked" attribute is deprecated, and has no effect in 5.10.0
137 and later. It was used as part of the now-removed "Perl 5.005
138 threads". It will disappear in Perl 5.28, after which its use will
139 be fatal.
140 const
142 This experimental attribute, introduced in Perl 5.22, only applies
143 to anonymous subroutines. It causes the subroutine to be called as
144 soon as the "sub" expression is evaluated. The return value is
145 captured and turned into a constant subroutine.
146 The following are the built-in attributes for variables:
148 shared
150 Indicates that the referenced variable can be shared across
151 different threads when used in conjunction with the threads and
152 threads::shared modules.
153 unique
155 The "unique" attribute is deprecated, and has no effect in 5.10.0
156 and later. It used to indicate that a single copy of an "our"
157 variable was to be used by all interpreters should the program
158 happen to be running in a multi-interpreter environment. It will
159 disappear in 5.28, after which its use will be fatal.
160 Available Subroutines
162 The following subroutines are available for general use once this
163 module has been loaded:
164 get This routine expects a single parameter--a reference to a
166 subroutine or variable. It returns a list of attributes, which may
167 be empty. If passed invalid arguments, it uses die() (via
168 Carp::croak) to raise a fatal exception. If it can find an
169 appropriate package name for a class method lookup, it will include
170 the results from a "FETCH_type_ATTRIBUTES" call in its return list,
171 as described in "Package-specific Attribute Handling" below.
172 Otherwise, only built-in attributes will be returned.
173 reftype
175 This routine expects a single parameter--a reference to a
176 subroutine or variable. It returns the built-in type of the
177 referenced variable, ignoring any package into which it might have
178 been blessed. This can be useful for determining the type value
179 which forms part of the method names described in "Package-specific
180 Attribute Handling" below.
181 Note that these routines are not exported by default.
183 Package-specific Attribute Handling
185 WARNING: the mechanisms described here are still experimental. Do not
186 rely on the current implementation. In particular, there is no
187 provision for applying package attributes to 'cloned' copies of
188 subroutines used as closures. (See "Making References" in perlref for
189 information on closures.) Package-specific attribute handling may
190 change incompatibly in a future release.
191 When an attribute list is present in a declaration, a check is made to
193 see whether an attribute 'modify' handler is present in the appropriate
194 package (or its @ISA inheritance tree). Similarly, when
195 "attributes::get" is called on a valid reference, a check is made for
196 an appropriate attribute 'fetch' handler. See "EXAMPLES" to see how
197 the "appropriate package" determination works.
198 The handler names are based on the underlying type of the variable
200 being declared or of the reference passed. Because these attributes
201 are associated with subroutine or variable declarations, this
202 deliberately ignores any possibility of being blessed into some
203 package. Thus, a subroutine declaration uses "CODE" as its type, and
204 even a blessed hash reference uses "HASH" as its type.
205 The class methods invoked for modifying and fetching are these:
207 FETCH_type_ATTRIBUTES
209 This method is called with two arguments: the relevant package
210 name, and a reference to a variable or subroutine for which
211 package-defined attributes are desired. The expected return value
212 is a list of associated attributes. This list may be empty.
213 MODIFY_type_ATTRIBUTES
215 This method is called with two fixed arguments, followed by the
216 list of attributes from the relevant declaration. The two fixed
217 arguments are the relevant package name and a reference to the
218 declared subroutine or variable. The expected return value is a
219 list of attributes which were not recognized by this handler. Note
220 that this allows for a derived class to delegate a call to its base
221 class, and then only examine the attributes which the base class
222 didn't already handle for it.
223 The call to this method is currently made during the processing of
225 the declaration. In particular, this means that a subroutine
226 reference will probably be for an undefined subroutine, even if
227 this declaration is actually part of the definition.
228 Calling "attributes::get()" from within the scope of a null package
230 declaration "package ;" for an unblessed variable reference will not
231 provide any starting package name for the 'fetch' method lookup. Thus,
232 this circumstance will not result in a method call for package-defined
233 attributes. A named subroutine knows to which symbol table entry it
234 belongs (or originally belonged), and it will use the corresponding
235 package. An anonymous subroutine knows the package name into which it
236 was compiled (unless it was also compiled with a null package
237 declaration), and so it will use that package name.
238 Syntax of Attribute Lists
240 An attribute list is a sequence of attribute specifications, separated
241 by whitespace or a colon (with optional whitespace). Each attribute
242 specification is a simple name, optionally followed by a parenthesised
243 parameter list. If such a parameter list is present, it is scanned
244 past as for the rules for the "q()" operator. (See "Quote and Quote-
245 like Operators" in perlop.) The parameter list is passed as it was
246 found, however, and not as per "q()".
247 Some examples of syntactically valid attribute lists:
249 switch(10,foo(7,3)) : expensive
251 Ugly('\(") :Bad
252 _5x5
253 lvalue method
254 Some examples of syntactically invalid attribute lists (with
256 annotation):
257 switch(10,foo() # ()-string not balanced
259 Ugly('(') # ()-string not balanced
260 5x5 # "5x5" not a valid identifier
261 Y2::north # "Y2::north" not a simple identifier
262 foo + bar # "+" neither a colon nor whitespace
263
265 Default exports
266 None.
267 Available exports
269 The routines "get" and "reftype" are exportable.
270 Export tags defined
272 The ":ALL" tag will get all of the above exports.
273
275 Here are some samples of syntactically valid declarations, with
276 annotation as to how they resolve internally into "use attributes"
277 invocations by perl. These examples are primarily useful to see how
278 the "appropriate package" is found for the possible method lookups for
279 package-defined attributes.
280 1. Code:
282 package Canine;
284 package Dog;
285 my Canine $spot : Watchful ;
286 Effect:
288 use attributes ();
290 attributes::->import(Canine => \$spot, "Watchful");
291 2. Code:
293 package Felis;
295 my $cat : Nervous;
296 Effect:
298 use attributes ();
300 attributes::->import(Felis => \$cat, "Nervous");
301 3. Code:
303 package X;
305 sub foo : lvalue ;
306 Effect:
308 use attributes X => \&foo, "lvalue";
310 4. Code:
312 package X;
314 sub Y::x : lvalue { 1 }
315 Effect:
317 use attributes Y => \&Y::x, "lvalue";
319 5. Code:
321 package X;
323 sub foo { 1 }
324 package Y;
326 BEGIN { *bar = \&X::foo; }
327 package Z;
329 sub Y::bar : lvalue ;
330 Effect:
332 use attributes X => \&X::foo, "lvalue";
334 This last example is purely for purposes of completeness. You should
336 not be trying to mess with the attributes of something in a package
337 that's not your own.
338
340 1.
341 sub MODIFY_CODE_ATTRIBUTES {
342 my ($class,$code,@attrs) = @_;
343 my $allowed = 'MyAttribute';
345 my @bad = grep { $_ ne $allowed } @attrs;
346 return @bad;
348 }
349 sub foo : MyAttribute {
351 print "foo\n";
352 }
353 This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is
355 called. In that subroutine, we check if any attribute is
356 disallowed and we return a list of these "bad attributes".
357 As we return an empty list, everything is fine.
359 2.
361 sub MODIFY_CODE_ATTRIBUTES {
362 my ($class,$code,@attrs) = @_;
363 my $allowed = 'MyAttribute';
365 my @bad = grep{ $_ ne $allowed }@attrs;
366 return @bad;
368 }
369 sub foo : MyAttribute Test {
371 print "foo\n";
372 }
373 This example is aborted at compile time as we use the attribute
375 "Test" which isn't allowed. "MODIFY_CODE_ATTRIBUTES" returns a
376 list that contains a single element ('Test').
377
379 "Private Variables via my()" in perlsub and "Subroutine Attributes" in
380 perlsub for details on the basic declarations; "use" in perlfunc for
381 details on the normal invocation mechanism.
382perl v5.26.3 2018-03-23 attributes(3pm)