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 const
136 This experimental attribute, introduced in Perl 5.22, only applies
137 to anonymous subroutines. It causes the subroutine to be called as
138 soon as the "sub" expression is evaluated. The return value is
139 captured and turned into a constant subroutine.
140 The following are the built-in attributes for variables:
142 shared
144 Indicates that the referenced variable can be shared across
145 different threads when used in conjunction with the threads and
146 threads::shared modules.
147 Available Subroutines
149 The following subroutines are available for general use once this
150 module has been loaded:
151 get This routine expects a single parameter--a reference to a
153 subroutine or variable. It returns a list of attributes, which may
154 be empty. If passed invalid arguments, it uses die() (via
155 Carp::croak) to raise a fatal exception. If it can find an
156 appropriate package name for a class method lookup, it will include
157 the results from a "FETCH_type_ATTRIBUTES" call in its return list,
158 as described in "Package-specific Attribute Handling" below.
159 Otherwise, only built-in attributes will be returned.
160 reftype
162 This routine expects a single parameter--a reference to a
163 subroutine or variable. It returns the built-in type of the
164 referenced variable, ignoring any package into which it might have
165 been blessed. This can be useful for determining the type value
166 which forms part of the method names described in "Package-specific
167 Attribute Handling" below.
168 Note that these routines are not exported by default.
170 Package-specific Attribute Handling
172 WARNING: the mechanisms described here are still experimental. Do not
173 rely on the current implementation. In particular, there is no
174 provision for applying package attributes to 'cloned' copies of
175 subroutines used as closures. (See "Making References" in perlref for
176 information on closures.) Package-specific attribute handling may
177 change incompatibly in a future release.
178 When an attribute list is present in a declaration, a check is made to
180 see whether an attribute 'modify' handler is present in the appropriate
181 package (or its @ISA inheritance tree). Similarly, when
182 "attributes::get" is called on a valid reference, a check is made for
183 an appropriate attribute 'fetch' handler. See "EXAMPLES" to see how
184 the "appropriate package" determination works.
185 The handler names are based on the underlying type of the variable
187 being declared or of the reference passed. Because these attributes
188 are associated with subroutine or variable declarations, this
189 deliberately ignores any possibility of being blessed into some
190 package. Thus, a subroutine declaration uses "CODE" as its type, and
191 even a blessed hash reference uses "HASH" as its type.
192 The class methods invoked for modifying and fetching are these:
194 FETCH_type_ATTRIBUTES
196 This method is called with two arguments: the relevant package
197 name, and a reference to a variable or subroutine for which
198 package-defined attributes are desired. The expected return value
199 is a list of associated attributes. This list may be empty.
200 MODIFY_type_ATTRIBUTES
202 This method is called with two fixed arguments, followed by the
203 list of attributes from the relevant declaration. The two fixed
204 arguments are the relevant package name and a reference to the
205 declared subroutine or variable. The expected return value is a
206 list of attributes which were not recognized by this handler. Note
207 that this allows for a derived class to delegate a call to its base
208 class, and then only examine the attributes which the base class
209 didn't already handle for it.
210 The call to this method is currently made during the processing of
212 the declaration. In particular, this means that a subroutine
213 reference will probably be for an undefined subroutine, even if
214 this declaration is actually part of the definition.
215 Calling "attributes::get()" from within the scope of a null package
217 declaration "package ;" for an unblessed variable reference will not
218 provide any starting package name for the 'fetch' method lookup. Thus,
219 this circumstance will not result in a method call for package-defined
220 attributes. A named subroutine knows to which symbol table entry it
221 belongs (or originally belonged), and it will use the corresponding
222 package. An anonymous subroutine knows the package name into which it
223 was compiled (unless it was also compiled with a null package
224 declaration), and so it will use that package name.
225 Syntax of Attribute Lists
227 An attribute list is a sequence of attribute specifications, separated
228 by whitespace or a colon (with optional whitespace). Each attribute
229 specification is a simple name, optionally followed by a parenthesised
230 parameter list. If such a parameter list is present, it is scanned
231 past as for the rules for the "q()" operator. (See "Quote and Quote-
232 like Operators" in perlop.) The parameter list is passed as it was
233 found, however, and not as per "q()".
234 Some examples of syntactically valid attribute lists:
236 switch(10,foo(7,3)) : expensive
238 Ugly('\(") :Bad
239 _5x5
240 lvalue method
241 Some examples of syntactically invalid attribute lists (with
243 annotation):
244 switch(10,foo() # ()-string not balanced
246 Ugly('(') # ()-string not balanced
247 5x5 # "5x5" not a valid identifier
248 Y2::north # "Y2::north" not a simple identifier
249 foo + bar # "+" neither a colon nor whitespace
250
252 Default exports
253 None.
254 Available exports
256 The routines "get" and "reftype" are exportable.
257 Export tags defined
259 The ":ALL" tag will get all of the above exports.
260
262 Here are some samples of syntactically valid declarations, with
263 annotation as to how they resolve internally into "use attributes"
264 invocations by perl. These examples are primarily useful to see how
265 the "appropriate package" is found for the possible method lookups for
266 package-defined attributes.
267 1. Code:
269 package Canine;
271 package Dog;
272 my Canine $spot : Watchful ;
273 Effect:
275 use attributes ();
277 attributes::->import(Canine => \$spot, "Watchful");
278 2. Code:
280 package Felis;
282 my $cat : Nervous;
283 Effect:
285 use attributes ();
287 attributes::->import(Felis => \$cat, "Nervous");
288 3. Code:
290 package X;
292 sub foo : lvalue ;
293 Effect:
295 use attributes X => \&foo, "lvalue";
297 4. Code:
299 package X;
301 sub Y::x : lvalue { 1 }
302 Effect:
304 use attributes Y => \&Y::x, "lvalue";
306 5. Code:
308 package X;
310 sub foo { 1 }
311 package Y;
313 BEGIN { *bar = \&X::foo; }
314 package Z;
316 sub Y::bar : lvalue ;
317 Effect:
319 use attributes X => \&X::foo, "lvalue";
321 This last example is purely for purposes of completeness. You should
323 not be trying to mess with the attributes of something in a package
324 that's not your own.
325
327 1.
328 sub MODIFY_CODE_ATTRIBUTES {
329 my ($class,$code,@attrs) = @_;
330 my $allowed = 'MyAttribute';
332 my @bad = grep { $_ ne $allowed } @attrs;
333 return @bad;
335 }
336 sub foo : MyAttribute {
338 print "foo\n";
339 }
340 This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is
342 called. In that subroutine, we check if any attribute is
343 disallowed and we return a list of these "bad attributes".
344 As we return an empty list, everything is fine.
346 2.
348 sub MODIFY_CODE_ATTRIBUTES {
349 my ($class,$code,@attrs) = @_;
350 my $allowed = 'MyAttribute';
352 my @bad = grep{ $_ ne $allowed }@attrs;
353 return @bad;
355 }
356 sub foo : MyAttribute Test {
358 print "foo\n";
359 }
360 This example is aborted at compile time as we use the attribute
362 "Test" which isn't allowed. "MODIFY_CODE_ATTRIBUTES" returns a
363 list that contains a single element ('Test').
364
366 "Private Variables via my()" in perlsub and "Subroutine Attributes" in
367 perlsub for details on the basic declarations; "use" in perlfunc for
368 details on the normal invocation mechanism.
369perl v5.30.2 2020-03-27 attributes(3pm)