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 locked
123 The "locked" attribute has no effect in 5.10.0 and later. It was
124 used as part of the now-removed "Perl 5.005 threads".
125 Available Subroutines
127 The following subroutines are available for general use once this
128 module has been loaded:
129 get This routine expects a single parameter--a reference to a
131 subroutine or variable. It returns a list of attributes, which may
132 be empty. If passed invalid arguments, it uses die() (via
133 Carp::croak) to raise a fatal exception. If it can find an
134 appropriate package name for a class method lookup, it will include
135 the results from a "FETCH_type_ATTRIBUTES" call in its return list,
136 as described in "Package-specific Attribute Handling" below.
137 Otherwise, only built-in attributes will be returned.
138 reftype
140 This routine expects a single parameter--a reference to a
141 subroutine or variable. It returns the built-in type of the
142 referenced variable, ignoring any package into which it might have
143 been blessed. This can be useful for determining the type value
144 which forms part of the method names described in "Package-specific
145 Attribute Handling" below.
146 Note that these routines are not exported by default.
148 Package-specific Attribute Handling
150 WARNING: the mechanisms described here are still experimental. Do not
151 rely on the current implementation. In particular, there is no
152 provision for applying package attributes to 'cloned' copies of
153 subroutines used as closures. (See "Making References" in perlref for
154 information on closures.) Package-specific attribute handling may
155 change incompatibly in a future release.
156 When an attribute list is present in a declaration, a check is made to
158 see whether an attribute 'modify' handler is present in the appropriate
159 package (or its @ISA inheritance tree). Similarly, when
160 "attributes::get" is called on a valid reference, a check is made for
161 an appropriate attribute 'fetch' handler. See "EXAMPLES" to see how
162 the "appropriate package" determination works.
163 The handler names are based on the underlying type of the variable
165 being declared or of the reference passed. Because these attributes
166 are associated with subroutine or variable declarations, this
167 deliberately ignores any possibility of being blessed into some
168 package. Thus, a subroutine declaration uses "CODE" as its type, and
169 even a blessed hash reference uses "HASH" as its type.
170 The class methods invoked for modifying and fetching are these:
172 FETCH_type_ATTRIBUTES
174 This method is called with two arguments: the relevant package
175 name, and a reference to a variable or subroutine for which
176 package-defined attributes are desired. The expected return value
177 is a list of associated attributes. This list may be empty.
178 MODIFY_type_ATTRIBUTES
180 This method is called with two fixed arguments, followed by the
181 list of attributes from the relevant declaration. The two fixed
182 arguments are the relevant package name and a reference to the
183 declared subroutine or variable. The expected return value is a
184 list of attributes which were not recognized by this handler. Note
185 that this allows for a derived class to delegate a call to its base
186 class, and then only examine the attributes which the base class
187 didn't already handle for it.
188 The call to this method is currently made during the processing of
190 the declaration. In particular, this means that a subroutine
191 reference will probably be for an undefined subroutine, even if
192 this declaration is actually part of the definition.
193 Calling "attributes::get()" from within the scope of a null package
195 declaration "package ;" for an unblessed variable reference will not
196 provide any starting package name for the 'fetch' method lookup. Thus,
197 this circumstance will not result in a method call for package-defined
198 attributes. A named subroutine knows to which symbol table entry it
199 belongs (or originally belonged), and it will use the corresponding
200 package. An anonymous subroutine knows the package name into which it
201 was compiled (unless it was also compiled with a null package
202 declaration), and so it will use that package name.
203 Syntax of Attribute Lists
205 An attribute list is a sequence of attribute specifications, separated
206 by whitespace or a colon (with optional whitespace). Each attribute
207 specification is a simple name, optionally followed by a parenthesised
208 parameter list. If such a parameter list is present, it is scanned
209 past as for the rules for the "q()" operator. (See "Quote and Quote-
210 like Operators" in perlop.) The parameter list is passed as it was
211 found, however, and not as per "q()".
212 Some examples of syntactically valid attribute lists:
214 switch(10,foo(7,3)) : expensive
216 Ugly('\(") :Bad
217 _5x5
218 lvalue method
219 Some examples of syntactically invalid attribute lists (with
221 annotation):
222 switch(10,foo() # ()-string not balanced
224 Ugly('(') # ()-string not balanced
225 5x5 # "5x5" not a valid identifier
226 Y2::north # "Y2::north" not a simple identifier
227 foo + bar # "+" neither a colon nor whitespace
228
230 Default exports
231 None.
232 Available exports
234 The routines "get" and "reftype" are exportable.
235 Export tags defined
237 The ":ALL" tag will get all of the above exports.
238
240 Here are some samples of syntactically valid declarations, with
241 annotation as to how they resolve internally into "use attributes"
242 invocations by perl. These examples are primarily useful to see how
243 the "appropriate package" is found for the possible method lookups for
244 package-defined attributes.
245 1. Code:
247 package Canine;
249 package Dog;
250 my Canine $spot : Watchful ;
251 Effect:
253 use attributes ();
255 attributes::->import(Canine => \$spot, "Watchful");
256 2. Code:
258 package Felis;
260 my $cat : Nervous;
261 Effect:
263 use attributes ();
265 attributes::->import(Felis => \$cat, "Nervous");
266 3. Code:
268 package X;
270 sub foo : lvalue ;
271 Effect:
273 use attributes X => \&foo, "lvalue";
275 4. Code:
277 package X;
279 sub Y::x : lvalue { 1 }
280 Effect:
282 use attributes Y => \&Y::x, "lvalue";
284 5. Code:
286 package X;
288 sub foo { 1 }
289 package Y;
291 BEGIN { *bar = \&X::foo; }
292 package Z;
294 sub Y::bar : lvalue ;
295 Effect:
297 use attributes X => \&X::foo, "lvalue";
299 This last example is purely for purposes of completeness. You should
301 not be trying to mess with the attributes of something in a package
302 that's not your own.
303
305 1.
306 sub MODIFY_CODE_ATTRIBUTES {
307 my ($class,$code,@attrs) = @_;
308 my $allowed = 'MyAttribute';
310 my @bad = grep { $_ ne $allowed } @attrs;
311 return @bad;
313 }
314 sub foo : MyAttribute {
316 print "foo\n";
317 }
318 This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is
320 called. In that subroutine, we check if any attribute is
321 disallowed and we return a list of these "bad attributes".
322 As we return an empty list, everything is fine.
324 2.
326 sub MODIFY_CODE_ATTRIBUTES {
327 my ($class,$code,@attrs) = @_;
328 my $allowed = 'MyAttribute';
330 my @bad = grep{ $_ ne $allowed }@attrs;
331 return @bad;
333 }
334 sub foo : MyAttribute Test {
336 print "foo\n";
337 }
338 This example is aborted at compile time as we use the attribute
340 "Test" which isn't allowed. "MODIFY_CODE_ATTRIBUTES" returns a
341 list that contains a single element ('Test').
342
344 "Private Variables via my()" in perlsub and "Subroutine Attributes" in
345 perlsub for details on the basic declarations; "use" in perlfunc for
346 details on the normal invocation mechanism.
347perl v5.16.3 2013-03-04 attributes(3pm)