attributes(3pm)

1attributes(3pm)        Perl Programmers Reference Guide        attributes(3pm)
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NAME

6       attributes - get/set subroutine or variable attributes
7

SYNOPSIS

9         sub foo : method ;
10         my ($x,@y,%z) : Bent = 1;
11         my $s = sub : method { ... };
12
13         use attributes ();    # optional, to get subroutine declarations
14         my @attrlist = attributes::get(\&foo);
15
16         use attributes 'get'; # import the attributes::get subroutine
17         my @attrlist = get \&foo;
18

DESCRIPTION

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
27           use attributes __PACKAGE__, \&foo, 'method';
28
29       The second example in the synopsis does something equivalent to this:
30
31           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
38       Yes, that's a lot of expansion.
39
40       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
46       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
51       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
57           my $x : Bent = 42 if 0;
58
59       will neither assign 42 to $x nor will it apply the "Bent" attribute to
60       the variable.
61
62       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
68   What "import" does
69       In the description it is mentioned that
70
71         sub foo : method;
72
73       is equivalent to
74
75         use attributes __PACKAGE__, \&foo, 'method';
76
77       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
81         attributes->import( __PACKAGE__, \&foo, 'method' );
82
83       So you want to know what "import" actually does?
84
85       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
92         MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' );
93
94       "MODIFY_<reftype>_ATTRIBUTES" has to return a list of all "bad
95       attributes".  If there are any bad attributes "import" croaks.
96
97       (See "Package-specific Attribute Handling" below.)
98
99   Built-in Attributes
100       The following are the built-in attributes for subroutines:
101
102       locked
103           5.005 threads only!  The use of the "locked" attribute currently
104           only makes sense if you are using the deprecated "Perl 5.005
105           threads" implementation of threads.
106
107           Setting this attribute is only meaningful when the subroutine or
108           method is to be called by multiple threads.  When set on a method
109           subroutine (i.e., one marked with the method attribute below), Perl
110           ensures that any invocation of it implicitly locks its first
111           argument before execution.  When set on a non-method subroutine,
112           Perl ensures that a lock is taken on the subroutine itself before
113           execution.  The semantics of the lock are exactly those of one
114           explicitly taken with the "lock" operator immediately after the
115           subroutine is entered.
116
117       method
118           Indicates that the referenced subroutine is a method.  This has a
119           meaning when taken together with the locked attribute, as described
120           there.  It also means that a subroutine so marked will not trigger
121           the "Ambiguous call resolved as CORE::%s" warning.
122
123       lvalue
124           Indicates that the referenced subroutine is a valid lvalue and can
125           be assigned to. The subroutine must return a modifiable value such
126           as a scalar variable, as described in perlsub.
127
128       For global variables there is "unique" attribute: see "our" in
129       perlfunc.
130
131   Available Subroutines
132       The following subroutines are available for general use once this
133       module has been loaded:
134
135       get This routine expects a single parameter--a reference to a
136           subroutine or variable.  It returns a list of attributes, which may
137           be empty.  If passed invalid arguments, it uses die() (via
138           Carp::croak) to raise a fatal exception.  If it can find an
139           appropriate package name for a class method lookup, it will include
140           the results from a "FETCH_type_ATTRIBUTES" call in its return list,
141           as described in "Package-specific Attribute Handling" below.
142           Otherwise, only built-in attributes will be returned.
143
144       reftype
145           This routine expects a single parameter--a reference to a
146           subroutine or variable.  It returns the built-in type of the
147           referenced variable, ignoring any package into which it might have
148           been blessed.  This can be useful for determining the type value
149           which forms part of the method names described in "Package-specific
150           Attribute Handling" below.
151
152       Note that these routines are not exported by default.
153
154   Package-specific Attribute Handling
155       WARNING: the mechanisms described here are still experimental.  Do not
156       rely on the current implementation.  In particular, there is no
157       provision for applying package attributes to 'cloned' copies of
158       subroutines used as closures.  (See "Making References" in perlref for
159       information on closures.)  Package-specific attribute handling may
160       change incompatibly in a future release.
161
162       When an attribute list is present in a declaration, a check is made to
163       see whether an attribute 'modify' handler is present in the appropriate
164       package (or its @ISA inheritance tree).  Similarly, when
165       "attributes::get" is called on a valid reference, a check is made for
166       an appropriate attribute 'fetch' handler.  See "EXAMPLES" to see how
167       the "appropriate package" determination works.
168
169       The handler names are based on the underlying type of the variable
170       being declared or of the reference passed.  Because these attributes
171       are associated with subroutine or variable declarations, this
172       deliberately ignores any possibility of being blessed into some
173       package.  Thus, a subroutine declaration uses "CODE" as its type, and
174       even a blessed hash reference uses "HASH" as its type.
175
176       The class methods invoked for modifying and fetching are these:
177
178       FETCH_type_ATTRIBUTES
179           This method is called with two arguments:  the relevant package
180           name, and a reference to a variable or subroutine for which
181           package-defined attributes are desired.  The expected return value
182           is a list of associated attributes.  This list may be empty.
183
184       MODIFY_type_ATTRIBUTES
185           This method is called with two fixed arguments, followed by the
186           list of attributes from the relevant declaration.  The two fixed
187           arguments are the relevant package name and a reference to the
188           declared subroutine or variable.  The expected return value is a
189           list of attributes which were not recognized by this handler.  Note
190           that this allows for a derived class to delegate a call to its base
191           class, and then only examine the attributes which the base class
192           didn't already handle for it.
193
194           The call to this method is currently made during the processing of
195           the declaration.  In particular, this means that a subroutine
196           reference will probably be for an undefined subroutine, even if
197           this declaration is actually part of the definition.
198
199       Calling "attributes::get()" from within the scope of a null package
200       declaration "package ;" for an unblessed variable reference will not
201       provide any starting package name for the 'fetch' method lookup.  Thus,
202       this circumstance will not result in a method call for package-defined
203       attributes.  A named subroutine knows to which symbol table entry it
204       belongs (or originally belonged), and it will use the corresponding
205       package.  An anonymous subroutine knows the package name into which it
206       was compiled (unless it was also compiled with a null package
207       declaration), and so it will use that package name.
208
209   Syntax of Attribute Lists
210       An attribute list is a sequence of attribute specifications, separated
211       by whitespace or a colon (with optional whitespace).  Each attribute
212       specification is a simple name, optionally followed by a parenthesised
213       parameter list.  If such a parameter list is present, it is scanned
214       past as for the rules for the "q()" operator.  (See "Quote and Quote-
215       like Operators" in perlop.)  The parameter list is passed as it was
216       found, however, and not as per "q()".
217
218       Some examples of syntactically valid attribute lists:
219
220           switch(10,foo(7,3))  :  expensive
221           Ugly('\(") :Bad
222           _5x5
223           locked method
224
225       Some examples of syntactically invalid attribute lists (with
226       annotation):
227
228           switch(10,foo()             # ()-string not balanced
229           Ugly('(')                   # ()-string not balanced
230           5x5                         # "5x5" not a valid identifier
231           Y2::north                   # "Y2::north" not a simple identifier
232           foo + bar                   # "+" neither a colon nor whitespace
233

EXPORTS

235   Default exports
236       None.
237
238   Available exports
239       The routines "get" and "reftype" are exportable.
240
241   Export tags defined
242       The ":ALL" tag will get all of the above exports.
243

EXAMPLES

245       Here are some samples of syntactically valid declarations, with
246       annotation as to how they resolve internally into "use attributes"
247       invocations by perl.  These examples are primarily useful to see how
248       the "appropriate package" is found for the possible method lookups for
249       package-defined attributes.
250
251       1.  Code:
252
253               package Canine;
254               package Dog;
255               my Canine $spot : Watchful ;
256
257           Effect:
258
259               use attributes ();
260               attributes::->import(Canine => \$spot, "Watchful");
261
262       2.  Code:
263
264               package Felis;
265               my $cat : Nervous;
266
267           Effect:
268
269               use attributes ();
270               attributes::->import(Felis => \$cat, "Nervous");
271
272       3.  Code:
273
274               package X;
275               sub foo : locked ;
276
277           Effect:
278
279               use attributes X => \&foo, "locked";
280
281       4.  Code:
282
283               package X;
284               sub Y::x : locked { 1 }
285
286           Effect:
287
288               use attributes Y => \&Y::x, "locked";
289
290       5.  Code:
291
292               package X;
293               sub foo { 1 }
294
295               package Y;
296               BEGIN { *bar = \&X::foo; }
297
298               package Z;
299               sub Y::bar : locked ;
300
301           Effect:
302
303               use attributes X => \&X::foo, "locked";
304
305       This last example is purely for purposes of completeness.  You should
306       not be trying to mess with the attributes of something in a package
307       that's not your own.
308

MORE EXAMPLES

310       1.
311               sub MODIFY_CODE_ATTRIBUTES {
312                  my ($class,$code,@attrs) = @_;
313
314                  my $allowed = 'MyAttribute';
315                  my @bad = grep { $_ ne $allowed } @attrs;
316
317                  return @bad;
318               }
319
320               sub foo : MyAttribute {
321                  print "foo\n";
322               }
323
324           This example runs. At compile time "MODIFY_CODE_ATTRIBUTES" is
325           called. In that subroutine, we check if any attribute is disallowed
326           and we return a list of these "bad attributes".
327
328           As we return an empty list, everything is fine.
329
330       2.
331             sub MODIFY_CODE_ATTRIBUTES {
332                my ($class,$code,@attrs) = @_;
333
334                my $allowed = 'MyAttribute';
335                my @bad = grep{ $_ ne $allowed }@attrs;
336
337                return @bad;
338             }
339
340             sub foo : MyAttribute Test {
341                print "foo\n";
342             }
343
344           This example is aborted at compile time as we use the attribute
345           "Test" which isn't allowed. "MODIFY_CODE_ATTRIBUTES" returns a list
346           that contains a single element ('Test').
347

NAME

SYNOPSIS

DESCRIPTION

EXPORTS

EXAMPLES

MORE EXAMPLES

SEE ALSO