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09bef843 SB |
1 | package attributes; |
2 | ||
0120eecf | 3 | $VERSION = 0.03; |
09bef843 | 4 | |
26f2972e GS |
5 | @EXPORT_OK = qw(get reftype); |
6 | @EXPORT = (); | |
7 | %EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]); | |
09bef843 SB |
8 | |
9 | use strict; | |
10 | ||
11 | sub croak { | |
12 | require Carp; | |
13 | goto &Carp::croak; | |
14 | } | |
15 | ||
16 | sub carp { | |
17 | require Carp; | |
18 | goto &Carp::carp; | |
19 | } | |
20 | ||
21 | ## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{} | |
22 | #sub reftype ($) ; | |
23 | #sub _fetch_attrs ($) ; | |
24 | #sub _guess_stash ($) ; | |
25 | #sub _modify_attrs ; | |
26 | #sub _warn_reserved () ; | |
27 | # | |
28 | # The extra trips through newATTRSUB in the interpreter wipe out any savings | |
29 | # from avoiding the BEGIN block. Just do the bootstrap now. | |
592f5969 | 30 | BEGIN { bootstrap attributes } |
09bef843 SB |
31 | |
32 | sub import { | |
26f2972e GS |
33 | @_ > 2 && ref $_[2] or do { |
34 | require Exporter; | |
35 | goto &Exporter::import; | |
c0c5a66b | 36 | }; |
09bef843 SB |
37 | my (undef,$home_stash,$svref,@attrs) = @_; |
38 | ||
39 | my $svtype = uc reftype($svref); | |
40 | my $pkgmeth; | |
41 | $pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES") | |
42 | if defined $home_stash && $home_stash ne ''; | |
43 | my @badattrs; | |
44 | if ($pkgmeth) { | |
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; | |
50 | if (@pkgattrs) { | |
51 | for my $attr (@pkgattrs) { | |
52 | $attr =~ s/\(.+\z//s; | |
53 | } | |
54 | my $s = ((@pkgattrs == 1) ? '' : 's'); | |
55 | carp "$svtype package attribute$s " . | |
56 | "may clash with future reserved word$s: " . | |
0120eecf | 57 | join(' : ' , @pkgattrs); |
09bef843 SB |
58 | } |
59 | } | |
60 | } | |
61 | else { | |
62 | @badattrs = _modify_attrs($svref, @attrs); | |
63 | } | |
64 | if (@badattrs) { | |
65 | croak "Invalid $svtype attribute" . | |
66 | (( @badattrs == 1 ) ? '' : 's') . | |
67 | ": " . | |
0120eecf | 68 | join(' : ', @badattrs); |
09bef843 SB |
69 | } |
70 | } | |
71 | ||
72 | sub get ($) { | |
73 | @_ == 1 && ref $_[0] or | |
74 | croak 'Usage: '.__PACKAGE__.'::get $ref'; | |
75 | my $svref = shift; | |
76 | my $svtype = uc reftype $svref; | |
77 | my $stash = _guess_stash $svref; | |
78 | $stash = caller unless defined $stash; | |
79 | my $pkgmeth; | |
80 | $pkgmeth = UNIVERSAL::can($stash, "FETCH_${svtype}_ATTRIBUTES") | |
81 | if defined $stash && $stash ne ''; | |
82 | return $pkgmeth ? | |
83 | (_fetch_attrs($svref), $pkgmeth->($stash, $svref)) : | |
84 | (_fetch_attrs($svref)) | |
85 | ; | |
86 | } | |
87 | ||
26f2972e | 88 | sub require_version { goto &UNIVERSAL::VERSION } |
09bef843 SB |
89 | |
90 | 1; | |
91 | __END__ | |
92 | #The POD goes here | |
93 | ||
94 | =head1 NAME | |
95 | ||
96 | attributes - get/set subroutine or variable attributes | |
97 | ||
98 | =head1 SYNOPSIS | |
99 | ||
100 | sub foo : method ; | |
101 | my ($x,@y,%z) : Bent ; | |
102 | my $s = sub : method { ... }; | |
103 | ||
104 | use attributes (); # optional, to get subroutine declarations | |
105 | my @attrlist = attributes::get(\&foo); | |
106 | ||
26f2972e GS |
107 | use attributes 'get'; # import the attributes::get subroutine |
108 | my @attrlist = get \&foo; | |
109 | ||
09bef843 SB |
110 | =head1 DESCRIPTION |
111 | ||
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 | |
26f2972e | 116 | list to this module. In particular, the first example above is equivalent to |
09bef843 SB |
117 | the following: |
118 | ||
119 | use attributes __PACKAGE__, \&foo, 'method'; | |
120 | ||
121 | The second example in the synopsis does something equivalent to this: | |
122 | ||
123 | use attributes __PACKAGE__, \$x, 'Bent'; | |
124 | use attributes __PACKAGE__, \@y, 'Bent'; | |
125 | use attributes __PACKAGE__, \%z, 'Bent'; | |
126 | ||
127 | Yes, that's three invocations. | |
128 | ||
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 | |
2752632f LB |
133 | implementation of this feature. Variable attributes are currently |
134 | not usable for tieing. | |
09bef843 SB |
135 | |
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.) | |
140 | ||
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 | |
145 | (such as "foo") | |
146 | will result in a warning with B<-w> or C<use warnings 'reserved'>. | |
147 | ||
148 | =head2 Built-in Attributes | |
149 | ||
150 | The following are the built-in attributes for subroutines: | |
151 | ||
152 | =over 4 | |
153 | ||
154 | =item locked | |
155 | ||
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. | |
165 | ||
166 | =item method | |
167 | ||
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. | |
172 | ||
89752b9c GS |
173 | =item lvalue |
174 | ||
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>. | |
178 | ||
09bef843 SB |
179 | =back |
180 | ||
181 | There are no built-in attributes for anything other than subroutines. | |
182 | ||
183 | =head2 Available Subroutines | |
184 | ||
185 | The following subroutines are available for general use once this module | |
186 | has been loaded: | |
187 | ||
188 | =over 4 | |
189 | ||
190 | =item get | |
191 | ||
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 | |
26f2972e | 198 | L<"Package-specific Attribute Handling"> below. |
09bef843 SB |
199 | Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned. |
200 | ||
201 | =item reftype | |
202 | ||
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 | |
26f2972e | 207 | the method names described in L<"Package-specific Attribute Handling"> below. |
09bef843 SB |
208 | |
209 | =back | |
210 | ||
26f2972e | 211 | Note that these routines are I<not> exported by default. |
09bef843 SB |
212 | |
213 | =head2 Package-specific Attribute Handling | |
214 | ||
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 | |
220 | release. | |
221 | ||
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" | |
227 | determination works. | |
228 | ||
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>. | |
235 | ||
236 | The class methods invoked for modifying and fetching are these: | |
237 | ||
238 | =over 4 | |
239 | ||
240 | =item FETCH_I<type>_ATTRIBUTES | |
241 | ||
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. | |
246 | ||
247 | =item MODIFY_I<type>_ATTRIBUTES | |
248 | ||
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. | |
256 | ||
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. | |
261 | ||
262 | =back | |
263 | ||
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. | |
273 | ||
274 | =head2 Syntax of Attribute Lists | |
275 | ||
276 | An attribute list is a sequence of attribute specifications, separated by | |
0120eecf GS |
277 | whitespace or a colon (with optional whitespace). |
278 | Each attribute specification is a simple | |
09bef843 SB |
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()>. | |
283 | ||
284 | Some examples of syntactically valid attribute lists: | |
285 | ||
0120eecf GS |
286 | switch(10,foo(7,3)) : expensive |
287 | Ugly('\(") :Bad | |
09bef843 SB |
288 | _5x5 |
289 | locked method | |
290 | ||
291 | Some examples of syntactically invalid attribute lists (with annotation): | |
292 | ||
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 | |
0120eecf | 297 | foo + bar # "+" neither a colon nor whitespace |
09bef843 | 298 | |
26f2972e GS |
299 | =head1 EXPORTS |
300 | ||
301 | =head2 Default exports | |
302 | ||
303 | None. | |
304 | ||
305 | =head2 Available exports | |
306 | ||
307 | The routines C<get> and C<reftype> are exportable. | |
308 | ||
309 | =head2 Export tags defined | |
310 | ||
311 | The C<:ALL> tag will get all of the above exports. | |
312 | ||
09bef843 SB |
313 | =head1 EXAMPLES |
314 | ||
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 | |
319 | attributes. | |
320 | ||
321 | =over 4 | |
322 | ||
323 | =item 1. | |
324 | ||
325 | Code: | |
326 | ||
327 | package Canine; | |
328 | package Dog; | |
329 | my Canine $spot : Watchful ; | |
330 | ||
331 | Effect: | |
332 | ||
333 | use attributes Canine => \$spot, "Watchful"; | |
334 | ||
335 | =item 2. | |
336 | ||
337 | Code: | |
338 | ||
339 | package Felis; | |
340 | my $cat : Nervous; | |
341 | ||
342 | Effect: | |
343 | ||
344 | use attributes Felis => \$cat, "Nervous"; | |
345 | ||
346 | =item 3. | |
347 | ||
348 | Code: | |
349 | ||
350 | package X; | |
351 | sub foo : locked ; | |
352 | ||
353 | Effect: | |
354 | ||
355 | use attributes X => \&foo, "locked"; | |
356 | ||
357 | =item 4. | |
358 | ||
359 | Code: | |
360 | ||
361 | package X; | |
362 | sub Y::x : locked { 1 } | |
363 | ||
364 | Effect: | |
365 | ||
366 | use attributes Y => \&Y::x, "locked"; | |
367 | ||
368 | =item 5. | |
369 | ||
370 | Code: | |
371 | ||
372 | package X; | |
373 | sub foo { 1 } | |
374 | ||
375 | package Y; | |
376 | BEGIN { *bar = \&X::foo; } | |
377 | ||
378 | package Z; | |
379 | sub Y::bar : locked ; | |
380 | ||
381 | Effect: | |
382 | ||
383 | use attributes X => \&X::foo, "locked"; | |
384 | ||
385 | =back | |
386 | ||
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 | |
389 | not your own. | |
390 | ||
391 | =head1 SEE ALSO | |
392 | ||
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. | |
398 | ||
399 | =cut | |
400 |