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1 | package attributes; |
2 | |
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3 | our $VERSION = 0.05; |
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4 | |
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5 | @EXPORT_OK = qw(get reftype); |
6 | @EXPORT = (); |
7 | %EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]); |
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8 | |
9 | use strict; |
10 | |
11 | sub croak { |
12 | require Carp; |
13 | goto &Carp::croak; |
14 | } |
15 | |
16 | sub carp { |
17 | require Carp; |
18 | goto &Carp::carp; |
19 | } |
20 | |
21 | ## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{} |
22 | #sub reftype ($) ; |
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. |
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30 | BEGIN { bootstrap attributes } |
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31 | |
32 | sub import { |
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33 | @_ > 2 && ref $_[2] or do { |
34 | require Exporter; |
35 | goto &Exporter::import; |
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36 | }; |
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37 | my (undef,$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: " . |
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57 | join(' : ' , @pkgattrs); |
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58 | } |
59 | } |
60 | } |
61 | else { |
62 | @badattrs = _modify_attrs($svref, @attrs); |
63 | } |
64 | if (@badattrs) { |
65 | croak "Invalid $svtype attribute" . |
66 | (( @badattrs == 1 ) ? '' : 's') . |
67 | ": " . |
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68 | join(' : ', @badattrs); |
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69 | } |
70 | } |
71 | |
72 | sub get ($) { |
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 | |
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88 | sub require_version { goto &UNIVERSAL::VERSION } |
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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 ; |
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101 | my ($x,@y,%z) : Bent = 1; |
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102 | my $s = sub : method { ... }; |
103 | |
104 | use attributes (); # optional, to get subroutine declarations |
105 | my @attrlist = attributes::get(\&foo); |
106 | |
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107 | use attributes 'get'; # import the attributes::get subroutine |
108 | my @attrlist = get \&foo; |
109 | |
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110 | =head1 DESCRIPTION |
111 | |
112 | Subroutine declarations and definitions may optionally have attribute lists |
113 | associated with them. (Variable C<my> declarations also may, but see the |
114 | warning below.) Perl handles these declarations by passing some information |
115 | about the call site and the thing being declared along with the attribute |
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116 | list to this module. In particular, the first example above is equivalent to |
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117 | the following: |
118 | |
119 | use attributes __PACKAGE__, \&foo, 'method'; |
120 | |
121 | The second example in the synopsis does something equivalent to this: |
122 | |
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123 | use attributes (); |
124 | my ($x,@y,%z); |
125 | attributes::->import(__PACKAGE__, \$x, 'Bent'); |
126 | attributes::->import(__PACKAGE__, \@y, 'Bent'); |
127 | attributes::->import(__PACKAGE__, \%z, 'Bent'); |
128 | ($x,@y,%z) = 1; |
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129 | |
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130 | Yes, that's a lot of expansion. |
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131 | |
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132 | B<WARNING>: attribute declarations for variables are still evolving. |
133 | The semantics and interfaces of such declarations could change in |
134 | future versions. They are present for purposes of experimentation |
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135 | with what the semantics ought to be. Do not rely on the current |
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136 | implementation of this feature. |
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137 | |
138 | There are only a few attributes currently handled by Perl itself (or |
139 | directly by this module, depending on how you look at it.) However, |
140 | package-specific attributes are allowed by an extension mechanism. |
141 | (See L<"Package-specific Attribute Handling"> below.) |
142 | |
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143 | The setting of subroutine attributes happens at compile time. |
144 | Variable attributes in C<our> declarations are also applied at compile time. |
145 | However, C<my> variables get their attributes applied at run-time. |
146 | This means that you have to I<reach> the run-time component of the C<my> |
147 | before those attributes will get applied. For example: |
148 | |
149 | my $x : Bent = 42 if 0; |
150 | |
151 | will neither assign 42 to $x I<nor> will it apply the C<Bent> attribute |
152 | to the variable. |
153 | |
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154 | An attempt to set an unrecognized attribute is a fatal error. (The |
155 | error is trappable, but it still stops the compilation within that |
156 | C<eval>.) Setting an attribute with a name that's all lowercase |
157 | letters that's not a built-in attribute (such as "foo") will result in |
158 | a warning with B<-w> or C<use warnings 'reserved'>. |
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159 | |
160 | =head2 Built-in Attributes |
161 | |
162 | The following are the built-in attributes for subroutines: |
163 | |
164 | =over 4 |
165 | |
166 | =item locked |
167 | |
168 | Setting this attribute is only meaningful when the subroutine or |
169 | method is to be called by multiple threads. When set on a method |
170 | subroutine (i.e., one marked with the B<method> attribute below), |
171 | Perl ensures that any invocation of it implicitly locks its first |
172 | argument before execution. When set on a non-method subroutine, |
173 | Perl ensures that a lock is taken on the subroutine itself before |
174 | execution. The semantics of the lock are exactly those of one |
175 | explicitly taken with the C<lock> operator immediately after the |
176 | subroutine is entered. |
177 | |
178 | =item method |
179 | |
180 | Indicates that the referenced subroutine is a method. |
181 | This has a meaning when taken together with the B<locked> attribute, |
182 | as described there. It also means that a subroutine so marked |
183 | will not trigger the "Ambiguous call resolved as CORE::%s" warning. |
184 | |
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185 | =item lvalue |
186 | |
187 | Indicates that the referenced subroutine is a valid lvalue and can |
188 | be assigned to. The subroutine must return a modifiable value such |
189 | as a scalar variable, as described in L<perlsub>. |
190 | |
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191 | =back |
192 | |
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193 | For global variables there is C<unique> attribute: see L<perlfunc/our>. |
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194 | |
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195 | =head2 Available Subroutines |
196 | |
197 | The following subroutines are available for general use once this module |
198 | has been loaded: |
199 | |
200 | =over 4 |
201 | |
202 | =item get |
203 | |
204 | This routine expects a single parameter--a reference to a |
205 | subroutine or variable. It returns a list of attributes, which may be |
206 | empty. If passed invalid arguments, it uses die() (via L<Carp::croak|Carp>) |
207 | to raise a fatal exception. If it can find an appropriate package name |
208 | for a class method lookup, it will include the results from a |
209 | C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in |
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210 | L<"Package-specific Attribute Handling"> below. |
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211 | Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned. |
212 | |
213 | =item reftype |
214 | |
215 | This routine expects a single parameter--a reference to a subroutine or |
216 | variable. It returns the built-in type of the referenced variable, |
217 | ignoring any package into which it might have been blessed. |
218 | This can be useful for determining the I<type> value which forms part of |
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219 | the method names described in L<"Package-specific Attribute Handling"> below. |
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220 | |
221 | =back |
222 | |
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223 | Note that these routines are I<not> exported by default. |
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224 | |
225 | =head2 Package-specific Attribute Handling |
226 | |
227 | B<WARNING>: the mechanisms described here are still experimental. Do not |
228 | rely on the current implementation. In particular, there is no provision |
229 | for applying package attributes to 'cloned' copies of subroutines used as |
230 | closures. (See L<perlref/"Making References"> for information on closures.) |
231 | Package-specific attribute handling may change incompatibly in a future |
232 | release. |
233 | |
234 | When an attribute list is present in a declaration, a check is made to see |
235 | whether an attribute 'modify' handler is present in the appropriate package |
236 | (or its @ISA inheritance tree). Similarly, when C<attributes::get> is |
237 | called on a valid reference, a check is made for an appropriate attribute |
238 | 'fetch' handler. See L<"EXAMPLES"> to see how the "appropriate package" |
239 | determination works. |
240 | |
241 | The handler names are based on the underlying type of the variable being |
242 | declared or of the reference passed. Because these attributes are |
243 | associated with subroutine or variable declarations, this deliberately |
244 | ignores any possibility of being blessed into some package. Thus, a |
245 | subroutine declaration uses "CODE" as its I<type>, and even a blessed |
246 | hash reference uses "HASH" as its I<type>. |
247 | |
248 | The class methods invoked for modifying and fetching are these: |
249 | |
250 | =over 4 |
251 | |
252 | =item FETCH_I<type>_ATTRIBUTES |
253 | |
254 | This method receives a single argument, which is a reference to the |
255 | variable or subroutine for which package-defined attributes are desired. |
256 | The expected return value is a list of associated attributes. |
257 | This list may be empty. |
258 | |
259 | =item MODIFY_I<type>_ATTRIBUTES |
260 | |
261 | This method is called with two fixed arguments, followed by the list of |
262 | attributes from the relevant declaration. The two fixed arguments are |
263 | the relevant package name and a reference to the declared subroutine or |
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264 | variable. The expected return value is a list of attributes which were |
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265 | not recognized by this handler. Note that this allows for a derived class |
266 | to delegate a call to its base class, and then only examine the attributes |
267 | which the base class didn't already handle for it. |
268 | |
269 | The call to this method is currently made I<during> the processing of the |
270 | declaration. In particular, this means that a subroutine reference will |
271 | probably be for an undefined subroutine, even if this declaration is |
272 | actually part of the definition. |
273 | |
274 | =back |
275 | |
276 | Calling C<attributes::get()> from within the scope of a null package |
277 | declaration C<package ;> for an unblessed variable reference will |
278 | not provide any starting package name for the 'fetch' method lookup. |
279 | Thus, this circumstance will not result in a method call for package-defined |
280 | attributes. A named subroutine knows to which symbol table entry it belongs |
281 | (or originally belonged), and it will use the corresponding package. |
282 | An anonymous subroutine knows the package name into which it was compiled |
283 | (unless it was also compiled with a null package declaration), and so it |
284 | will use that package name. |
285 | |
286 | =head2 Syntax of Attribute Lists |
287 | |
288 | An attribute list is a sequence of attribute specifications, separated by |
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289 | whitespace or a colon (with optional whitespace). |
290 | Each attribute specification is a simple |
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291 | name, optionally followed by a parenthesised parameter list. |
292 | If such a parameter list is present, it is scanned past as for the rules |
293 | for the C<q()> operator. (See L<perlop/"Quote and Quote-like Operators">.) |
294 | The parameter list is passed as it was found, however, and not as per C<q()>. |
295 | |
296 | Some examples of syntactically valid attribute lists: |
297 | |
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298 | switch(10,foo(7,3)) : expensive |
299 | Ugly('\(") :Bad |
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300 | _5x5 |
301 | locked method |
302 | |
303 | Some examples of syntactically invalid attribute lists (with annotation): |
304 | |
305 | switch(10,foo() # ()-string not balanced |
306 | Ugly('(') # ()-string not balanced |
307 | 5x5 # "5x5" not a valid identifier |
308 | Y2::north # "Y2::north" not a simple identifier |
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309 | foo + bar # "+" neither a colon nor whitespace |
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310 | |
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311 | =head1 EXPORTS |
312 | |
313 | =head2 Default exports |
314 | |
315 | None. |
316 | |
317 | =head2 Available exports |
318 | |
319 | The routines C<get> and C<reftype> are exportable. |
320 | |
321 | =head2 Export tags defined |
322 | |
323 | The C<:ALL> tag will get all of the above exports. |
324 | |
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325 | =head1 EXAMPLES |
326 | |
327 | Here are some samples of syntactically valid declarations, with annotation |
328 | as to how they resolve internally into C<use attributes> invocations by |
329 | perl. These examples are primarily useful to see how the "appropriate |
330 | package" is found for the possible method lookups for package-defined |
331 | attributes. |
332 | |
333 | =over 4 |
334 | |
335 | =item 1. |
336 | |
337 | Code: |
338 | |
339 | package Canine; |
340 | package Dog; |
341 | my Canine $spot : Watchful ; |
342 | |
343 | Effect: |
344 | |
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345 | use attributes (); |
346 | attributes::->import(Canine => \$spot, "Watchful"); |
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347 | |
348 | =item 2. |
349 | |
350 | Code: |
351 | |
352 | package Felis; |
353 | my $cat : Nervous; |
354 | |
355 | Effect: |
356 | |
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357 | use attributes (); |
358 | attributes::->import(Felis => \$cat, "Nervous"); |
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359 | |
360 | =item 3. |
361 | |
362 | Code: |
363 | |
364 | package X; |
365 | sub foo : locked ; |
366 | |
367 | Effect: |
368 | |
369 | use attributes X => \&foo, "locked"; |
370 | |
371 | =item 4. |
372 | |
373 | Code: |
374 | |
375 | package X; |
376 | sub Y::x : locked { 1 } |
377 | |
378 | Effect: |
379 | |
380 | use attributes Y => \&Y::x, "locked"; |
381 | |
382 | =item 5. |
383 | |
384 | Code: |
385 | |
386 | package X; |
387 | sub foo { 1 } |
388 | |
389 | package Y; |
390 | BEGIN { *bar = \&X::foo; } |
391 | |
392 | package Z; |
393 | sub Y::bar : locked ; |
394 | |
395 | Effect: |
396 | |
397 | use attributes X => \&X::foo, "locked"; |
398 | |
399 | =back |
400 | |
401 | This last example is purely for purposes of completeness. You should not |
402 | be trying to mess with the attributes of something in a package that's |
403 | not your own. |
404 | |
405 | =head1 SEE ALSO |
406 | |
407 | L<perlsub/"Private Variables via my()"> and |
408 | L<perlsub/"Subroutine Attributes"> for details on the basic declarations; |
409 | L<attrs> for the obsolescent form of subroutine attribute specification |
410 | which this module replaces; |
411 | L<perlfunc/use> for details on the normal invocation mechanism. |
412 | |
413 | =cut |
414 | |