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1 | package attributes; |
2 | |
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3 | our $VERSION = 0.10; |
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4 | |
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5 | @EXPORT_OK = qw(get reftype); |
6 | @EXPORT = (); |
7 | %EXPORT_TAGS = (ALL => [@EXPORT, @EXPORT_OK]); |
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8 | |
9 | use strict; |
10 | |
11 | sub croak { |
12 | require Carp; |
13 | goto &Carp::croak; |
14 | } |
15 | |
16 | sub carp { |
17 | require Carp; |
18 | goto &Carp::carp; |
19 | } |
20 | |
21 | ## forward declaration(s) rather than wrapping the bootstrap call in BEGIN{} |
22 | #sub reftype ($) ; |
23 | #sub _fetch_attrs ($) ; |
24 | #sub _guess_stash ($) ; |
25 | #sub _modify_attrs ; |
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26 | # |
27 | # The extra trips through newATTRSUB in the interpreter wipe out any savings |
28 | # from avoiding the BEGIN block. Just do the bootstrap now. |
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29 | BEGIN { bootstrap attributes } |
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30 | |
31 | sub import { |
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32 | @_ > 2 && ref $_[2] or do { |
33 | require Exporter; |
34 | goto &Exporter::import; |
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35 | }; |
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36 | my (undef,$home_stash,$svref,@attrs) = @_; |
37 | |
38 | my $svtype = uc reftype($svref); |
39 | my $pkgmeth; |
40 | $pkgmeth = UNIVERSAL::can($home_stash, "MODIFY_${svtype}_ATTRIBUTES") |
41 | if defined $home_stash && $home_stash ne ''; |
42 | my @badattrs; |
43 | if ($pkgmeth) { |
44 | my @pkgattrs = _modify_attrs($svref, @attrs); |
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45 | @badattrs = $pkgmeth->($home_stash, $svref, @pkgattrs); |
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46 | if (!@badattrs && @pkgattrs) { |
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47 | require warnings; |
48 | return unless warnings::enabled('reserved'); |
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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 | |
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160 | =head2 What C<import> does |
161 | |
162 | In the description it is mentioned that |
163 | |
164 | sub foo : method; |
165 | |
166 | is equivalent to |
167 | |
168 | use attributes __PACKAGE__, \&foo, 'method'; |
169 | |
170 | As you might know this calls the C<import> function of C<attributes> at compile |
171 | time with these parameters: 'attributes', the caller's package name, the reference |
172 | to the code and 'method'. |
173 | |
174 | attributes->import( __PACKAGE__, \&foo, 'method' ); |
175 | |
176 | So you want to know what C<import> actually does? |
177 | |
178 | First of all C<import> gets the type of the third parameter ('CODE' in this case). |
179 | C<attributes.pm> checks if there is a subroutine called C<< MODIFY_<reftype>_ATTRIBUTES >> |
180 | in the caller's namespace (here: 'main'). In this case a subroutine C<MODIFY_CODE_ATTRIBUTES> is |
181 | required. Then this method is called to check if you have used a "bad attribute". |
182 | The subroutine call in this example would look like |
183 | |
184 | MODIFY_CODE_ATTRIBUTES( 'main', \&foo, 'method' ); |
185 | |
186 | C<< MODIFY_<reftype>_ATTRIBUTES >> has to return a list of all "bad attributes". |
187 | If there are any bad attributes C<import> croaks. |
188 | |
189 | (See L<"Package-specific Attribute Handling"> below.) |
190 | |
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191 | =head2 Built-in Attributes |
192 | |
193 | The following are the built-in attributes for subroutines: |
194 | |
195 | =over 4 |
196 | |
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197 | =item lvalue |
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198 | |
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199 | Indicates that the referenced subroutine is a valid lvalue and can |
200 | be assigned to. The subroutine must return a modifiable value such |
201 | as a scalar variable, as described in L<perlsub>. |
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202 | |
203 | =item method |
204 | |
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205 | Indicates that the referenced subroutine is a method. A subroutine so marked |
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206 | will not trigger the "Ambiguous call resolved as CORE::%s" warning. |
207 | |
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208 | =item locked |
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209 | |
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210 | The "locked" attribute has no effect in 5.10.0 and later. It was used as part |
211 | of the now-removed "Perl 5.005 threads". |
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212 | |
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213 | =back |
214 | |
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215 | For global variables there is C<unique> attribute: see L<perlfunc/our>. |
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216 | |
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217 | =head2 Available Subroutines |
218 | |
219 | The following subroutines are available for general use once this module |
220 | has been loaded: |
221 | |
222 | =over 4 |
223 | |
224 | =item get |
225 | |
226 | This routine expects a single parameter--a reference to a |
227 | subroutine or variable. It returns a list of attributes, which may be |
228 | empty. If passed invalid arguments, it uses die() (via L<Carp::croak|Carp>) |
229 | to raise a fatal exception. If it can find an appropriate package name |
230 | for a class method lookup, it will include the results from a |
231 | C<FETCH_I<type>_ATTRIBUTES> call in its return list, as described in |
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232 | L<"Package-specific Attribute Handling"> below. |
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233 | Otherwise, only L<built-in attributes|"Built-in Attributes"> will be returned. |
234 | |
235 | =item reftype |
236 | |
237 | This routine expects a single parameter--a reference to a subroutine or |
238 | variable. It returns the built-in type of the referenced variable, |
239 | ignoring any package into which it might have been blessed. |
240 | This can be useful for determining the I<type> value which forms part of |
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241 | the method names described in L<"Package-specific Attribute Handling"> below. |
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242 | |
243 | =back |
244 | |
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245 | Note that these routines are I<not> exported by default. |
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246 | |
247 | =head2 Package-specific Attribute Handling |
248 | |
249 | B<WARNING>: the mechanisms described here are still experimental. Do not |
250 | rely on the current implementation. In particular, there is no provision |
251 | for applying package attributes to 'cloned' copies of subroutines used as |
252 | closures. (See L<perlref/"Making References"> for information on closures.) |
253 | Package-specific attribute handling may change incompatibly in a future |
254 | release. |
255 | |
256 | When an attribute list is present in a declaration, a check is made to see |
257 | whether an attribute 'modify' handler is present in the appropriate package |
258 | (or its @ISA inheritance tree). Similarly, when C<attributes::get> is |
259 | called on a valid reference, a check is made for an appropriate attribute |
260 | 'fetch' handler. See L<"EXAMPLES"> to see how the "appropriate package" |
261 | determination works. |
262 | |
263 | The handler names are based on the underlying type of the variable being |
264 | declared or of the reference passed. Because these attributes are |
265 | associated with subroutine or variable declarations, this deliberately |
266 | ignores any possibility of being blessed into some package. Thus, a |
267 | subroutine declaration uses "CODE" as its I<type>, and even a blessed |
268 | hash reference uses "HASH" as its I<type>. |
269 | |
270 | The class methods invoked for modifying and fetching are these: |
271 | |
272 | =over 4 |
273 | |
274 | =item FETCH_I<type>_ATTRIBUTES |
275 | |
630ad279 |
276 | This method is called with two arguments: the relevant package name, |
277 | and a reference to a variable or subroutine for which package-defined |
278 | attributes are desired. The expected return value is a list of |
279 | associated attributes. This list may be empty. |
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280 | |
281 | =item MODIFY_I<type>_ATTRIBUTES |
282 | |
283 | This method is called with two fixed arguments, followed by the list of |
284 | attributes from the relevant declaration. The two fixed arguments are |
285 | the relevant package name and a reference to the declared subroutine or |
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286 | variable. The expected return value is a list of attributes which were |
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287 | not recognized by this handler. Note that this allows for a derived class |
288 | to delegate a call to its base class, and then only examine the attributes |
289 | which the base class didn't already handle for it. |
290 | |
291 | The call to this method is currently made I<during> the processing of the |
292 | declaration. In particular, this means that a subroutine reference will |
293 | probably be for an undefined subroutine, even if this declaration is |
294 | actually part of the definition. |
295 | |
296 | =back |
297 | |
298 | Calling C<attributes::get()> from within the scope of a null package |
299 | declaration C<package ;> for an unblessed variable reference will |
300 | not provide any starting package name for the 'fetch' method lookup. |
301 | Thus, this circumstance will not result in a method call for package-defined |
302 | attributes. A named subroutine knows to which symbol table entry it belongs |
303 | (or originally belonged), and it will use the corresponding package. |
304 | An anonymous subroutine knows the package name into which it was compiled |
305 | (unless it was also compiled with a null package declaration), and so it |
306 | will use that package name. |
307 | |
308 | =head2 Syntax of Attribute Lists |
309 | |
310 | An attribute list is a sequence of attribute specifications, separated by |
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311 | whitespace or a colon (with optional whitespace). |
312 | Each attribute specification is a simple |
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313 | name, optionally followed by a parenthesised parameter list. |
314 | If such a parameter list is present, it is scanned past as for the rules |
315 | for the C<q()> operator. (See L<perlop/"Quote and Quote-like Operators">.) |
316 | The parameter list is passed as it was found, however, and not as per C<q()>. |
317 | |
318 | Some examples of syntactically valid attribute lists: |
319 | |
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320 | switch(10,foo(7,3)) : expensive |
321 | Ugly('\(") :Bad |
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322 | _5x5 |
323 | locked method |
324 | |
325 | Some examples of syntactically invalid attribute lists (with annotation): |
326 | |
327 | switch(10,foo() # ()-string not balanced |
328 | Ugly('(') # ()-string not balanced |
329 | 5x5 # "5x5" not a valid identifier |
330 | Y2::north # "Y2::north" not a simple identifier |
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331 | foo + bar # "+" neither a colon nor whitespace |
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332 | |
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333 | =head1 EXPORTS |
334 | |
335 | =head2 Default exports |
336 | |
337 | None. |
338 | |
339 | =head2 Available exports |
340 | |
341 | The routines C<get> and C<reftype> are exportable. |
342 | |
343 | =head2 Export tags defined |
344 | |
345 | The C<:ALL> tag will get all of the above exports. |
346 | |
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347 | =head1 EXAMPLES |
348 | |
349 | Here are some samples of syntactically valid declarations, with annotation |
350 | as to how they resolve internally into C<use attributes> invocations by |
351 | perl. These examples are primarily useful to see how the "appropriate |
352 | package" is found for the possible method lookups for package-defined |
353 | attributes. |
354 | |
355 | =over 4 |
356 | |
357 | =item 1. |
358 | |
359 | Code: |
360 | |
361 | package Canine; |
362 | package Dog; |
363 | my Canine $spot : Watchful ; |
364 | |
365 | Effect: |
366 | |
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367 | use attributes (); |
368 | attributes::->import(Canine => \$spot, "Watchful"); |
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369 | |
370 | =item 2. |
371 | |
372 | Code: |
373 | |
374 | package Felis; |
375 | my $cat : Nervous; |
376 | |
377 | Effect: |
378 | |
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379 | use attributes (); |
380 | attributes::->import(Felis => \$cat, "Nervous"); |
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381 | |
382 | =item 3. |
383 | |
384 | Code: |
385 | |
386 | package X; |
387 | sub foo : locked ; |
388 | |
389 | Effect: |
390 | |
391 | use attributes X => \&foo, "locked"; |
392 | |
393 | =item 4. |
394 | |
395 | Code: |
396 | |
397 | package X; |
398 | sub Y::x : locked { 1 } |
399 | |
400 | Effect: |
401 | |
402 | use attributes Y => \&Y::x, "locked"; |
403 | |
404 | =item 5. |
405 | |
406 | Code: |
407 | |
408 | package X; |
409 | sub foo { 1 } |
410 | |
411 | package Y; |
412 | BEGIN { *bar = \&X::foo; } |
413 | |
414 | package Z; |
415 | sub Y::bar : locked ; |
416 | |
417 | Effect: |
418 | |
419 | use attributes X => \&X::foo, "locked"; |
420 | |
421 | =back |
422 | |
423 | This last example is purely for purposes of completeness. You should not |
424 | be trying to mess with the attributes of something in a package that's |
425 | not your own. |
426 | |
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427 | =head1 MORE EXAMPLES |
428 | |
429 | =over 4 |
430 | |
431 | =item 1. |
432 | |
433 | sub MODIFY_CODE_ATTRIBUTES { |
434 | my ($class,$code,@attrs) = @_; |
435 | |
436 | my $allowed = 'MyAttribute'; |
437 | my @bad = grep { $_ ne $allowed } @attrs; |
438 | |
439 | return @bad; |
440 | } |
441 | |
442 | sub foo : MyAttribute { |
443 | print "foo\n"; |
444 | } |
445 | |
446 | This example runs. At compile time C<MODIFY_CODE_ATTRIBUTES> is called. In that |
447 | subroutine, we check if any attribute is disallowed and we return a list of |
448 | these "bad attributes". |
449 | |
450 | As we return an empty list, everything is fine. |
451 | |
452 | =item 2. |
453 | |
454 | sub MODIFY_CODE_ATTRIBUTES { |
455 | my ($class,$code,@attrs) = @_; |
456 | |
457 | my $allowed = 'MyAttribute'; |
458 | my @bad = grep{ $_ ne $allowed }@attrs; |
459 | |
460 | return @bad; |
461 | } |
462 | |
463 | sub foo : MyAttribute Test { |
464 | print "foo\n"; |
465 | } |
466 | |
467 | This example is aborted at compile time as we use the attribute "Test" which |
468 | isn't allowed. C<MODIFY_CODE_ATTRIBUTES> returns a list that contains a single |
469 | element ('Test'). |
470 | |
471 | =back |
472 | |
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473 | =head1 SEE ALSO |
474 | |
475 | L<perlsub/"Private Variables via my()"> and |
476 | L<perlsub/"Subroutine Attributes"> for details on the basic declarations; |
477 | L<attrs> for the obsolescent form of subroutine attribute specification |
478 | which this module replaces; |
479 | L<perlfunc/use> for details on the normal invocation mechanism. |
480 | |
481 | =cut |
482 | |