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0453d815 | 1 | =head1 NAME |
d74e8afc | 2 | X<warning, lexical> X<warnings> X<warning> |
0453d815 PM |
3 | |
4 | perllexwarn - Perl Lexical Warnings | |
5 | ||
6 | =head1 DESCRIPTION | |
5a3e7812 | 7 | |
00eb3f2b RGS |
8 | The C<use warnings> pragma enables to control precisely what warnings are |
9 | to be enabled in which parts of a Perl program. It's a more flexible | |
10 | alternative for both the command line flag B<-w> and the equivalent Perl | |
11 | variable, C<$^W>. | |
0453d815 | 12 | |
00eb3f2b | 13 | This pragma works just like the C<strict> pragma. |
0453d815 | 14 | This means that the scope of the warning pragma is limited to the |
106325ad | 15 | enclosing block. It also means that the pragma setting will not |
0453d815 PM |
16 | leak across files (via C<use>, C<require> or C<do>). This allows |
17 | authors to independently define the degree of warning checks that will | |
18 | be applied to their module. | |
19 | ||
20 | By default, optional warnings are disabled, so any legacy code that | |
21 | doesn't attempt to control the warnings will work unchanged. | |
22 | ||
23 | All warnings are enabled in a block by either of these: | |
c47ff5f1 | 24 | |
4358a253 SS |
25 | use warnings; |
26 | use warnings 'all'; | |
c47ff5f1 | 27 | |
0453d815 PM |
28 | Similarly all warnings are disabled in a block by either of these: |
29 | ||
4358a253 SS |
30 | no warnings; |
31 | no warnings 'all'; | |
0453d815 PM |
32 | |
33 | For example, consider the code below: | |
34 | ||
4358a253 SS |
35 | use warnings; |
36 | my @a; | |
0453d815 | 37 | { |
4358a253 SS |
38 | no warnings; |
39 | my $b = @a[0]; | |
0453d815 | 40 | } |
f1f33818 | 41 | my $c = @a[0]; |
0453d815 PM |
42 | |
43 | The code in the enclosing block has warnings enabled, but the inner | |
f1f33818 PM |
44 | block has them disabled. In this case that means the assignment to the |
45 | scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]"> | |
46 | warning, but the assignment to the scalar C<$b> will not. | |
0453d815 PM |
47 | |
48 | =head2 Default Warnings and Optional Warnings | |
49 | ||
50 | Before the introduction of lexical warnings, Perl had two classes of | |
51 | warnings: mandatory and optional. | |
52 | ||
53 | As its name suggests, if your code tripped a mandatory warning, you | |
54 | would get a warning whether you wanted it or not. | |
252aa082 JH |
55 | For example, the code below would always produce an C<"isn't numeric"> |
56 | warning about the "2:". | |
0453d815 | 57 | |
252aa082 | 58 | my $a = "2:" + 3; |
0453d815 | 59 | |
0453d815 PM |
60 | With the introduction of lexical warnings, mandatory warnings now become |
61 | I<default> warnings. The difference is that although the previously | |
62 | mandatory warnings are still enabled by default, they can then be | |
63 | subsequently enabled or disabled with the lexical warning pragma. For | |
e476b1b5 | 64 | example, in the code below, an C<"isn't numeric"> warning will only |
0453d815 PM |
65 | be reported for the C<$a> variable. |
66 | ||
252aa082 | 67 | my $a = "2:" + 3; |
4358a253 | 68 | no warnings; |
252aa082 | 69 | my $b = "2:" + 3; |
0453d815 PM |
70 | |
71 | Note that neither the B<-w> flag or the C<$^W> can be used to | |
72 | disable/enable default warnings. They are still mandatory in this case. | |
73 | ||
74 | =head2 What's wrong with B<-w> and C<$^W> | |
75 | ||
76 | Although very useful, the big problem with using B<-w> on the command | |
77 | line to enable warnings is that it is all or nothing. Take the typical | |
78 | scenario when you are writing a Perl program. Parts of the code you | |
79 | will write yourself, but it's very likely that you will make use of | |
80 | pre-written Perl modules. If you use the B<-w> flag in this case, you | |
81 | end up enabling warnings in pieces of code that you haven't written. | |
82 | ||
83 | Similarly, using C<$^W> to either disable or enable blocks of code is | |
84 | fundamentally flawed. For a start, say you want to disable warnings in | |
85 | a block of code. You might expect this to be enough to do the trick: | |
86 | ||
87 | { | |
4358a253 SS |
88 | local ($^W) = 0; |
89 | my $a =+ 2; | |
90 | my $b; chop $b; | |
0453d815 PM |
91 | } |
92 | ||
93 | When this code is run with the B<-w> flag, a warning will be produced | |
ac036724 | 94 | for the C<$a> line: C<"Reversed += operator">. |
0453d815 PM |
95 | |
96 | The problem is that Perl has both compile-time and run-time warnings. To | |
97 | disable compile-time warnings you need to rewrite the code like this: | |
98 | ||
99 | { | |
100 | BEGIN { $^W = 0 } | |
4358a253 SS |
101 | my $a =+ 2; |
102 | my $b; chop $b; | |
0453d815 PM |
103 | } |
104 | ||
f1f33818 | 105 | The other big problem with C<$^W> is the way you can inadvertently |
0453d815 PM |
106 | change the warning setting in unexpected places in your code. For example, |
107 | when the code below is run (without the B<-w> flag), the second call | |
108 | to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas | |
109 | the first will not. | |
110 | ||
111 | sub doit | |
112 | { | |
4358a253 | 113 | my $b; chop $b; |
0453d815 PM |
114 | } |
115 | ||
4358a253 | 116 | doit(); |
0453d815 PM |
117 | |
118 | { | |
4358a253 | 119 | local ($^W) = 1; |
0453d815 PM |
120 | doit() |
121 | } | |
122 | ||
123 | This is a side-effect of C<$^W> being dynamically scoped. | |
124 | ||
125 | Lexical warnings get around these limitations by allowing finer control | |
126 | over where warnings can or can't be tripped. | |
127 | ||
128 | =head2 Controlling Warnings from the Command Line | |
129 | ||
130 | There are three Command Line flags that can be used to control when | |
131 | warnings are (or aren't) produced: | |
132 | ||
133 | =over 5 | |
134 | ||
135 | =item B<-w> | |
d74e8afc | 136 | X<-w> |
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137 | |
138 | This is the existing flag. If the lexical warnings pragma is B<not> | |
139 | used in any of you code, or any of the modules that you use, this flag | |
140 | will enable warnings everywhere. See L<Backward Compatibility> for | |
141 | details of how this flag interacts with lexical warnings. | |
142 | ||
143 | =item B<-W> | |
d74e8afc | 144 | X<-W> |
c47ff5f1 | 145 | |
0453d815 PM |
146 | If the B<-W> flag is used on the command line, it will enable all warnings |
147 | throughout the program regardless of whether warnings were disabled | |
4438c4b7 | 148 | locally using C<no warnings> or C<$^W =0>. This includes all files that get |
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149 | included via C<use>, C<require> or C<do>. |
150 | Think of it as the Perl equivalent of the "lint" command. | |
151 | ||
152 | =item B<-X> | |
d74e8afc | 153 | X<-X> |
0453d815 PM |
154 | |
155 | Does the exact opposite to the B<-W> flag, i.e. it disables all warnings. | |
156 | ||
157 | =back | |
158 | ||
159 | =head2 Backward Compatibility | |
160 | ||
1dc2b704 | 161 | If you are used to working with a version of Perl prior to the |
0453d815 PM |
162 | introduction of lexically scoped warnings, or have code that uses both |
163 | lexical warnings and C<$^W>, this section will describe how they interact. | |
164 | ||
165 | How Lexical Warnings interact with B<-w>/C<$^W>: | |
5a3e7812 | 166 | |
0453d815 PM |
167 | =over 5 |
168 | ||
169 | =item 1. | |
170 | ||
171 | If none of the three command line flags (B<-w>, B<-W> or B<-X>) that | |
1dc2b704 | 172 | control warnings is used and neither C<$^W> nor the C<warnings> pragma |
e476b1b5 GS |
173 | are used, then default warnings will be enabled and optional warnings |
174 | disabled. | |
0453d815 PM |
175 | This means that legacy code that doesn't attempt to control the warnings |
176 | will work unchanged. | |
177 | ||
178 | =item 2. | |
179 | ||
ac036724 | 180 | The B<-w> flag just sets the global C<$^W> variable as in 5.005. This |
0453d815 PM |
181 | means that any legacy code that currently relies on manipulating C<$^W> |
182 | to control warning behavior will still work as is. | |
183 | ||
184 | =item 3. | |
c47ff5f1 | 185 | |
0453d815 PM |
186 | Apart from now being a boolean, the C<$^W> variable operates in exactly |
187 | the same horrible uncontrolled global way, except that it cannot | |
188 | disable/enable default warnings. | |
189 | ||
190 | =item 4. | |
c47ff5f1 | 191 | |
e476b1b5 | 192 | If a piece of code is under the control of the C<warnings> pragma, |
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193 | both the C<$^W> variable and the B<-w> flag will be ignored for the |
194 | scope of the lexical warning. | |
195 | ||
196 | =item 5. | |
c47ff5f1 | 197 | |
0453d815 PM |
198 | The only way to override a lexical warnings setting is with the B<-W> |
199 | or B<-X> command line flags. | |
200 | ||
201 | =back | |
202 | ||
106325ad | 203 | The combined effect of 3 & 4 is that it will allow code which uses |
e476b1b5 | 204 | the C<warnings> pragma to control the warning behavior of $^W-type |
0453d815 PM |
205 | code (using a C<local $^W=0>) if it really wants to, but not vice-versa. |
206 | ||
0453d815 | 207 | =head2 Category Hierarchy |
d74e8afc | 208 | X<warning, categories> |
c47ff5f1 | 209 | |
e476b1b5 GS |
210 | A hierarchy of "categories" have been defined to allow groups of warnings |
211 | to be enabled/disabled in isolation. | |
212 | ||
213 | The current hierarchy is: | |
214 | ||
6f87cb12 FC |
215 | all -+ |
216 | | | |
217 | +- closure | |
218 | | | |
219 | +- deprecated | |
220 | | | |
221 | +- exiting | |
222 | | | |
f1d34ca8 FC |
223 | +- experimental --+ |
224 | | | | |
225 | | +- experimental::lexical_subs | |
6f87cb12 FC |
226 | | |
227 | +- glob | |
228 | | | |
229 | +- imprecision | |
230 | | | |
231 | +- io ------------+ | |
232 | | | | |
233 | | +- closed | |
234 | | | | |
235 | | +- exec | |
236 | | | | |
237 | | +- layer | |
238 | | | | |
239 | | +- newline | |
240 | | | | |
241 | | +- pipe | |
242 | | | | |
243 | | +- unopened | |
244 | | | |
245 | +- misc | |
246 | | | |
247 | +- numeric | |
248 | | | |
249 | +- once | |
250 | | | |
251 | +- overflow | |
252 | | | |
253 | +- pack | |
254 | | | |
255 | +- portable | |
256 | | | |
257 | +- recursion | |
258 | | | |
259 | +- redefine | |
260 | | | |
261 | +- regexp | |
262 | | | |
263 | +- severe --------+ | |
264 | | | | |
265 | | +- debugging | |
266 | | | | |
267 | | +- inplace | |
268 | | | | |
269 | | +- internal | |
270 | | | | |
271 | | +- malloc | |
272 | | | |
273 | +- signal | |
274 | | | |
275 | +- substr | |
276 | | | |
277 | +- syntax --------+ | |
278 | | | | |
279 | | +- ambiguous | |
280 | | | | |
281 | | +- bareword | |
282 | | | | |
283 | | +- digit | |
284 | | | | |
285 | | +- illegalproto | |
286 | | | | |
287 | | +- parenthesis | |
288 | | | | |
289 | | +- precedence | |
290 | | | | |
291 | | +- printf | |
292 | | | | |
293 | | +- prototype | |
294 | | | | |
295 | | +- qw | |
296 | | | | |
297 | | +- reserved | |
298 | | | | |
299 | | +- semicolon | |
300 | | | |
301 | +- taint | |
302 | | | |
303 | +- threads | |
304 | | | |
305 | +- uninitialized | |
306 | | | |
307 | +- unpack | |
308 | | | |
309 | +- untie | |
310 | | | |
311 | +- utf8 ----------+ | |
312 | | | | |
313 | | +- non_unicode | |
314 | | | | |
315 | | +- nonchar | |
316 | | | | |
317 | | +- surrogate | |
318 | | | |
319 | +- void | |
0453d815 | 320 | |
4438c4b7 JH |
321 | Just like the "strict" pragma any of these categories can be combined |
322 | ||
4358a253 SS |
323 | use warnings qw(void redefine); |
324 | no warnings qw(io syntax untie); | |
4438c4b7 JH |
325 | |
326 | Also like the "strict" pragma, if there is more than one instance of the | |
e476b1b5 | 327 | C<warnings> pragma in a given scope the cumulative effect is additive. |
4438c4b7 | 328 | |
4358a253 | 329 | use warnings qw(void); # only "void" warnings enabled |
4438c4b7 | 330 | ... |
4358a253 | 331 | use warnings qw(io); # only "void" & "io" warnings enabled |
4438c4b7 | 332 | ... |
4358a253 | 333 | no warnings qw(void); # only "io" warnings enabled |
4438c4b7 | 334 | |
e476b1b5 GS |
335 | To determine which category a specific warning has been assigned to see |
336 | L<perldiag>. | |
0453d815 | 337 | |
12bcd1a6 PM |
338 | Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a |
339 | sub-category of the "syntax" category. It is now a top-level category | |
340 | in its own right. | |
341 | ||
0453d815 | 342 | =head2 Fatal Warnings |
d74e8afc | 343 | X<warning, fatal> |
c47ff5f1 | 344 | |
0453d815 | 345 | The presence of the word "FATAL" in the category list will escalate any |
e476b1b5 | 346 | warnings detected from the categories specified in the lexical scope |
f1f33818 PM |
347 | into fatal errors. In the code below, the use of C<time>, C<length> |
348 | and C<join> can all produce a C<"Useless use of xxx in void context"> | |
349 | warning. | |
4438c4b7 | 350 | |
4358a253 | 351 | use warnings; |
cea6626f | 352 | |
4358a253 | 353 | time; |
cea6626f | 354 | |
0453d815 | 355 | { |
4358a253 SS |
356 | use warnings FATAL => qw(void); |
357 | length "abc"; | |
0453d815 | 358 | } |
cea6626f | 359 | |
4358a253 | 360 | join "", 1,2,3; |
cea6626f | 361 | |
4358a253 | 362 | print "done\n"; |
f1f33818 PM |
363 | |
364 | When run it produces this output | |
365 | ||
366 | Useless use of time in void context at fatal line 3. | |
367 | Useless use of length in void context at fatal line 7. | |
368 | ||
369 | The scope where C<length> is used has escalated the C<void> warnings | |
370 | category into a fatal error, so the program terminates immediately it | |
371 | encounters the warning. | |
c47ff5f1 | 372 | |
6e9af7e4 PM |
373 | To explicitly turn off a "FATAL" warning you just disable the warning |
374 | it is associated with. So, for example, to disable the "void" warning | |
375 | in the example above, either of these will do the trick: | |
08540116 PM |
376 | |
377 | no warnings qw(void); | |
378 | no warnings FATAL => qw(void); | |
0453d815 | 379 | |
6e9af7e4 PM |
380 | If you want to downgrade a warning that has been escalated into a fatal |
381 | error back to a normal warning, you can use the "NONFATAL" keyword. For | |
382 | example, the code below will promote all warnings into fatal errors, | |
383 | except for those in the "syntax" category. | |
384 | ||
385 | use warnings FATAL => 'all', NONFATAL => 'syntax'; | |
386 | ||
e476b1b5 | 387 | =head2 Reporting Warnings from a Module |
d74e8afc | 388 | X<warning, reporting> X<warning, registering> |
e476b1b5 | 389 | |
d3a7d8c7 GS |
390 | The C<warnings> pragma provides a number of functions that are useful for |
391 | module authors. These are used when you want to report a module-specific | |
7e6d00f8 | 392 | warning to a calling module has enabled warnings via the C<warnings> |
d3a7d8c7 | 393 | pragma. |
e476b1b5 | 394 | |
d3a7d8c7 | 395 | Consider the module C<MyMod::Abc> below. |
e476b1b5 | 396 | |
d3a7d8c7 | 397 | package MyMod::Abc; |
e476b1b5 | 398 | |
d3a7d8c7 GS |
399 | use warnings::register; |
400 | ||
401 | sub open { | |
4358a253 | 402 | my $path = shift; |
7ddf7bb5 | 403 | if ($path !~ m#^/#) { |
2359510d | 404 | warnings::warn("changing relative path to /var/abc") |
7ddf7bb5 | 405 | if warnings::enabled(); |
2359510d | 406 | $path = "/var/abc/$path"; |
d3a7d8c7 GS |
407 | } |
408 | } | |
409 | ||
4358a253 | 410 | 1; |
d3a7d8c7 GS |
411 | |
412 | The call to C<warnings::register> will create a new warnings category | |
c901ad27 | 413 | called "MyMod::Abc", i.e. the new category name matches the current |
7e6d00f8 PM |
414 | package name. The C<open> function in the module will display a warning |
415 | message if it gets given a relative path as a parameter. This warnings | |
416 | will only be displayed if the code that uses C<MyMod::Abc> has actually | |
417 | enabled them with the C<warnings> pragma like below. | |
d3a7d8c7 GS |
418 | |
419 | use MyMod::Abc; | |
420 | use warnings 'MyMod::Abc'; | |
421 | ... | |
422 | abc::open("../fred.txt"); | |
423 | ||
424 | It is also possible to test whether the pre-defined warnings categories are | |
425 | set in the calling module with the C<warnings::enabled> function. Consider | |
426 | this snippet of code: | |
427 | ||
428 | package MyMod::Abc; | |
429 | ||
430 | sub open { | |
7e6d00f8 | 431 | warnings::warnif("deprecated", |
4358a253 SS |
432 | "open is deprecated, use new instead"); |
433 | new(@_); | |
e476b1b5 | 434 | } |
6bc102ca | 435 | |
e476b1b5 GS |
436 | sub new |
437 | ... | |
4358a253 | 438 | 1; |
e476b1b5 GS |
439 | |
440 | The function C<open> has been deprecated, so code has been included to | |
441 | display a warning message whenever the calling module has (at least) the | |
442 | "deprecated" warnings category enabled. Something like this, say. | |
443 | ||
444 | use warnings 'deprecated'; | |
d3a7d8c7 | 445 | use MyMod::Abc; |
e476b1b5 | 446 | ... |
4358a253 | 447 | MyMod::Abc::open($filename); |
e476b1b5 | 448 | |
7e6d00f8 PM |
449 | Either the C<warnings::warn> or C<warnings::warnif> function should be |
450 | used to actually display the warnings message. This is because they can | |
451 | make use of the feature that allows warnings to be escalated into fatal | |
452 | errors. So in this case | |
e476b1b5 | 453 | |
d3a7d8c7 GS |
454 | use MyMod::Abc; |
455 | use warnings FATAL => 'MyMod::Abc'; | |
e476b1b5 | 456 | ... |
d3a7d8c7 | 457 | MyMod::Abc::open('../fred.txt'); |
e476b1b5 | 458 | |
7e6d00f8 | 459 | the C<warnings::warnif> function will detect this and die after |
d3a7d8c7 | 460 | displaying the warning message. |
e476b1b5 | 461 | |
7e6d00f8 PM |
462 | The three warnings functions, C<warnings::warn>, C<warnings::warnif> |
463 | and C<warnings::enabled> can optionally take an object reference in place | |
464 | of a category name. In this case the functions will use the class name | |
465 | of the object as the warnings category. | |
466 | ||
467 | Consider this example: | |
468 | ||
4358a253 | 469 | package Original; |
7e6d00f8 | 470 | |
4358a253 SS |
471 | no warnings; |
472 | use warnings::register; | |
7e6d00f8 PM |
473 | |
474 | sub new | |
475 | { | |
4358a253 SS |
476 | my $class = shift; |
477 | bless [], $class; | |
7e6d00f8 PM |
478 | } |
479 | ||
480 | sub check | |
481 | { | |
4358a253 SS |
482 | my $self = shift; |
483 | my $value = shift; | |
7e6d00f8 PM |
484 | |
485 | if ($value % 2 && warnings::enabled($self)) | |
486 | { warnings::warn($self, "Odd numbers are unsafe") } | |
487 | } | |
488 | ||
489 | sub doit | |
490 | { | |
4358a253 SS |
491 | my $self = shift; |
492 | my $value = shift; | |
493 | $self->check($value); | |
7e6d00f8 PM |
494 | # ... |
495 | } | |
496 | ||
4358a253 | 497 | 1; |
7e6d00f8 | 498 | |
4358a253 | 499 | package Derived; |
7e6d00f8 | 500 | |
4358a253 SS |
501 | use warnings::register; |
502 | use Original; | |
503 | our @ISA = qw( Original ); | |
7e6d00f8 PM |
504 | sub new |
505 | { | |
4358a253 SS |
506 | my $class = shift; |
507 | bless [], $class; | |
7e6d00f8 PM |
508 | } |
509 | ||
13a2d996 | 510 | |
4358a253 | 511 | 1; |
7e6d00f8 PM |
512 | |
513 | The code below makes use of both modules, but it only enables warnings from | |
514 | C<Derived>. | |
515 | ||
4358a253 SS |
516 | use Original; |
517 | use Derived; | |
7e6d00f8 | 518 | use warnings 'Derived'; |
63acfd00 | 519 | my $a = Original->new(); |
4358a253 | 520 | $a->doit(1); |
63acfd00 | 521 | my $b = Derived->new(); |
4358a253 | 522 | $a->doit(1); |
7e6d00f8 PM |
523 | |
524 | When this code is run only the C<Derived> object, C<$b>, will generate | |
525 | a warning. | |
526 | ||
527 | Odd numbers are unsafe at main.pl line 7 | |
528 | ||
529 | Notice also that the warning is reported at the line where the object is first | |
530 | used. | |
531 | ||
572bfd36 RS |
532 | When registering new categories of warning, you can supply more names to |
533 | warnings::register like this: | |
534 | ||
535 | package MyModule; | |
536 | use warnings::register qw(format precision); | |
537 | ||
538 | ... | |
539 | ||
540 | warnings::warnif('MyModule::format', '...'); | |
541 | ||
0453d815 PM |
542 | =head1 SEE ALSO |
543 | ||
e476b1b5 | 544 | L<warnings>, L<perldiag>. |
c47ff5f1 | 545 | |
0453d815 | 546 | =head1 AUTHOR |
c47ff5f1 | 547 | |
0453d815 | 548 | Paul Marquess |