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