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[p5sagit/p5-mst-13.2.git] / pod / perllexwarn.pod
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0453d815 1=head1 NAME
d74e8afc 2X<warning, lexical> X<warnings> X<warning>
0453d815 3
4perllexwarn - Perl Lexical Warnings
5
6=head1 DESCRIPTION
5a3e7812 7
00eb3f2b 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
0453d815 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
4358a253 25 use warnings;
26 use warnings 'all';
c47ff5f1 27
0453d815 28Similarly all warnings are disabled in a block by either of these:
29
4358a253 30 no warnings;
31 no warnings 'all';
0453d815 32
33For example, consider the code below:
34
4358a253 35 use warnings;
36 my @a;
0453d815 37 {
4358a253 38 no warnings;
39 my $b = @a[0];
0453d815 40 }
f1f33818 41 my $c = @a[0];
0453d815 42
43The code in the enclosing block has warnings enabled, but the inner
f1f33818 44block has them disabled. In this case that means the assignment to the
45scalar C<$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.
0453d815 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.
252aa082 55For example, the code below would always produce an C<"isn't numeric">
56warning about the "2:".
0453d815 57
252aa082 58 my $a = "2:" + 3;
0453d815 59
0453d815 60With the introduction of lexical warnings, mandatory warnings now become
61I<default> warnings. The difference is that although the previously
62mandatory warnings are still enabled by default, they can then be
63subsequently enabled or disabled with the lexical warning pragma. For
e476b1b5 64example, in the code below, an C<"isn't numeric"> warning will only
0453d815 65be reported for the C<$a> variable.
66
252aa082 67 my $a = "2:" + 3;
4358a253 68 no warnings;
252aa082 69 my $b = "2:" + 3;
0453d815 70
71Note that neither the B<-w> flag or the C<$^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 {
4358a253 88 local ($^W) = 0;
89 my $a =+ 2;
90 my $b; chop $b;
0453d815 91 }
92
93When this code is run with the B<-w> flag, a warning will be produced
ac036724 94for the C<$a> line: C<"Reversed += operator">.
0453d815 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 }
4358a253 101 my $a =+ 2;
102 my $b; chop $b;
0453d815 103 }
104
f1f33818 105The other big problem with C<$^W> is the way you can inadvertently
0453d815 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 $b;
0453d815 114 }
115
4358a253 116 doit();
0453d815 117
118 {
4358a253 119 local ($^W) = 1;
0453d815 120 doit()
121 }
122
123This is a side-effect of C<$^W> being dynamically scoped.
124
125Lexical warnings get around these limitations by allowing finer control
126over where warnings can or can't be tripped.
127
128=head2 Controlling Warnings from the Command Line
129
130There are three Command Line flags that can be used to control when
131warnings are (or aren't) produced:
132
133=over 5
134
135=item B<-w>
d74e8afc 136X<-w>
0453d815 137
138This is the existing flag. If the lexical warnings pragma is B<not>
139used in any of you code, or any of the modules that you use, this flag
140will enable warnings everywhere. See L<Backward Compatibility> for
141details of how this flag interacts with lexical warnings.
142
143=item B<-W>
d74e8afc 144X<-W>
c47ff5f1 145
0453d815 146If the B<-W> flag is used on the command line, it will enable all warnings
147throughout the program regardless of whether warnings were disabled
4438c4b7 148locally using C<no warnings> or C<$^W =0>. This includes all files that get
0453d815 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>
0453d815 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
0453d815 167=over 5
168
169=item 1.
170
171If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
e476b1b5 172control warnings is used and neither C<$^W> or the C<warnings> pragma
173are used, then default warnings will be enabled and optional warnings
174disabled.
0453d815 175This means that legacy code that doesn't attempt to control the warnings
176will work unchanged.
177
178=item 2.
179
ac036724 180The B<-w> flag just sets the global C<$^W> variable as in 5.005. This
0453d815 181means that any legacy code that currently relies on manipulating C<$^W>
182to control warning behavior will still work as is.
183
184=item 3.
c47ff5f1 185
0453d815 186Apart from now being a boolean, the C<$^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,
0453d815 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
0453d815 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
0453d815 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
e476b1b5 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 |
e476b1b5 217 +- closure
218 |
12bcd1a6 219 +- deprecated
220 |
e476b1b5 221 +- exiting
222 |
223 +- glob
224 |
225 +- io -----------+
226 | |
227 | +- closed
228 | |
229 | +- exec
230 | |
99ef548b 231 | +- layer
232 | |
e476b1b5 233 | +- newline
234 | |
235 | +- pipe
236 | |
237 | +- unopened
238 |
b88df990 239 +- imprecision
240 |
e476b1b5 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 | |
e476b1b5 279 | +- digit
280 | |
197afce1 281 | +- illegalproto
282 | |
e476b1b5 283 | +- parenthesis
284 | |
285 | +- precedence
286 | |
287 | +- printf
288 | |
289 | +- prototype
290 | |
291 | +- qw
292 | |
293 | +- reserved
294 | |
295 | +- semicolon
296 |
297 +- taint
298 |
38875929 299 +- threads
300 |
e476b1b5 301 +- uninitialized
302 |
303 +- unpack
304 |
305 +- untie
306 |
307 +- utf8
308 |
309 +- void
0453d815 310
4438c4b7 311Just like the "strict" pragma any of these categories can be combined
312
4358a253 313 use warnings qw(void redefine);
314 no warnings qw(io syntax untie);
4438c4b7 315
316Also like the "strict" pragma, if there is more than one instance of the
e476b1b5 317C<warnings> pragma in a given scope the cumulative effect is additive.
4438c4b7 318
4358a253 319 use warnings qw(void); # only "void" warnings enabled
4438c4b7 320 ...
4358a253 321 use warnings qw(io); # only "void" & "io" warnings enabled
4438c4b7 322 ...
4358a253 323 no warnings qw(void); # only "io" warnings enabled
4438c4b7 324
e476b1b5 325To determine which category a specific warning has been assigned to see
326L<perldiag>.
0453d815 327
12bcd1a6 328Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
329sub-category of the "syntax" category. It is now a top-level category
330in its own right.
331
332
0453d815 333=head2 Fatal Warnings
d74e8afc 334X<warning, fatal>
c47ff5f1 335
0453d815 336The presence of the word "FATAL" in the category list will escalate any
e476b1b5 337warnings detected from the categories specified in the lexical scope
f1f33818 338into fatal errors. In the code below, the use of C<time>, C<length>
339and C<join> can all produce a C<"Useless use of xxx in void context">
340warning.
4438c4b7 341
4358a253 342 use warnings;
cea6626f 343
4358a253 344 time;
cea6626f 345
0453d815 346 {
4358a253 347 use warnings FATAL => qw(void);
348 length "abc";
0453d815 349 }
cea6626f 350
4358a253 351 join "", 1,2,3;
cea6626f 352
4358a253 353 print "done\n";
f1f33818 354
355When run it produces this output
356
357 Useless use of time in void context at fatal line 3.
358 Useless use of length in void context at fatal line 7.
359
360The scope where C<length> is used has escalated the C<void> warnings
361category into a fatal error, so the program terminates immediately it
362encounters the warning.
c47ff5f1 363
6e9af7e4 364To explicitly turn off a "FATAL" warning you just disable the warning
365it is associated with. So, for example, to disable the "void" warning
366in the example above, either of these will do the trick:
08540116 367
368 no warnings qw(void);
369 no warnings FATAL => qw(void);
0453d815 370
6e9af7e4 371If you want to downgrade a warning that has been escalated into a fatal
372error back to a normal warning, you can use the "NONFATAL" keyword. For
373example, the code below will promote all warnings into fatal errors,
374except for those in the "syntax" category.
375
376 use warnings FATAL => 'all', NONFATAL => 'syntax';
377
e476b1b5 378=head2 Reporting Warnings from a Module
d74e8afc 379X<warning, reporting> X<warning, registering>
e476b1b5 380
d3a7d8c7 381The C<warnings> pragma provides a number of functions that are useful for
382module authors. These are used when you want to report a module-specific
7e6d00f8 383warning to a calling module has enabled warnings via the C<warnings>
d3a7d8c7 384pragma.
e476b1b5 385
d3a7d8c7 386Consider the module C<MyMod::Abc> below.
e476b1b5 387
d3a7d8c7 388 package MyMod::Abc;
e476b1b5 389
d3a7d8c7 390 use warnings::register;
391
392 sub open {
4358a253 393 my $path = shift;
7ddf7bb5 394 if ($path !~ m#^/#) {
2359510d 395 warnings::warn("changing relative path to /var/abc")
7ddf7bb5 396 if warnings::enabled();
2359510d 397 $path = "/var/abc/$path";
d3a7d8c7 398 }
399 }
400
4358a253 401 1;
d3a7d8c7 402
403The call to C<warnings::register> will create a new warnings category
c901ad27 404called "MyMod::Abc", i.e. the new category name matches the current
7e6d00f8 405package name. The C<open> function in the module will display a warning
406message if it gets given a relative path as a parameter. This warnings
407will only be displayed if the code that uses C<MyMod::Abc> has actually
408enabled them with the C<warnings> pragma like below.
d3a7d8c7 409
410 use MyMod::Abc;
411 use warnings 'MyMod::Abc';
412 ...
413 abc::open("../fred.txt");
414
415It is also possible to test whether the pre-defined warnings categories are
416set in the calling module with the C<warnings::enabled> function. Consider
417this snippet of code:
418
419 package MyMod::Abc;
420
421 sub open {
7e6d00f8 422 warnings::warnif("deprecated",
4358a253 423 "open is deprecated, use new instead");
424 new(@_);
e476b1b5 425 }
6bc102ca 426
e476b1b5 427 sub new
428 ...
4358a253 429 1;
e476b1b5 430
431The function C<open> has been deprecated, so code has been included to
432display a warning message whenever the calling module has (at least) the
433"deprecated" warnings category enabled. Something like this, say.
434
435 use warnings 'deprecated';
d3a7d8c7 436 use MyMod::Abc;
e476b1b5 437 ...
4358a253 438 MyMod::Abc::open($filename);
e476b1b5 439
7e6d00f8 440Either the C<warnings::warn> or C<warnings::warnif> function should be
441used to actually display the warnings message. This is because they can
442make use of the feature that allows warnings to be escalated into fatal
443errors. So in this case
e476b1b5 444
d3a7d8c7 445 use MyMod::Abc;
446 use warnings FATAL => 'MyMod::Abc';
e476b1b5 447 ...
d3a7d8c7 448 MyMod::Abc::open('../fred.txt');
e476b1b5 449
7e6d00f8 450the C<warnings::warnif> function will detect this and die after
d3a7d8c7 451displaying the warning message.
e476b1b5 452
7e6d00f8 453The three warnings functions, C<warnings::warn>, C<warnings::warnif>
454and C<warnings::enabled> can optionally take an object reference in place
455of a category name. In this case the functions will use the class name
456of the object as the warnings category.
457
458Consider this example:
459
4358a253 460 package Original;
7e6d00f8 461
4358a253 462 no warnings;
463 use warnings::register;
7e6d00f8 464
465 sub new
466 {
4358a253 467 my $class = shift;
468 bless [], $class;
7e6d00f8 469 }
470
471 sub check
472 {
4358a253 473 my $self = shift;
474 my $value = shift;
7e6d00f8 475
476 if ($value % 2 && warnings::enabled($self))
477 { warnings::warn($self, "Odd numbers are unsafe") }
478 }
479
480 sub doit
481 {
4358a253 482 my $self = shift;
483 my $value = shift;
484 $self->check($value);
7e6d00f8 485 # ...
486 }
487
4358a253 488 1;
7e6d00f8 489
4358a253 490 package Derived;
7e6d00f8 491
4358a253 492 use warnings::register;
493 use Original;
494 our @ISA = qw( Original );
7e6d00f8 495 sub new
496 {
4358a253 497 my $class = shift;
498 bless [], $class;
7e6d00f8 499 }
500
13a2d996 501
4358a253 502 1;
7e6d00f8 503
504The code below makes use of both modules, but it only enables warnings from
505C<Derived>.
506
4358a253 507 use Original;
508 use Derived;
7e6d00f8 509 use warnings 'Derived';
63acfd00 510 my $a = Original->new();
4358a253 511 $a->doit(1);
63acfd00 512 my $b = Derived->new();
4358a253 513 $a->doit(1);
7e6d00f8 514
515When this code is run only the C<Derived> object, C<$b>, will generate
516a warning.
517
518 Odd numbers are unsafe at main.pl line 7
519
520Notice also that the warning is reported at the line where the object is first
521used.
522
0453d815 523=head1 SEE ALSO
524
e476b1b5 525L<warnings>, L<perldiag>.
c47ff5f1 526
0453d815 527=head1 AUTHOR
c47ff5f1 528
0453d815 529Paul Marquess