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0453d815 1=head1 NAME
2
3perllexwarn - Perl Lexical Warnings
4
5=head1 DESCRIPTION
5a3e7812 6
4438c4b7 7The C<use warnings> pragma is a replacement for both the command line
0453d815 8flag B<-w> and the equivalent Perl variable, C<$^W>.
9
10The pragma works just like the existing "strict" pragma.
11This means that the scope of the warning pragma is limited to the
106325ad 12enclosing block. It also means that the pragma setting will not
0453d815 13leak across files (via C<use>, C<require> or C<do>). This allows
14authors to independently define the degree of warning checks that will
15be applied to their module.
16
17By default, optional warnings are disabled, so any legacy code that
18doesn't attempt to control the warnings will work unchanged.
19
20All warnings are enabled in a block by either of these:
c47ff5f1 21
4438c4b7 22 use warnings ;
23 use warnings 'all' ;
c47ff5f1 24
0453d815 25Similarly all warnings are disabled in a block by either of these:
26
4438c4b7 27 no warnings ;
28 no warnings 'all' ;
0453d815 29
30For example, consider the code below:
31
4438c4b7 32 use warnings ;
f1f33818 33 my @a ;
0453d815 34 {
4438c4b7 35 no warnings ;
f1f33818 36 my $b = @a[0] ;
0453d815 37 }
f1f33818 38 my $c = @a[0];
0453d815 39
40The code in the enclosing block has warnings enabled, but the inner
f1f33818 41block has them disabled. In this case that means the assignment to the
42scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
43warning, but the assignment to the scalar C<$b> will not.
0453d815 44
45=head2 Default Warnings and Optional Warnings
46
47Before the introduction of lexical warnings, Perl had two classes of
48warnings: mandatory and optional.
49
50As its name suggests, if your code tripped a mandatory warning, you
51would get a warning whether you wanted it or not.
252aa082 52For example, the code below would always produce an C<"isn't numeric">
53warning about the "2:".
0453d815 54
252aa082 55 my $a = "2:" + 3;
0453d815 56
0453d815 57With the introduction of lexical warnings, mandatory warnings now become
58I<default> warnings. The difference is that although the previously
59mandatory warnings are still enabled by default, they can then be
60subsequently enabled or disabled with the lexical warning pragma. For
e476b1b5 61example, in the code below, an C<"isn't numeric"> warning will only
0453d815 62be reported for the C<$a> variable.
63
252aa082 64 my $a = "2:" + 3;
4438c4b7 65 no warnings ;
252aa082 66 my $b = "2:" + 3;
0453d815 67
68Note that neither the B<-w> flag or the C<$^W> can be used to
69disable/enable default warnings. They are still mandatory in this case.
70
71=head2 What's wrong with B<-w> and C<$^W>
72
73Although very useful, the big problem with using B<-w> on the command
74line to enable warnings is that it is all or nothing. Take the typical
75scenario when you are writing a Perl program. Parts of the code you
76will write yourself, but it's very likely that you will make use of
77pre-written Perl modules. If you use the B<-w> flag in this case, you
78end up enabling warnings in pieces of code that you haven't written.
79
80Similarly, using C<$^W> to either disable or enable blocks of code is
81fundamentally flawed. For a start, say you want to disable warnings in
82a block of code. You might expect this to be enough to do the trick:
83
84 {
85 local ($^W) = 0 ;
86 my $a =+ 2 ;
87 my $b ; chop $b ;
88 }
89
90When this code is run with the B<-w> flag, a warning will be produced
91for the C<$a> line -- C<"Reversed += operator">.
92
93The problem is that Perl has both compile-time and run-time warnings. To
94disable compile-time warnings you need to rewrite the code like this:
95
96 {
97 BEGIN { $^W = 0 }
98 my $a =+ 2 ;
99 my $b ; chop $b ;
100 }
101
f1f33818 102The other big problem with C<$^W> is the way you can inadvertently
0453d815 103change the warning setting in unexpected places in your code. For example,
104when the code below is run (without the B<-w> flag), the second call
105to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
106the first will not.
107
108 sub doit
109 {
110 my $b ; chop $b ;
111 }
112
113 doit() ;
114
115 {
116 local ($^W) = 1 ;
117 doit()
118 }
119
120This is a side-effect of C<$^W> being dynamically scoped.
121
122Lexical warnings get around these limitations by allowing finer control
123over where warnings can or can't be tripped.
124
125=head2 Controlling Warnings from the Command Line
126
127There are three Command Line flags that can be used to control when
128warnings are (or aren't) produced:
129
130=over 5
131
132=item B<-w>
133
134This is the existing flag. If the lexical warnings pragma is B<not>
135used in any of you code, or any of the modules that you use, this flag
136will enable warnings everywhere. See L<Backward Compatibility> for
137details of how this flag interacts with lexical warnings.
138
139=item B<-W>
c47ff5f1 140
0453d815 141If the B<-W> flag is used on the command line, it will enable all warnings
142throughout the program regardless of whether warnings were disabled
4438c4b7 143locally using C<no warnings> or C<$^W =0>. This includes all files that get
0453d815 144included via C<use>, C<require> or C<do>.
145Think of it as the Perl equivalent of the "lint" command.
146
147=item B<-X>
148
149Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
150
151=back
152
153=head2 Backward Compatibility
154
155If you are used with working with a version of Perl prior to the
156introduction of lexically scoped warnings, or have code that uses both
157lexical warnings and C<$^W>, this section will describe how they interact.
158
159How Lexical Warnings interact with B<-w>/C<$^W>:
5a3e7812 160
0453d815 161=over 5
162
163=item 1.
164
165If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
e476b1b5 166control warnings is used and neither C<$^W> or the C<warnings> pragma
167are used, then default warnings will be enabled and optional warnings
168disabled.
0453d815 169This means that legacy code that doesn't attempt to control the warnings
170will work unchanged.
171
172=item 2.
173
174The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
175means that any legacy code that currently relies on manipulating C<$^W>
176to control warning behavior will still work as is.
177
178=item 3.
c47ff5f1 179
0453d815 180Apart from now being a boolean, the C<$^W> variable operates in exactly
181the same horrible uncontrolled global way, except that it cannot
182disable/enable default warnings.
183
184=item 4.
c47ff5f1 185
e476b1b5 186If a piece of code is under the control of the C<warnings> pragma,
0453d815 187both the C<$^W> variable and the B<-w> flag will be ignored for the
188scope of the lexical warning.
189
190=item 5.
c47ff5f1 191
0453d815 192The only way to override a lexical warnings setting is with the B<-W>
193or B<-X> command line flags.
194
195=back
196
106325ad 197The combined effect of 3 & 4 is that it will allow code which uses
e476b1b5 198the C<warnings> pragma to control the warning behavior of $^W-type
0453d815 199code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
200
0453d815 201=head2 Category Hierarchy
c47ff5f1 202
e476b1b5 203A hierarchy of "categories" have been defined to allow groups of warnings
204to be enabled/disabled in isolation.
205
206The current hierarchy is:
207
208 all -+
209 |
e476b1b5 210 +- closure
211 |
12bcd1a6 212 +- deprecated
213 |
e476b1b5 214 +- exiting
215 |
216 +- glob
217 |
218 +- io -----------+
219 | |
220 | +- closed
221 | |
222 | +- exec
223 | |
224 | +- newline
225 | |
226 | +- pipe
227 | |
228 | +- unopened
229 |
230 +- misc
231 |
232 +- numeric
233 |
234 +- once
235 |
236 +- overflow
237 |
238 +- pack
239 |
240 +- portable
241 |
242 +- recursion
243 |
244 +- redefine
245 |
246 +- regexp
247 |
248 +- severe -------+
249 | |
250 | +- debugging
251 | |
252 | +- inplace
253 | |
254 | +- internal
255 | |
256 | +- malloc
257 |
258 +- signal
259 |
260 +- substr
261 |
262 +- syntax -------+
263 | |
264 | +- ambiguous
265 | |
266 | +- bareword
267 | |
e476b1b5 268 | +- digit
269 | |
270 | +- parenthesis
271 | |
272 | +- precedence
273 | |
274 | +- printf
275 | |
276 | +- prototype
277 | |
278 | +- qw
279 | |
280 | +- reserved
281 | |
282 | +- semicolon
283 |
284 +- taint
285 |
e476b1b5 286 +- uninitialized
287 |
288 +- unpack
289 |
290 +- untie
291 |
292 +- utf8
293 |
294 +- void
295 |
296 +- y2k
0453d815 297
4438c4b7 298Just like the "strict" pragma any of these categories can be combined
299
300 use warnings qw(void redefine) ;
301 no warnings qw(io syntax untie) ;
302
303Also like the "strict" pragma, if there is more than one instance of the
e476b1b5 304C<warnings> pragma in a given scope the cumulative effect is additive.
4438c4b7 305
306 use warnings qw(void) ; # only "void" warnings enabled
307 ...
308 use warnings qw(io) ; # only "void" & "io" warnings enabled
309 ...
310 no warnings qw(void) ; # only "io" warnings enabled
311
e476b1b5 312To determine which category a specific warning has been assigned to see
313L<perldiag>.
0453d815 314
12bcd1a6 315Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
316sub-category of the "syntax" category. It is now a top-level category
317in its own right.
318
319
0453d815 320=head2 Fatal Warnings
c47ff5f1 321
0453d815 322The presence of the word "FATAL" in the category list will escalate any
e476b1b5 323warnings detected from the categories specified in the lexical scope
f1f33818 324into fatal errors. In the code below, the use of C<time>, C<length>
325and C<join> can all produce a C<"Useless use of xxx in void context">
326warning.
4438c4b7 327
328 use warnings ;
cea6626f 329
f1f33818 330 time ;
cea6626f 331
0453d815 332 {
f1f33818 333 use warnings FATAL => qw(void) ;
334 length "abc" ;
0453d815 335 }
cea6626f 336
f1f33818 337 join "", 1,2,3 ;
cea6626f 338
f1f33818 339 print "done\n" ;
340
341When run it produces this output
342
343 Useless use of time in void context at fatal line 3.
344 Useless use of length in void context at fatal line 7.
345
346The scope where C<length> is used has escalated the C<void> warnings
347category into a fatal error, so the program terminates immediately it
348encounters the warning.
c47ff5f1 349
08540116 350To explicitly disable a "FATAL" warning you just disable the warning it is
351associated with. So, for example, to disable the "void" warning in the
352example above, either of these will do the trick:
353
354 no warnings qw(void);
355 no warnings FATAL => qw(void);
0453d815 356
e476b1b5 357=head2 Reporting Warnings from a Module
358
d3a7d8c7 359The C<warnings> pragma provides a number of functions that are useful for
360module authors. These are used when you want to report a module-specific
7e6d00f8 361warning to a calling module has enabled warnings via the C<warnings>
d3a7d8c7 362pragma.
e476b1b5 363
d3a7d8c7 364Consider the module C<MyMod::Abc> below.
e476b1b5 365
d3a7d8c7 366 package MyMod::Abc;
e476b1b5 367
d3a7d8c7 368 use warnings::register;
369
370 sub open {
371 my $path = shift ;
372 if (warnings::enabled() && $path !~ m#^/#) {
373 warnings::warn("changing relative path to /tmp/");
374 $path = "/tmp/$path" ;
375 }
376 }
377
378 1 ;
379
380The call to C<warnings::register> will create a new warnings category
7e6d00f8 381called "MyMod::abc", i.e. the new category name matches the current
382package name. The C<open> function in the module will display a warning
383message if it gets given a relative path as a parameter. This warnings
384will only be displayed if the code that uses C<MyMod::Abc> has actually
385enabled them with the C<warnings> pragma like below.
d3a7d8c7 386
387 use MyMod::Abc;
388 use warnings 'MyMod::Abc';
389 ...
390 abc::open("../fred.txt");
391
392It is also possible to test whether the pre-defined warnings categories are
393set in the calling module with the C<warnings::enabled> function. Consider
394this snippet of code:
395
396 package MyMod::Abc;
397
398 sub open {
7e6d00f8 399 warnings::warnif("deprecated",
400 "open is deprecated, use new instead") ;
e476b1b5 401 new(@_) ;
402 }
6bc102ca 403
e476b1b5 404 sub new
405 ...
406 1 ;
407
408The function C<open> has been deprecated, so code has been included to
409display a warning message whenever the calling module has (at least) the
410"deprecated" warnings category enabled. Something like this, say.
411
412 use warnings 'deprecated';
d3a7d8c7 413 use MyMod::Abc;
e476b1b5 414 ...
d3a7d8c7 415 MyMod::Abc::open($filename) ;
e476b1b5 416
7e6d00f8 417Either the C<warnings::warn> or C<warnings::warnif> function should be
418used to actually display the warnings message. This is because they can
419make use of the feature that allows warnings to be escalated into fatal
420errors. So in this case
e476b1b5 421
d3a7d8c7 422 use MyMod::Abc;
423 use warnings FATAL => 'MyMod::Abc';
e476b1b5 424 ...
d3a7d8c7 425 MyMod::Abc::open('../fred.txt');
e476b1b5 426
7e6d00f8 427the C<warnings::warnif> function will detect this and die after
d3a7d8c7 428displaying the warning message.
e476b1b5 429
7e6d00f8 430The three warnings functions, C<warnings::warn>, C<warnings::warnif>
431and C<warnings::enabled> can optionally take an object reference in place
432of a category name. In this case the functions will use the class name
433of the object as the warnings category.
434
435Consider this example:
436
437 package Original ;
438
439 no warnings ;
440 use warnings::register ;
441
442 sub new
443 {
444 my $class = shift ;
445 bless [], $class ;
446 }
447
448 sub check
449 {
450 my $self = shift ;
451 my $value = shift ;
452
453 if ($value % 2 && warnings::enabled($self))
454 { warnings::warn($self, "Odd numbers are unsafe") }
455 }
456
457 sub doit
458 {
459 my $self = shift ;
460 my $value = shift ;
461 $self->check($value) ;
462 # ...
463 }
464
465 1 ;
466
467 package Derived ;
468
469 use warnings::register ;
470 use Original ;
471 our @ISA = qw( Original ) ;
472 sub new
473 {
474 my $class = shift ;
475 bless [], $class ;
476 }
477
13a2d996 478
7e6d00f8 479 1 ;
480
481The code below makes use of both modules, but it only enables warnings from
482C<Derived>.
483
484 use Original ;
485 use Derived ;
486 use warnings 'Derived';
487 my $a = new Original ;
488 $a->doit(1) ;
489 my $b = new Derived ;
490 $a->doit(1) ;
491
492When this code is run only the C<Derived> object, C<$b>, will generate
493a warning.
494
495 Odd numbers are unsafe at main.pl line 7
496
497Notice also that the warning is reported at the line where the object is first
498used.
499
e476b1b5 500=head1 TODO
c47ff5f1 501
0453d815 502 perl5db.pl
503 The debugger saves and restores C<$^W> at runtime. I haven't checked
504 whether the debugger will still work with the lexical warnings
505 patch applied.
506
507 diagnostics.pm
508 I *think* I've got diagnostics to work with the lexical warnings
509 patch, but there were design decisions made in diagnostics to work
510 around the limitations of C<$^W>. Now that those limitations are gone,
511 the module should be revisited.
512
7e6d00f8 513 document calling the warnings::* functions from XS
514
0453d815 515=head1 SEE ALSO
516
e476b1b5 517L<warnings>, L<perldiag>.
c47ff5f1 518
0453d815 519=head1 AUTHOR
c47ff5f1 520
0453d815 521Paul Marquess