2 X<warning, lexical> X<warnings> X<warning>
4 perllexwarn - Perl Lexical Warnings
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
13 This pragma works just like the C<strict> pragma.
14 This means that the scope of the warning pragma is limited to the
15 enclosing block. It also means that the pragma setting will not
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.
20 By default, optional warnings are disabled, so any legacy code that
21 doesn't attempt to control the warnings will work unchanged.
23 All warnings are enabled in a block by either of these:
28 Similarly all warnings are disabled in a block by either of these:
33 For example, consider the code below:
43 The code in the enclosing block has warnings enabled, but the inner
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.
48 =head2 Default Warnings and Optional Warnings
50 Before the introduction of lexical warnings, Perl had two classes of
51 warnings: mandatory and optional.
53 As its name suggests, if your code tripped a mandatory warning, you
54 would get a warning whether you wanted it or not.
55 For example, the code below would always produce an C<"isn't numeric">
56 warning about the "2:".
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
64 example, in the code below, an C<"isn't numeric"> warning will only
65 be reported for the C<$a> variable.
71 Note that neither the B<-w> flag or the C<$^W> can be used to
72 disable/enable default warnings. They are still mandatory in this case.
74 =head2 What's wrong with B<-w> and C<$^W>
76 Although very useful, the big problem with using B<-w> on the command
77 line to enable warnings is that it is all or nothing. Take the typical
78 scenario when you are writing a Perl program. Parts of the code you
79 will write yourself, but it's very likely that you will make use of
80 pre-written Perl modules. If you use the B<-w> flag in this case, you
81 end up enabling warnings in pieces of code that you haven't written.
83 Similarly, using C<$^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:
93 When this code is run with the B<-w> flag, a warning will be produced
94 for the C<$a> line: C<"Reversed += operator">.
96 The problem is that Perl has both compile-time and run-time warnings. To
97 disable compile-time warnings you need to rewrite the code like this:
105 The other big problem with C<$^W> is the way you can inadvertently
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
123 This is a side-effect of C<$^W> being dynamically scoped.
125 Lexical warnings get around these limitations by allowing finer control
126 over where warnings can or can't be tripped.
128 =head2 Controlling Warnings from the Command Line
130 There are three Command Line flags that can be used to control when
131 warnings are (or aren't) produced:
138 This is the existing flag. If the lexical warnings pragma is B<not>
139 used in any of you code, or any of the modules that you use, this flag
140 will enable warnings everywhere. See L<Backward Compatibility> for
141 details of how this flag interacts with lexical warnings.
146 If the B<-W> flag is used on the command line, it will enable all warnings
147 throughout the program regardless of whether warnings were disabled
148 locally using C<no warnings> or C<$^W =0>.
149 This includes all files that get
150 included via C<use>, C<require> or C<do>.
151 Think of it as the Perl equivalent of the "lint" command.
156 Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
160 =head2 Backward Compatibility
162 If you are used to working with a version of Perl prior to the
163 introduction of lexically scoped warnings, or have code that uses both
164 lexical warnings and C<$^W>, this section will describe how they interact.
166 How Lexical Warnings interact with B<-w>/C<$^W>:
172 If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
173 control warnings is used and neither C<$^W> nor the C<warnings> pragma
174 are used, then default warnings will be enabled and optional warnings
176 This means that legacy code that doesn't attempt to control the warnings
181 The B<-w> flag just sets the global C<$^W> variable as in 5.005. This
182 means that any legacy code that currently relies on manipulating C<$^W>
183 to control warning behavior will still work as is.
187 Apart from now being a boolean, the C<$^W> variable operates in exactly
188 the same horrible uncontrolled global way, except that it cannot
189 disable/enable default warnings.
193 If a piece of code is under the control of the C<warnings> pragma,
194 both the C<$^W> variable and the B<-w> flag will be ignored for the
195 scope of the lexical warning.
199 The only way to override a lexical warnings setting is with the B<-W>
200 or B<-X> command line flags.
204 The combined effect of 3 & 4 is that it will allow code which uses
205 the C<warnings> pragma to control the warning behavior of $^W-type
206 code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
208 =head2 Category Hierarchy
209 X<warning, categories>
211 A hierarchy of "categories" have been defined to allow groups of warnings
212 to be enabled/disabled in isolation.
214 The current hierarchy is:
217 This tree is generated by regen/warnings.pl. Any changes made here
220 =for warnings.pl begin
232 | +- experimental::autoderef
234 | +- experimental::lexical_subs
236 | +- experimental::lexical_topic
238 | +- experimental::postderef
240 | +- experimental::regex_sets
242 | +- experimental::signatures
244 | +- experimental::smartmatch
344 Just like the "strict" pragma any of these categories can be combined
346 use warnings qw(void redefine);
347 no warnings qw(io syntax untie);
349 Also like the "strict" pragma, if there is more than one instance of the
350 C<warnings> pragma in a given scope the cumulative effect is additive.
352 use warnings qw(void); # only "void" warnings enabled
354 use warnings qw(io); # only "void" & "io" warnings enabled
356 no warnings qw(void); # only "io" warnings enabled
358 To determine which category a specific warning has been assigned to see
361 Note: Before Perl 5.8.0, the lexical warnings category "deprecated" was a
362 sub-category of the "syntax" category. It is now a top-level category
365 =head2 Fatal Warnings
368 The presence of the word "FATAL" in the category list will escalate any
369 warnings detected from the categories specified in the lexical scope
370 into fatal errors. In the code below, the use of C<time>, C<length>
371 and C<join> can all produce a C<"Useless use of xxx in void context">
379 use warnings FATAL => qw(void);
387 When run it produces this output
389 Useless use of time in void context at fatal line 3.
390 Useless use of length in void context at fatal line 7.
392 The scope where C<length> is used has escalated the C<void> warnings
393 category into a fatal error, so the program terminates immediately when it
394 encounters the warning.
396 To explicitly turn off a "FATAL" warning you just disable the warning
397 it is associated with. So, for example, to disable the "void" warning
398 in the example above, either of these will do the trick:
400 no warnings qw(void);
401 no warnings FATAL => qw(void);
403 If you want to downgrade a warning that has been escalated into a fatal
404 error back to a normal warning, you can use the "NONFATAL" keyword. For
405 example, the code below will promote all warnings into fatal errors,
406 except for those in the "syntax" category.
408 use warnings FATAL => 'all', NONFATAL => 'syntax';
410 As of Perl 5.20, instead of C<< use warnings FATAL => 'all'; >> you can
413 use v5.20; # Perl 5.20 or greater is required for the following
414 use warnings 'FATAL'; # short form of "use warnings FATAL => 'all';"
416 If you want your program to be compatible with versions of Perl before
417 5.20, you must use C<< use warnings FATAL => 'all'; >> instead. (In
418 previous versions of Perl, the behavior of the statements
419 C<< use warnings 'FATAL'; >>, C<< use warnings 'NONFATAL'; >> and
420 C<< no warnings 'FATAL'; >> was unspecified; they did not behave as if
421 they included the C<< => 'all' >> portion. As of 5.20, they do.)
423 B<NOTE:> Users of FATAL warnings, especially
424 those using C<< FATAL => 'all' >>
425 should be fully aware that they are risking future portability of their
426 programs by doing so. Perl makes absolutely no commitments to not
427 introduce new warnings, or warnings categories in the future, and indeed
428 we explicitly reserve the right to do so. Code that may not warn now may
429 warn in a future release of Perl if the Perl5 development team deems it
430 in the best interests of the community to do so. Should code using FATAL
431 warnings break due to the introduction of a new warning we will NOT
432 consider it an incompatible change. Users of FATAL warnings should take
433 special caution during upgrades to check to see if their code triggers
434 any new warnings and should pay particular attention to the fine print of
435 the documentation of the features they use to ensure they do not exploit
436 features that are documented as risky, deprecated, or unspecified, or where
437 the documentation says "so don't do that", or anything with the same sense
438 and spirit. Use of such features in combination with FATAL warnings is
439 ENTIRELY AT THE USER'S RISK.
441 =head2 Reporting Warnings from a Module
442 X<warning, reporting> X<warning, registering>
444 The C<warnings> pragma provides a number of functions that are useful for
445 module authors. These are used when you want to report a module-specific
446 warning to a calling module has enabled warnings via the C<warnings>
449 Consider the module C<MyMod::Abc> below.
453 use warnings::register;
457 if ($path !~ m#^/#) {
458 warnings::warn("changing relative path to /var/abc")
459 if warnings::enabled();
460 $path = "/var/abc/$path";
466 The call to C<warnings::register> will create a new warnings category
467 called "MyMod::Abc", i.e. the new category name matches the current
468 package name. The C<open> function in the module will display a warning
469 message if it gets given a relative path as a parameter. This warnings
470 will only be displayed if the code that uses C<MyMod::Abc> has actually
471 enabled them with the C<warnings> pragma like below.
474 use warnings 'MyMod::Abc';
476 abc::open("../fred.txt");
478 It is also possible to test whether the pre-defined warnings categories are
479 set in the calling module with the C<warnings::enabled> function. Consider
480 this snippet of code:
485 warnings::warnif("deprecated",
486 "open is deprecated, use new instead");
494 The function C<open> has been deprecated, so code has been included to
495 display a warning message whenever the calling module has (at least) the
496 "deprecated" warnings category enabled. Something like this, say.
498 use warnings 'deprecated';
501 MyMod::Abc::open($filename);
503 Either the C<warnings::warn> or C<warnings::warnif> function should be
504 used to actually display the warnings message. This is because they can
505 make use of the feature that allows warnings to be escalated into fatal
506 errors. So in this case
509 use warnings FATAL => 'MyMod::Abc';
511 MyMod::Abc::open('../fred.txt');
513 the C<warnings::warnif> function will detect this and die after
514 displaying the warning message.
516 The three warnings functions, C<warnings::warn>, C<warnings::warnif>
517 and C<warnings::enabled> can optionally take an object reference in place
518 of a category name. In this case the functions will use the class name
519 of the object as the warnings category.
521 Consider this example:
526 use warnings::register;
539 if ($value % 2 && warnings::enabled($self))
540 { warnings::warn($self, "Odd numbers are unsafe") }
547 $self->check($value);
555 use warnings::register;
557 our @ISA = qw( Original );
567 The code below makes use of both modules, but it only enables warnings from
572 use warnings 'Derived';
573 my $a = Original->new();
575 my $b = Derived->new();
578 When this code is run only the C<Derived> object, C<$b>, will generate
581 Odd numbers are unsafe at main.pl line 7
583 Notice also that the warning is reported at the line where the object is first
586 When registering new categories of warning, you can supply more names to
587 warnings::register like this:
590 use warnings::register qw(format precision);
594 warnings::warnif('MyModule::format', '...');
598 L<warnings>, L<perldiag>.