2 X<warning, lexical> X<warnings> X<warning>
4 perllexwarn - Perl Lexical Warnings
8 The C<use warnings> pragma is a replacement for both the command line
9 flag B<-w> and the equivalent Perl variable, C<$^W>.
11 The pragma works just like the existing "strict" pragma.
12 This means that the scope of the warning pragma is limited to the
13 enclosing block. It also means that the pragma setting will not
14 leak across files (via C<use>, C<require> or C<do>). This allows
15 authors to independently define the degree of warning checks that will
16 be applied to their module.
18 By default, optional warnings are disabled, so any legacy code that
19 doesn't attempt to control the warnings will work unchanged.
21 All warnings are enabled in a block by either of these:
26 Similarly all warnings are disabled in a block by either of these:
31 For example, consider the code below:
41 The code in the enclosing block has warnings enabled, but the inner
42 block has them disabled. In this case that means the assignment to the
43 scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
44 warning, but the assignment to the scalar C<$b> will not.
46 =head2 Default Warnings and Optional Warnings
48 Before the introduction of lexical warnings, Perl had two classes of
49 warnings: mandatory and optional.
51 As its name suggests, if your code tripped a mandatory warning, you
52 would get a warning whether you wanted it or not.
53 For example, the code below would always produce an C<"isn't numeric">
54 warning about the "2:".
58 With the introduction of lexical warnings, mandatory warnings now become
59 I<default> warnings. The difference is that although the previously
60 mandatory warnings are still enabled by default, they can then be
61 subsequently enabled or disabled with the lexical warning pragma. For
62 example, in the code below, an C<"isn't numeric"> warning will only
63 be reported for the C<$a> variable.
69 Note that neither the B<-w> flag or the C<$^W> can be used to
70 disable/enable default warnings. They are still mandatory in this case.
72 =head2 What's wrong with B<-w> and C<$^W>
74 Although very useful, the big problem with using B<-w> on the command
75 line to enable warnings is that it is all or nothing. Take the typical
76 scenario when you are writing a Perl program. Parts of the code you
77 will write yourself, but it's very likely that you will make use of
78 pre-written Perl modules. If you use the B<-w> flag in this case, you
79 end up enabling warnings in pieces of code that you haven't written.
81 Similarly, using C<$^W> to either disable or enable blocks of code is
82 fundamentally flawed. For a start, say you want to disable warnings in
83 a block of code. You might expect this to be enough to do the trick:
91 When this code is run with the B<-w> flag, a warning will be produced
92 for the C<$a> line -- C<"Reversed += operator">.
94 The problem is that Perl has both compile-time and run-time warnings. To
95 disable compile-time warnings you need to rewrite the code like this:
103 The other big problem with C<$^W> is the way you can inadvertently
104 change the warning setting in unexpected places in your code. For example,
105 when the code below is run (without the B<-w> flag), the second call
106 to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
121 This is a side-effect of C<$^W> being dynamically scoped.
123 Lexical warnings get around these limitations by allowing finer control
124 over where warnings can or can't be tripped.
126 =head2 Controlling Warnings from the Command Line
128 There are three Command Line flags that can be used to control when
129 warnings are (or aren't) produced:
136 This is the existing flag. If the lexical warnings pragma is B<not>
137 used in any of you code, or any of the modules that you use, this flag
138 will enable warnings everywhere. See L<Backward Compatibility> for
139 details of how this flag interacts with lexical warnings.
144 If the B<-W> flag is used on the command line, it will enable all warnings
145 throughout the program regardless of whether warnings were disabled
146 locally using C<no warnings> or C<$^W =0>. This includes all files that get
147 included via C<use>, C<require> or C<do>.
148 Think of it as the Perl equivalent of the "lint" command.
153 Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
157 =head2 Backward Compatibility
159 If you are used with working with a version of Perl prior to the
160 introduction of lexically scoped warnings, or have code that uses both
161 lexical warnings and C<$^W>, this section will describe how they interact.
163 How Lexical Warnings interact with B<-w>/C<$^W>:
169 If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
170 control warnings is used and neither C<$^W> or the C<warnings> pragma
171 are used, then default warnings will be enabled and optional warnings
173 This means that legacy code that doesn't attempt to control the warnings
178 The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
179 means that any legacy code that currently relies on manipulating C<$^W>
180 to control warning behavior will still work as is.
184 Apart from now being a boolean, the C<$^W> variable operates in exactly
185 the same horrible uncontrolled global way, except that it cannot
186 disable/enable default warnings.
190 If a piece of code is under the control of the C<warnings> pragma,
191 both the C<$^W> variable and the B<-w> flag will be ignored for the
192 scope of the lexical warning.
196 The only way to override a lexical warnings setting is with the B<-W>
197 or B<-X> command line flags.
201 The combined effect of 3 & 4 is that it will allow code which uses
202 the C<warnings> pragma to control the warning behavior of $^W-type
203 code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
205 =head2 Category Hierarchy
206 X<warning, categories>
208 A hierarchy of "categories" have been defined to allow groups of warnings
209 to be enabled/disabled in isolation.
211 The current hierarchy is:
307 Just like the "strict" pragma any of these categories can be combined
309 use warnings qw(void redefine);
310 no warnings qw(io syntax untie);
312 Also like the "strict" pragma, if there is more than one instance of the
313 C<warnings> pragma in a given scope the cumulative effect is additive.
315 use warnings qw(void); # only "void" warnings enabled
317 use warnings qw(io); # only "void" & "io" warnings enabled
319 no warnings qw(void); # only "io" warnings enabled
321 To determine which category a specific warning has been assigned to see
324 Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
325 sub-category of the "syntax" category. It is now a top-level category
329 =head2 Fatal Warnings
332 The presence of the word "FATAL" in the category list will escalate any
333 warnings detected from the categories specified in the lexical scope
334 into fatal errors. In the code below, the use of C<time>, C<length>
335 and C<join> can all produce a C<"Useless use of xxx in void context">
343 use warnings FATAL => qw(void);
351 When run it produces this output
353 Useless use of time in void context at fatal line 3.
354 Useless use of length in void context at fatal line 7.
356 The scope where C<length> is used has escalated the C<void> warnings
357 category into a fatal error, so the program terminates immediately it
358 encounters the warning.
360 To explicitly turn off a "FATAL" warning you just disable the warning
361 it is associated with. So, for example, to disable the "void" warning
362 in the example above, either of these will do the trick:
364 no warnings qw(void);
365 no warnings FATAL => qw(void);
367 If you want to downgrade a warning that has been escalated into a fatal
368 error back to a normal warning, you can use the "NONFATAL" keyword. For
369 example, the code below will promote all warnings into fatal errors,
370 except for those in the "syntax" category.
372 use warnings FATAL => 'all', NONFATAL => 'syntax';
374 =head2 Reporting Warnings from a Module
375 X<warning, reporting> X<warning, registering>
377 The C<warnings> pragma provides a number of functions that are useful for
378 module authors. These are used when you want to report a module-specific
379 warning to a calling module has enabled warnings via the C<warnings>
382 Consider the module C<MyMod::Abc> below.
386 use warnings::register;
390 if ($path !~ m#^/#) {
391 warnings::warn("changing relative path to /var/abc")
392 if warnings::enabled();
393 $path = "/var/abc/$path";
399 The call to C<warnings::register> will create a new warnings category
400 called "MyMod::abc", i.e. the new category name matches the current
401 package name. The C<open> function in the module will display a warning
402 message if it gets given a relative path as a parameter. This warnings
403 will only be displayed if the code that uses C<MyMod::Abc> has actually
404 enabled them with the C<warnings> pragma like below.
407 use warnings 'MyMod::Abc';
409 abc::open("../fred.txt");
411 It is also possible to test whether the pre-defined warnings categories are
412 set in the calling module with the C<warnings::enabled> function. Consider
413 this snippet of code:
418 warnings::warnif("deprecated",
419 "open is deprecated, use new instead");
427 The function C<open> has been deprecated, so code has been included to
428 display a warning message whenever the calling module has (at least) the
429 "deprecated" warnings category enabled. Something like this, say.
431 use warnings 'deprecated';
434 MyMod::Abc::open($filename);
436 Either the C<warnings::warn> or C<warnings::warnif> function should be
437 used to actually display the warnings message. This is because they can
438 make use of the feature that allows warnings to be escalated into fatal
439 errors. So in this case
442 use warnings FATAL => 'MyMod::Abc';
444 MyMod::Abc::open('../fred.txt');
446 the C<warnings::warnif> function will detect this and die after
447 displaying the warning message.
449 The three warnings functions, C<warnings::warn>, C<warnings::warnif>
450 and C<warnings::enabled> can optionally take an object reference in place
451 of a category name. In this case the functions will use the class name
452 of the object as the warnings category.
454 Consider this example:
459 use warnings::register;
472 if ($value % 2 && warnings::enabled($self))
473 { warnings::warn($self, "Odd numbers are unsafe") }
480 $self->check($value);
488 use warnings::register;
490 our @ISA = qw( Original );
500 The code below makes use of both modules, but it only enables warnings from
505 use warnings 'Derived';
506 my $a = Original->new();
508 my $b = Derived->new();
511 When this code is run only the C<Derived> object, C<$b>, will generate
514 Odd numbers are unsafe at main.pl line 7
516 Notice also that the warning is reported at the line where the object is first
522 The debugger saves and restores C<$^W> at runtime. I haven't checked
523 whether the debugger will still work with the lexical warnings
527 I *think* I've got diagnostics to work with the lexical warnings
528 patch, but there were design decisions made in diagnostics to work
529 around the limitations of C<$^W>. Now that those limitations are gone,
530 the module should be revisited.
532 document calling the warnings::* functions from XS
536 L<warnings>, L<perldiag>.