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:
305 Just like the "strict" pragma any of these categories can be combined
307 use warnings qw(void redefine);
308 no warnings qw(io syntax untie);
310 Also like the "strict" pragma, if there is more than one instance of the
311 C<warnings> pragma in a given scope the cumulative effect is additive.
313 use warnings qw(void); # only "void" warnings enabled
315 use warnings qw(io); # only "void" & "io" warnings enabled
317 no warnings qw(void); # only "io" warnings enabled
319 To determine which category a specific warning has been assigned to see
322 Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
323 sub-category of the "syntax" category. It is now a top-level category
327 =head2 Fatal Warnings
330 The presence of the word "FATAL" in the category list will escalate any
331 warnings detected from the categories specified in the lexical scope
332 into fatal errors. In the code below, the use of C<time>, C<length>
333 and C<join> can all produce a C<"Useless use of xxx in void context">
341 use warnings FATAL => qw(void);
349 When run it produces this output
351 Useless use of time in void context at fatal line 3.
352 Useless use of length in void context at fatal line 7.
354 The scope where C<length> is used has escalated the C<void> warnings
355 category into a fatal error, so the program terminates immediately it
356 encounters the warning.
358 To explicitly turn off a "FATAL" warning you just disable the warning
359 it is associated with. So, for example, to disable the "void" warning
360 in the example above, either of these will do the trick:
362 no warnings qw(void);
363 no warnings FATAL => qw(void);
365 If you want to downgrade a warning that has been escalated into a fatal
366 error back to a normal warning, you can use the "NONFATAL" keyword. For
367 example, the code below will promote all warnings into fatal errors,
368 except for those in the "syntax" category.
370 use warnings FATAL => 'all', NONFATAL => 'syntax';
372 =head2 Reporting Warnings from a Module
373 X<warning, reporting> X<warning, registering>
375 The C<warnings> pragma provides a number of functions that are useful for
376 module authors. These are used when you want to report a module-specific
377 warning to a calling module has enabled warnings via the C<warnings>
380 Consider the module C<MyMod::Abc> below.
384 use warnings::register;
388 if ($path !~ m#^/#) {
389 warnings::warn("changing relative path to /var/abc")
390 if warnings::enabled();
391 $path = "/var/abc/$path";
397 The call to C<warnings::register> will create a new warnings category
398 called "MyMod::abc", i.e. the new category name matches the current
399 package name. The C<open> function in the module will display a warning
400 message if it gets given a relative path as a parameter. This warnings
401 will only be displayed if the code that uses C<MyMod::Abc> has actually
402 enabled them with the C<warnings> pragma like below.
405 use warnings 'MyMod::Abc';
407 abc::open("../fred.txt");
409 It is also possible to test whether the pre-defined warnings categories are
410 set in the calling module with the C<warnings::enabled> function. Consider
411 this snippet of code:
416 warnings::warnif("deprecated",
417 "open is deprecated, use new instead");
425 The function C<open> has been deprecated, so code has been included to
426 display a warning message whenever the calling module has (at least) the
427 "deprecated" warnings category enabled. Something like this, say.
429 use warnings 'deprecated';
432 MyMod::Abc::open($filename);
434 Either the C<warnings::warn> or C<warnings::warnif> function should be
435 used to actually display the warnings message. This is because they can
436 make use of the feature that allows warnings to be escalated into fatal
437 errors. So in this case
440 use warnings FATAL => 'MyMod::Abc';
442 MyMod::Abc::open('../fred.txt');
444 the C<warnings::warnif> function will detect this and die after
445 displaying the warning message.
447 The three warnings functions, C<warnings::warn>, C<warnings::warnif>
448 and C<warnings::enabled> can optionally take an object reference in place
449 of a category name. In this case the functions will use the class name
450 of the object as the warnings category.
452 Consider this example:
457 use warnings::register;
470 if ($value % 2 && warnings::enabled($self))
471 { warnings::warn($self, "Odd numbers are unsafe") }
478 $self->check($value);
486 use warnings::register;
488 our @ISA = qw( Original );
498 The code below makes use of both modules, but it only enables warnings from
503 use warnings 'Derived';
504 my $a = Original->new();
506 my $b = Derived->new();
509 When this code is run only the C<Derived> object, C<$b>, will generate
512 Odd numbers are unsafe at main.pl line 7
514 Notice also that the warning is reported at the line where the object is first
520 The debugger saves and restores C<$^W> at runtime. I haven't checked
521 whether the debugger will still work with the lexical warnings
525 I *think* I've got diagnostics to work with the lexical warnings
526 patch, but there were design decisions made in diagnostics to work
527 around the limitations of C<$^W>. Now that those limitations are gone,
528 the module should be revisited.
530 document calling the warnings::* functions from XS
534 L<warnings>, L<perldiag>.