1PERLLEXWARN(1) Perl Programmers Reference Guide PERLLEXWARN(1)
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6 perllexwarn - Perl Lexical Warnings
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9 The "use warnings" pragma is a replacement for both the command line
10 flag -w and the equivalent Perl variable, $^W.
11 The pragma works just like the existing "strict" pragma. This means
13 that the scope of the warning pragma is limited to the enclosing block.
14 It also means that the pragma setting will not leak across files (via
15 "use", "require" or "do"). This allows authors to independently define
16 the degree of warning checks that will be applied to their module.
17 By default, optional warnings are disabled, so any legacy code that
19 doesn't attempt to control the warnings will work unchanged.
20 All warnings are enabled in a block by either of these:
22 use warnings;
24 use warnings 'all';
25 Similarly all warnings are disabled in a block by either of these:
27 no warnings;
29 no warnings 'all';
30 For example, consider the code below:
32 use warnings;
34 my @a;
35 {
36 no warnings;
37 my $b = @a[0];
38 }
39 my $c = @a[0];
40 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 will trip the "Scalar value @a[0] better written as $a[0]"
44 warning, but the assignment to the scalar $b will not.
45 Default Warnings and Optional Warnings
47 Before the introduction of lexical warnings, Perl had two classes of
49 warnings: mandatory and optional.
50 As its name suggests, if your code tripped a mandatory warning, you
52 would get a warning whether you wanted it or not. For example, the
53 code below would always produce an "isn't numeric" warning about the
54 "2:".
55 my $a = "2:" + 3;
57 With the introduction of lexical warnings, mandatory warnings now
59 become default warnings. The difference is that although the previously
60 mandatory warnings are still enabled by default, they can then be sub‐
61 sequently enabled or disabled with the lexical warning pragma. For
62 example, in the code below, an "isn't numeric" warning will only be
63 reported for the $a variable.
64 my $a = "2:" + 3;
66 no warnings;
67 my $b = "2:" + 3;
68 Note that neither the -w flag or the $^W can be used to disable/enable
70 default warnings. They are still mandatory in this case.
71 What's wrong with -w and $^W
73 Although very useful, the big problem with using -w on the command line
75 to enable warnings is that it is all or nothing. Take the typical sce‐
76 nario when you are writing a Perl program. Parts of the code you will
77 write yourself, but it's very likely that you will make use of pre-
78 written Perl modules. If you use the -w flag in this case, you end up
79 enabling warnings in pieces of code that you haven't written.
80 Similarly, using $^W to either disable or enable blocks of code is fun‐
82 damentally flawed. For a start, say you want to disable warnings in a
83 block of code. You might expect this to be enough to do the trick:
84 {
86 local ($^W) = 0;
87 my $a =+ 2;
88 my $b; chop $b;
89 }
90 When this code is run with the -w flag, a warning will be produced for
92 the $a line -- "Reversed += operator".
93 The problem is that Perl has both compile-time and run-time warnings.
95 To disable compile-time warnings you need to rewrite the code like
96 this:
97 {
99 BEGIN { $^W = 0 }
100 my $a =+ 2;
101 my $b; chop $b;
102 }
103 The other big problem with $^W is the way you can inadvertently change
105 the warning setting in unexpected places in your code. For example,
106 when the code below is run (without the -w flag), the second call to
107 "doit" will trip a "Use of uninitialized value" warning, whereas the
108 first will not.
109 sub doit
111 {
112 my $b; chop $b;
113 }
114 doit();
116 {
118 local ($^W) = 1;
119 doit()
120 }
121 This is a side-effect of $^W being dynamically scoped.
123 Lexical warnings get around these limitations by allowing finer control
125 over where warnings can or can't be tripped.
126 Controlling Warnings from the Command Line
128 There are three Command Line flags that can be used to control when
130 warnings are (or aren't) produced:
131 -w This is the existing flag. If the lexical warnings pragma is not
133 used in any of you code, or any of the modules that you use, this
134 flag will enable warnings everywhere. See "Backward Compatibility"
135 for details of how this flag interacts with lexical warnings.
136 -W If the -W flag is used on the command line, it will enable all
138 warnings throughout the program regardless of whether warnings
139 were disabled locally using "no warnings" or "$^W =0". This
140 includes all files that get included via "use", "require" or "do".
141 Think of it as the Perl equivalent of the "lint" command.
142 -X Does the exact opposite to the -W flag, i.e. it disables all warn‐
144 ings.
145 Backward Compatibility
147 If you are used with working with a version of Perl prior to the intro‐
149 duction of lexically scoped warnings, or have code that uses both lexi‐
150 cal warnings and $^W, this section will describe how they interact.
151 How Lexical Warnings interact with -w/$^W:
153 1. If none of the three command line flags (-w, -W or -X) that con‐
155 trol warnings is used and neither $^W or the "warnings" pragma are
156 used, then default warnings will be enabled and optional warnings
157 disabled. This means that legacy code that doesn't attempt to
158 control the warnings will work unchanged.
159 2. The -w flag just sets the global $^W variable as in 5.005 -- this
161 means that any legacy code that currently relies on manipulating
162 $^W to control warning behavior will still work as is.
163 3. Apart from now being a boolean, the $^W variable operates in
165 exactly the same horrible uncontrolled global way, except that it
166 cannot disable/enable default warnings.
167 4. If a piece of code is under the control of the "warnings" pragma,
169 both the $^W variable and the -w flag will be ignored for the
170 scope of the lexical warning.
171 5. The only way to override a lexical warnings setting is with the -W
173 or -X command line flags.
174 The combined effect of 3 & 4 is that it will allow code which uses the
176 "warnings" pragma to control the warning behavior of $^W-type code
177 (using a "local $^W=0") if it really wants to, but not vice-versa.
178 Category Hierarchy
180 A hierarchy of "categories" have been defined to allow groups of warn‐
182 ings to be enabled/disabled in isolation.
183 The current hierarchy is:
185 all -+
187 ⎪
188 +- closure
189 ⎪
190 +- deprecated
191 ⎪
192 +- exiting
193 ⎪
194 +- glob
195 ⎪
196 +- io -----------+
197 ⎪ ⎪
198 ⎪ +- closed
199 ⎪ ⎪
200 ⎪ +- exec
201 ⎪ ⎪
202 ⎪ +- layer
203 ⎪ ⎪
204 ⎪ +- newline
205 ⎪ ⎪
206 ⎪ +- pipe
207 ⎪ ⎪
208 ⎪ +- unopened
209 ⎪
210 +- misc
211 ⎪
212 +- numeric
213 ⎪
214 +- once
215 ⎪
216 +- overflow
217 ⎪
218 +- pack
219 ⎪
220 +- portable
221 ⎪
222 +- recursion
223 ⎪
224 +- redefine
225 ⎪
226 +- regexp
227 ⎪
228 +- severe -------+
229 ⎪ ⎪
230 ⎪ +- debugging
231 ⎪ ⎪
232 ⎪ +- inplace
233 ⎪ ⎪
234 ⎪ +- internal
235 ⎪ ⎪
236 ⎪ +- malloc
237 ⎪
238 +- signal
239 ⎪
240 +- substr
241 ⎪
242 +- syntax -------+
243 ⎪ ⎪
244 ⎪ +- ambiguous
245 ⎪ ⎪
246 ⎪ +- bareword
247 ⎪ ⎪
248 ⎪ +- digit
249 ⎪ ⎪
250 ⎪ +- parenthesis
251 ⎪ ⎪
252 ⎪ +- precedence
253 ⎪ ⎪
254 ⎪ +- printf
255 ⎪ ⎪
256 ⎪ +- prototype
257 ⎪ ⎪
258 ⎪ +- qw
259 ⎪ ⎪
260 ⎪ +- reserved
261 ⎪ ⎪
262 ⎪ +- semicolon
263 ⎪
264 +- taint
265 ⎪
266 +- threads
267 ⎪
268 +- uninitialized
269 ⎪
270 +- unpack
271 ⎪
272 +- untie
273 ⎪
274 +- utf8
275 ⎪
276 +- void
277 ⎪
278 +- y2k
279 Just like the "strict" pragma any of these categories can be combined
281 use warnings qw(void redefine);
283 no warnings qw(io syntax untie);
284 Also like the "strict" pragma, if there is more than one instance of
286 the "warnings" pragma in a given scope the cumulative effect is addi‐
287 tive.
288 use warnings qw(void); # only "void" warnings enabled
290 ...
291 use warnings qw(io); # only "void" & "io" warnings enabled
292 ...
293 no warnings qw(void); # only "io" warnings enabled
294 To determine which category a specific warning has been assigned to see
296 perldiag.
297 Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
299 sub-category of the "syntax" category. It is now a top-level category
300 in its own right.
301 Fatal Warnings
303 The presence of the word "FATAL" in the category list will escalate any
305 warnings detected from the categories specified in the lexical scope
306 into fatal errors. In the code below, the use of "time", "length" and
307 "join" can all produce a "Useless use of xxx in void context" warning.
308 use warnings;
310 time;
312 {
314 use warnings FATAL => qw(void);
315 length "abc";
316 }
317 join "", 1,2,3;
319 print "done\n";
321 When run it produces this output
323 Useless use of time in void context at fatal line 3.
325 Useless use of length in void context at fatal line 7.
326 The scope where "length" is used has escalated the "void" warnings cat‐
328 egory into a fatal error, so the program terminates immediately it
329 encounters the warning.
330 To explicitly turn off a "FATAL" warning you just disable the warning
332 it is associated with. So, for example, to disable the "void" warning
333 in the example above, either of these will do the trick:
334 no warnings qw(void);
336 no warnings FATAL => qw(void);
337 If you want to downgrade a warning that has been escalated into a fatal
339 error back to a normal warning, you can use the "NONFATAL" keyword. For
340 example, the code below will promote all warnings into fatal errors,
341 except for those in the "syntax" category.
342 use warnings FATAL => 'all', NONFATAL => 'syntax';
344 Reporting Warnings from a Module
346 The "warnings" pragma provides a number of functions that are useful
348 for module authors. These are used when you want to report a module-
349 specific warning to a calling module has enabled warnings via the
350 "warnings" pragma.
351 Consider the module "MyMod::Abc" below.
353 package MyMod::Abc;
355 use warnings::register;
357 sub open {
359 my $path = shift;
360 if ($path !~ m#^/#) {
361 warnings::warn("changing relative path to /var/abc")
362 if warnings::enabled();
363 $path = "/var/abc/$path";
364 }
365 }
366 1;
368 The call to "warnings::register" will create a new warnings category
370 called "MyMod::abc", i.e. the new category name matches the current
371 package name. The "open" function in the module will display a warning
372 message if it gets given a relative path as a parameter. This warnings
373 will only be displayed if the code that uses "MyMod::Abc" has actually
374 enabled them with the "warnings" pragma like below.
375 use MyMod::Abc;
377 use warnings 'MyMod::Abc';
378 ...
379 abc::open("../fred.txt");
380 It is also possible to test whether the pre-defined warnings categories
382 are set in the calling module with the "warnings::enabled" function.
383 Consider this snippet of code:
384 package MyMod::Abc;
386 sub open {
388 warnings::warnif("deprecated",
389 "open is deprecated, use new instead");
390 new(@_);
391 }
392 sub new
394 ...
395 1;
396 The function "open" has been deprecated, so code has been included to
398 display a warning message whenever the calling module has (at least)
399 the "deprecated" warnings category enabled. Something like this, say.
400 use warnings 'deprecated';
402 use MyMod::Abc;
403 ...
404 MyMod::Abc::open($filename);
405 Either the "warnings::warn" or "warnings::warnif" function should be
407 used to actually display the warnings message. This is because they can
408 make use of the feature that allows warnings to be escalated into fatal
409 errors. So in this case
410 use MyMod::Abc;
412 use warnings FATAL => 'MyMod::Abc';
413 ...
414 MyMod::Abc::open('../fred.txt');
415 the "warnings::warnif" function will detect this and die after display‐
417 ing the warning message.
418 The three warnings functions, "warnings::warn", "warnings::warnif" and
420 "warnings::enabled" can optionally take an object reference in place of
421 a category name. In this case the functions will use the class name of
422 the object as the warnings category.
423 Consider this example:
425 package Original;
427 no warnings;
429 use warnings::register;
430 sub new
432 {
433 my $class = shift;
434 bless [], $class;
435 }
436 sub check
438 {
439 my $self = shift;
440 my $value = shift;
441 if ($value % 2 && warnings::enabled($self))
443 { warnings::warn($self, "Odd numbers are unsafe") }
444 }
445 sub doit
447 {
448 my $self = shift;
449 my $value = shift;
450 $self->check($value);
451 # ...
452 }
453 1;
455 package Derived;
457 use warnings::register;
459 use Original;
460 our @ISA = qw( Original );
461 sub new
462 {
463 my $class = shift;
464 bless [], $class;
465 }
466 1;
468 The code below makes use of both modules, but it only enables warnings
470 from "Derived".
471 use Original;
473 use Derived;
474 use warnings 'Derived';
475 my $a = new Original;
476 $a->doit(1);
477 my $b = new Derived;
478 $a->doit(1);
479 When this code is run only the "Derived" object, $b, will generate a
481 warning.
482 Odd numbers are unsafe at main.pl line 7
484 Notice also that the warning is reported at the line where the object
486 is first used.
487
489 perl5db.pl
490 The debugger saves and restores C<$^W> at runtime. I haven't checked
491 whether the debugger will still work with the lexical warnings
492 patch applied.
493 diagnostics.pm
495 I *think* I've got diagnostics to work with the lexical warnings
496 patch, but there were design decisions made in diagnostics to work
497 around the limitations of C<$^W>. Now that those limitations are gone,
498 the module should be revisited.
499 document calling the warnings::* functions from XS
501
503 warnings, perldiag.
504
506 Paul Marquess
507perl v5.8.8 2006-01-07 PERLLEXWARN(1)