mingw/html/pod/perllexwarn.html

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  13 <big><strong><span class="block">&nbsp;perllexwarn - Perl Lexical Warnings</span></strong></big>
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  17 <p><a name="__index__"></a></p>
  18 <!-- INDEX BEGIN -->
  19
  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#description">DESCRIPTION</a></li>
  24         <ul>
  25
  26                 <li><a href="#default_warnings_and_optional_warnings">Default Warnings and Optional Warnings</a></li>
  27                 <li><a href="#what_s_wrong_with_w_and___w">What's wrong with <strong>-w</strong> and <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a></a></li>
  28                 <li><a href="#controlling_warnings_from_the_command_line">Controlling Warnings from the Command Line</a></li>
  29                 <li><a href="#backward_compatibility">Backward Compatibility</a></li>
  30                 <li><a href="#category_hierarchy">Category Hierarchy</a></li>
  31                 <li><a href="#fatal_warnings">Fatal Warnings</a></li>
  32                 <li><a href="#reporting_warnings_from_a_module">Reporting Warnings from a Module</a></li>
  33         </ul>
  34
  35         <li><a href="#todo">TODO</a></li>
  36         <li><a href="#see_also">SEE ALSO</a></li>
  37         <li><a href="#author">AUTHOR</a></li>
  38 </ul>
  39 <!-- INDEX END -->
  40
  41 <hr />
  42 <p>
  43 </p>
  44 <h1><a name="name_x_warning__lexical__x_warnings__x_warning_">NAME
  45   </a></h1>
  46 <p>perllexwarn - Perl Lexical Warnings</p>
  47 <p>
  48 </p>
  49 <hr />
  50 <h1><a name="description">DESCRIPTION</a></h1>
  51 <p>The <code>use warnings</code> pragma is a replacement for both the command line
  52 flag <strong>-w</strong> and the equivalent Perl variable, <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a>.</p>
  53 <p>The pragma works just like the existing ``strict'' pragma.
  54 This means that the scope of the warning pragma is limited to the
  55 enclosing block. It also means that the pragma setting will not
  56 leak across files (via <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_use"><code>use</code></a>, <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_require"><code>require</code></a> or <code>do</code>). This allows
  57 authors to independently define the degree of warning checks that will
  58 be applied to their module.</p>
  59 <p>By default, optional warnings are disabled, so any legacy code that
  60 doesn't attempt to control the warnings will work unchanged.</p>
  61 <p>All warnings are enabled in a block by either of these:</p>
  62 <pre>
  63     use warnings;
  64     use warnings 'all';</pre>
  65 <p>Similarly all warnings are disabled in a block by either of these:</p>
  66 <pre>
  67     no warnings;
  68     no warnings 'all';</pre>
  69 <p>For example, consider the code below:</p>
  70 <pre>
  71     use warnings;
  72     my @a;
  73     {
  74         no warnings;
  75         my $b = @a[0];
  76     }
  77     my $c = @a[0];</pre>
  78 <p>The code in the enclosing block has warnings enabled, but the inner
  79 block has them disabled. In this case that means the assignment to the
  80 scalar <code>$c</code> will trip the <code>&quot;Scalar value @a[0] better written as $a[0]&quot;</code>
  81 warning, but the assignment to the scalar <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__b"><code>$b</code></a> will not.</p>
  82 <p>
  83 </p>
  84 <h2><a name="default_warnings_and_optional_warnings">Default Warnings and Optional Warnings</a></h2>
  85 <p>Before the introduction of lexical warnings, Perl had two classes of
  86 warnings: mandatory and optional.</p>
  87 <p>As its name suggests, if your code tripped a mandatory warning, you
  88 would get a warning whether you wanted it or not.
  89 For example, the code below would always produce an <code>&quot;isn't numeric&quot;</code>
  90 warning about the ``2:''.</p>
  91 <pre>
  92     my $a = &quot;2:&quot; + 3;</pre>
  93 <p>With the introduction of lexical warnings, mandatory warnings now become
  94 <em>default</em> warnings. The difference is that although the previously
  95 mandatory warnings are still enabled by default, they can then be
  96 subsequently enabled or disabled with the lexical warning pragma. For
  97 example, in the code below, an <code>&quot;isn't numeric&quot;</code> warning will only
  98 be reported for the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__a"><code>$a</code></a> variable.</p>
  99 <pre>
 100     my $a = &quot;2:&quot; + 3;
 101     no warnings;
 102     my $b = &quot;2:&quot; + 3;</pre>
 103 <p>Note that neither the <strong>-w</strong> flag or the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> can be used to
 104 disable/enable default warnings. They are still mandatory in this case.</p>
 105 <p>
 106 </p>
 107 <h2><a name="what_s_wrong_with_w_and___w">What's wrong with <strong>-w</strong> and <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a></a></h2>
 108 <p>Although very useful, the big problem with using <strong>-w</strong> on the command
 109 line to enable warnings is that it is all or nothing. Take the typical
 110 scenario when you are writing a Perl program. Parts of the code you
 111 will write yourself, but it's very likely that you will make use of
 112 pre-written Perl modules. If you use the <strong>-w</strong> flag in this case, you
 113 end up enabling warnings in pieces of code that you haven't written.</p>
 114 <p>Similarly, using <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> to either disable or enable blocks of code is
 115 fundamentally flawed. For a start, say you want to disable warnings in
 116 a block of code. You might expect this to be enough to do the trick:</p>
 117 <pre>
 118      {
 119          local ($^W) = 0;
 120          my $a =+ 2;
 121          my $b; chop $b;
 122      }</pre>
 123 <p>When this code is run with the <strong>-w</strong> flag, a warning will be produced
 124 for the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__a"><code>$a</code></a> line -- <code>&quot;Reversed += operator&quot;</code>.</p>
 125 <p>The problem is that Perl has both compile-time and run-time warnings. To
 126 disable compile-time warnings you need to rewrite the code like this:</p>
 127 <pre>
 128      {
 129          BEGIN { $^W = 0 }
 130          my $a =+ 2;
 131          my $b; chop $b;
 132      }</pre>
 133 <p>The other big problem with <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> is the way you can inadvertently
 134 change the warning setting in unexpected places in your code. For example,
 135 when the code below is run (without the <strong>-w</strong> flag), the second call
 136 to <code>doit</code> will trip a <code>&quot;Use of uninitialized value&quot;</code> warning, whereas
 137 the first will not.</p>
 138 <pre>
 139     sub doit
 140     {
 141         my $b; chop $b;
 142     }</pre>
 143 <pre>
 144     doit();</pre>
 145 <pre>
 146     {
 147         local ($^W) = 1;
 148         doit()
 149     }</pre>
 150 <p>This is a side-effect of <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> being dynamically scoped.</p>
 151 <p>Lexical warnings get around these limitations by allowing finer control
 152 over where warnings can or can't be tripped.</p>
 153 <p>
 154 </p>
 155 <h2><a name="controlling_warnings_from_the_command_line">Controlling Warnings from the Command Line</a></h2>
 156 <p>There are three Command Line flags that can be used to control when
 157 warnings are (or aren't) produced:</p>
 158 <dl>
 159 <dt><strong><a name="item__2dw_x_3c_2dw_3e"><strong>-w</strong>
 160 </a></strong>
 161
 162 <dd>
 163 <p>This is  the existing flag. If the lexical warnings pragma is <strong>not</strong>
 164 used in any of you code, or any of the modules that you use, this flag
 165 will enable warnings everywhere. See <a href="#backward_compatibility">Backward Compatibility</a> for
 166 details of how this flag interacts with lexical warnings.</p>
 167 </dd>
 168 </li>
 169 <dt><strong><a name="item__2dw_x_3c_2dw_3e"><strong>-W</strong>
 170 </a></strong>
 171
 172 <dd>
 173 <p>If the <strong>-W</strong> flag is used on the command line, it will enable all warnings
 174 throughout the program regardless of whether warnings were disabled
 175 locally using <code>no warnings</code> or <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W =0</code></a>. This includes all files that get
 176 included via <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_use"><code>use</code></a>, <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_require"><code>require</code></a> or <code>do</code>.
 177 Think of it as the Perl equivalent of the ``lint'' command.</p>
 178 </dd>
 179 </li>
 180 <dt><strong><a name="item__2dx_x_3c_2dx_3e"><strong>-X</strong>
 181 </a></strong>
 182
 183 <dd>
 184 <p>Does the exact opposite to the <strong>-W</strong> flag, i.e. it disables all warnings.</p>
 185 </dd>
 186 </li>
 187 </dl>
 188 <p>
 189 </p>
 190 <h2><a name="backward_compatibility">Backward Compatibility</a></h2>
 191 <p>If you are used with working with a version of Perl prior to the
 192 introduction of lexically scoped warnings, or have code that uses both
 193 lexical warnings and <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a>, this section will describe how they interact.</p>
 194 <p>How Lexical Warnings interact with <strong>-w</strong>/<a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a>:</p>
 195 <ol>
 196 <li>
 197 <p>If none of the three command line flags (<strong>-w</strong>, <strong>-W</strong> or <strong>-X</strong>) that
 198 control warnings is used and neither <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> or the <code>warnings</code> pragma
 199 are used, then default warnings will be enabled and optional warnings
 200 disabled.
 201 This means that legacy code that doesn't attempt to control the warnings
 202 will work unchanged.</p>
 203 </li>
 204 <li>
 205 <p>The <strong>-w</strong> flag just sets the global <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> variable as in 5.005 -- this
 206 means that any legacy code that currently relies on manipulating <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a>
 207 to control warning behavior will still work as is.</p>
 208 </li>
 209 <li>
 210 <p>Apart from now being a boolean, the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> variable operates in exactly
 211 the same horrible uncontrolled global way, except that it cannot
 212 disable/enable default warnings.</p>
 213 </li>
 214 <li>
 215 <p>If a piece of code is under the control of the <code>warnings</code> pragma,
 216 both the <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item___w"><code>$^W</code></a> variable and the <strong>-w</strong> flag will be ignored for the
 217 scope of the lexical warning.</p>
 218 </li>
 219 <li>
 220 <p>The only way to override a lexical warnings setting is with the <strong>-W</strong>
 221 or <strong>-X</strong> command line flags.</p>
 222 </li>
 223 </ol>
 224 <p>The combined effect of 3 &amp; 4 is that it will allow code which uses
 225 the <code>warnings</code> pragma to control the warning behavior of $^W-type
 226 code (using a <code>local $^W=0</code>) if it really wants to, but not vice-versa.</p>
 227 <p>
 228 </p>
 229 <h2><a name="category_hierarchy_x_warning__categories_">Category Hierarchy
 230 </a></h2>
 231 <p>A hierarchy of ``categories'' have been defined to allow groups of warnings
 232 to be enabled/disabled in isolation.</p>
 233 <p>The current hierarchy is:</p>
 234 <pre>
 235   all -+
 236        |
 237        +- closure
 238        |
 239        +- deprecated
 240        |
 241        +- exiting
 242        |
 243        +- glob
 244        |
 245        +- io -----------+
 246        |                |
 247        |                +- closed
 248        |                |
 249        |                +- exec
 250        |                |
 251        |                +- layer
 252        |                |
 253        |                +- newline
 254        |                |
 255        |                +- pipe
 256        |                |
 257        |                +- unopened
 258        |
 259        +- misc
 260        |
 261        +- numeric
 262        |
 263        +- once
 264        |
 265        +- overflow
 266        |
 267        +- pack
 268        |
 269        +- portable
 270        |
 271        +- recursion
 272        |
 273        +- redefine
 274        |
 275        +- regexp
 276        |
 277        +- severe -------+
 278        |                |
 279        |                +- debugging
 280        |                |
 281        |                +- inplace
 282        |                |
 283        |                +- internal
 284        |                |
 285        |                +- malloc
 286        |
 287        +- signal
 288        |
 289        +- substr
 290        |
 291        +- syntax -------+
 292        |                |
 293        |                +- ambiguous
 294        |                |
 295        |                +- bareword
 296        |                |
 297        |                +- digit
 298        |                |
 299        |                +- parenthesis
 300        |                |
 301        |                +- precedence
 302        |                |
 303        |                +- printf
 304        |                |
 305        |                +- prototype
 306        |                |
 307        |                +- qw
 308        |                |
 309        |                +- reserved
 310        |                |
 311        |                +- semicolon
 312        |
 313        +- taint
 314        |
 315        +- threads
 316        |
 317        +- uninitialized
 318        |
 319        +- unpack
 320        |
 321        +- untie
 322        |
 323        +- utf8
 324        |
 325        +- void
 326        |
 327        +- y2k</pre>
 328 <p>Just like the ``strict'' pragma any of these categories can be combined</p>
 329 <pre>
 330     use warnings qw(void redefine);
 331     no warnings qw(io syntax untie);</pre>
 332 <p>Also like the ``strict'' pragma, if there is more than one instance of the
 333 <code>warnings</code> pragma in a given scope the cumulative effect is additive.</p>
 334 <pre>
 335     use warnings qw(void); # only &quot;void&quot; warnings enabled
 336     ...
 337     use warnings qw(io);   # only &quot;void&quot; &amp; &quot;io&quot; warnings enabled
 338     ...
 339     no warnings qw(void);  # only &quot;io&quot; warnings enabled</pre>
 340 <p>To determine which category a specific warning has been assigned to see
 341 <a href="file://C|\msysgit\mingw\html/pod/perldiag.html">the perldiag manpage</a>.</p>
 342 <p>Note: In Perl 5.6.1, the lexical warnings category ``deprecated'' was a
 343 sub-category of the ``syntax'' category. It is now a top-level category
 344 in its own right.</p>
 345 <p>
 346 </p>
 347 <h2><a name="fatal_warnings_x_warning__fatal_">Fatal Warnings
 348 </a></h2>
 349 <p>The presence of the word ``FATAL'' in the category list will escalate any
 350 warnings detected from the categories specified in the lexical scope
 351 into fatal errors. In the code below, the use of <code>time</code>, <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_length"><code>length</code></a>
 352 and <code>join</code> can all produce a <code>&quot;Useless use of xxx in void context&quot;</code>
 353 warning.</p>
 354 <pre>
 355     use warnings;</pre>
 356 <pre>
 357     time;</pre>
 358 <pre>
 359     {
 360         use warnings FATAL =&gt; qw(void);
 361         length &quot;abc&quot;;
 362     }</pre>
 363 <pre>
 364     join &quot;&quot;, 1,2,3;</pre>
 365 <pre>
 366     print &quot;done\n&quot;;</pre>
 367 <p>When run it produces this output</p>
 368 <pre>
 369     Useless use of time in void context at fatal line 3.
 370     Useless use of length in void context at fatal line 7.</pre>
 371 <p>The scope where <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_length"><code>length</code></a> is used has escalated the <code>void</code> warnings
 372 category into a fatal error, so the program terminates immediately it
 373 encounters the warning.</p>
 374 <p>To explicitly turn off a ``FATAL'' warning you just disable the warning
 375 it is associated with.  So, for example, to disable the ``void'' warning
 376 in the example above, either of these will do the trick:</p>
 377 <pre>
 378     no warnings qw(void);
 379     no warnings FATAL =&gt; qw(void);</pre>
 380 <p>If you want to downgrade a warning that has been escalated into a fatal
 381 error back to a normal warning, you can use the ``NONFATAL'' keyword. For
 382 example, the code below will promote all warnings into fatal errors,
 383 except for those in the ``syntax'' category.</p>
 384 <pre>
 385     use warnings FATAL =&gt; 'all', NONFATAL =&gt; 'syntax';</pre>
 386 <p>
 387 </p>
 388 <h2><a name="reporting_warnings_from_a_module_x_warning__reporting__x_warning__registering_">Reporting Warnings from a Module
 389  </a></h2>
 390 <p>The <code>warnings</code> pragma provides a number of functions that are useful for
 391 module authors. These are used when you want to report a module-specific
 392 warning to a calling module has enabled warnings via the <code>warnings</code>
 393 pragma.</p>
 394 <p>Consider the module <code>MyMod::Abc</code> below.</p>
 395 <pre>
 396     package MyMod::Abc;</pre>
 397 <pre>
 398     use warnings::register;</pre>
 399 <pre>
 400     sub open {
 401         my $path = shift;
 402         if ($path !~ m#^/#) {
 403             warnings::warn(&quot;changing relative path to /var/abc&quot;)
 404                 if warnings::enabled();
 405             $path = &quot;/var/abc/$path&quot;;
 406         }
 407     }</pre>
 408 <pre>
 409     1;</pre>
 410 <p>The call to <code>warnings::register</code> will create a new warnings category
 411 called ``MyMod::abc'', i.e. the new category name matches the current
 412 package name. The <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open</code></a> function in the module will display a warning
 413 message if it gets given a relative path as a parameter. This warnings
 414 will only be displayed if the code that uses <code>MyMod::Abc</code> has actually
 415 enabled them with the <code>warnings</code> pragma like below.</p>
 416 <pre>
 417     use MyMod::Abc;
 418     use warnings 'MyMod::Abc';
 419     ...
 420     abc::open(&quot;../fred.txt&quot;);</pre>
 421 <p>It is also possible to test whether the pre-defined warnings categories are
 422 set in the calling module with the <code>warnings::enabled</code> function. Consider
 423 this snippet of code:</p>
 424 <pre>
 425     package MyMod::Abc;</pre>
 426 <pre>
 427     sub open {
 428         warnings::warnif(&quot;deprecated&quot;,
 429                          &quot;open is deprecated, use new instead&quot;);
 430         new(@_);
 431     }</pre>
 432 <pre>
 433     sub new
 434     ...
 435     1;</pre>
 436 <p>The function <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_open"><code>open</code></a> has been deprecated, so code has been included to
 437 display a warning message whenever the calling module has (at least) the
 438 ``deprecated'' warnings category enabled. Something like this, say.</p>
 439 <pre>
 440     use warnings 'deprecated';
 441     use MyMod::Abc;
 442     ...
 443     MyMod::Abc::open($filename);</pre>
 444 <p>Either the <code>warnings::warn</code> or <code>warnings::warnif</code> function should be
 445 used to actually display the warnings message. This is because they can
 446 make use of the feature that allows warnings to be escalated into fatal
 447 errors. So in this case</p>
 448 <pre>
 449     use MyMod::Abc;
 450     use warnings FATAL =&gt; 'MyMod::Abc';
 451     ...
 452     MyMod::Abc::open('../fred.txt');</pre>
 453 <p>the <code>warnings::warnif</code> function will detect this and die after
 454 displaying the warning message.</p>
 455 <p>The three warnings functions, <code>warnings::warn</code>, <code>warnings::warnif</code>
 456 and <code>warnings::enabled</code> can optionally take an object reference in place
 457 of a category name. In this case the functions will use the class name
 458 of the object as the warnings category.</p>
 459 <p>Consider this example:</p>
 460 <pre>
 461     package Original;</pre>
 462 <pre>
 463     no warnings;
 464     use warnings::register;</pre>
 465 <pre>
 466     sub new
 467     {
 468         my $class = shift;
 469         bless [], $class;
 470     }</pre>
 471 <pre>
 472     sub check
 473     {
 474         my $self = shift;
 475         my $value = shift;</pre>
 476 <pre>
 477         if ($value % 2 &amp;&amp; warnings::enabled($self))
 478           { warnings::warn($self, &quot;Odd numbers are unsafe&quot;) }
 479     }</pre>
 480 <pre>
 481     sub doit
 482     {
 483         my $self = shift;
 484         my $value = shift;
 485         $self-&gt;check($value);
 486         # ...
 487     }</pre>
 488 <pre>
 489     1;</pre>
 490 <pre>
 491     package Derived;</pre>
 492 <pre>
 493     use warnings::register;
 494     use Original;
 495     our @ISA = qw( Original );
 496     sub new
 497     {
 498         my $class = shift;
 499         bless [], $class;
 500     }</pre>
 501 <pre>
 502     1;</pre>
 503 <p>The code below makes use of both modules, but it only enables warnings from
 504 <code>Derived</code>.</p>
 505 <pre>
 506     use Original;
 507     use Derived;
 508     use warnings 'Derived';
 509     my $a = new Original;
 510     $a-&gt;doit(1);
 511     my $b = new Derived;
 512     $a-&gt;doit(1);</pre>
 513 <p>When this code is run only the <code>Derived</code> object, <a href="file://C|\msysgit\mingw\html/pod/perlvar.html#item__b"><code>$b</code></a>, will generate
 514 a warning.</p>
 515 <pre>
 516     Odd numbers are unsafe at main.pl line 7</pre>
 517 <p>Notice also that the warning is reported at the line where the object is first
 518 used.</p>
 519 <p>
 520 </p>
 521 <hr />
 522 <h1><a name="todo">TODO</a></h1>
 523 <pre>
 524   perl5db.pl
 525     The debugger saves and restores C&lt;$^W&gt; at runtime. I haven't checked
 526     whether the debugger will still work with the lexical warnings
 527     patch applied.</pre>
 528 <pre>
 529   diagnostics.pm
 530     I *think* I've got diagnostics to work with the lexical warnings
 531     patch, but there were design decisions made in diagnostics to work
 532     around the limitations of C&lt;$^W&gt;. Now that those limitations are gone,
 533     the module should be revisited.</pre>
 534 <pre>
 535   document calling the warnings::* functions from XS</pre>
 536 <p>
 537 </p>
 538 <hr />
 539 <h1><a name="see_also">SEE ALSO</a></h1>
 540 <p><a href="file://C|\msysgit\mingw\html/lib/warnings.html">the warnings manpage</a>, <a href="file://C|\msysgit\mingw\html/pod/perldiag.html">the perldiag manpage</a>.</p>
 541 <p>
 542 </p>
 543 <hr />
 544 <h1><a name="author">AUTHOR</a></h1>
 545 <p>Paul Marquess</p>
 546 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 547 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 548 <big><strong><span class="block">&nbsp;perllexwarn - Perl Lexical Warnings</span></strong></big>
 549 </td></tr>
 550 </table>
 551
 552 </body>
 553
 554 </html>