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PERLLEXWARN(1)         Perl Programmers Reference Guide         PERLLEXWARN(1)



NAME
       perllexwarn - Perl Lexical Warnings

DESCRIPTION
       The "use warnings" pragma is a replacement for both the command line flag -w and the equivalent Perl variable,
       $^W.

       The pragma works just like the existing "strict" pragma.  This means that the scope of the warning pragma is
       limited to the enclosing block. It also means that the pragma setting will not leak across files (via "use",
       "require" or "do"). This allows authors to independently define the degree of warning checks that will be
       applied to their module.

       By default, optional warnings are disabled, so any legacy code that doesn't attempt to control the warnings
       will work unchanged.

       All warnings are enabled in a block by either of these:

           use warnings;
           use warnings 'all';

       Similarly all warnings are disabled in a block by either of these:

           no warnings;
           no warnings 'all';

       For example, consider the code below:

           use warnings;
           my @a;
           {
               no warnings;
               my $b = @a[0];
           }
           my $c = @a[0];

       The code in the enclosing block has warnings enabled, but the inner block has them disabled. In this case that
       means the assignment to the scalar $c will trip the "Scalar value @a[0] better written as $a[0]" warning, but
       the assignment to the scalar $b will not.

       Default Warnings and Optional Warnings

       Before the introduction of lexical warnings, Perl had two classes of warnings: mandatory and optional.

       As its name suggests, if your code tripped a mandatory warning, you would get a warning whether you wanted it
       or not.  For example, the code below would always produce an "isn't numeric" warning about the "2:".

           my $a = "2:" + 3;

       With the introduction of lexical warnings, mandatory warnings now become default warnings. The difference is
       that although the previously mandatory warnings are still enabled by default, they can then be subsequently
       enabled or disabled with the lexical warning pragma. For example, in the code below, an "isn't numeric" warning
       will only be reported for the $a variable.

           my $a = "2:" + 3;
           no warnings;
           my $b = "2:" + 3;

       Note that neither the -w flag or the $^W can be used to disable/enable default warnings. They are still manda-
       tory in this case.

       What's wrong with -w and $^W

       Although very useful, the big problem with using -w on the command line to enable warnings is that it is all or
       nothing. Take the typical scenario when you are writing a Perl program. Parts of the code you will write your-
       self, but it's very likely that you will make use of pre-written Perl modules. If you use the -w flag in this
       case, you end up enabling warnings in pieces of code that you haven't written.

       Similarly, using $^W to either disable or enable blocks of code is fundamentally flawed. For a start, say you
       want to disable warnings in a block of code. You might expect this to be enough to do the trick:

            {
                local ($^W) = 0;
                my $a =+ 2;
                my $b; chop $b;
            }

       When this code is run with the -w flag, a warning will be produced for the $a line -- "Reversed += operator".

       The problem is that Perl has both compile-time and run-time warnings. To disable compile-time warnings you need
       to rewrite the code like this:

            {
                BEGIN { $^W = 0 }
                my $a =+ 2;
                my $b; chop $b;
            }

       The other big problem with $^W is the way you can inadvertently change the warning setting in unexpected places
       in your code. For example, when the code below is run (without the -w flag), the second call to "doit" will
       trip a "Use of uninitialized value" warning, whereas the first will not.

           sub doit
           {
               my $b; chop $b;
           }

           doit();

           {
               local ($^W) = 1;
               doit()
           }

       This is a side-effect of $^W being dynamically scoped.

       Lexical warnings get around these limitations by allowing finer control over where warnings can or can't be
       tripped.

       Controlling Warnings from the Command Line

       There are three Command Line flags that can be used to control when warnings are (or aren't) produced:

       -w   This is  the existing flag. If the lexical warnings pragma is not used in any of you code, or any of the
            modules that you use, this flag will enable warnings everywhere. See "Backward Compatibility" for details
            of how this flag interacts with lexical warnings.

       -W   If the -W flag is used on the command line, it will enable all warnings throughout the program regardless
            of whether warnings were disabled locally using "no warnings" or "$^W =0". This includes all files that
            get included via "use", "require" or "do".  Think of it as the Perl equivalent of the "lint" command.

       -X   Does the exact opposite to the -W flag, i.e. it disables all warnings.

       Backward Compatibility

       If you are used with working with a version of Perl prior to the introduction of lexically scoped warnings, or
       have code that uses both lexical warnings and $^W, this section will describe how they interact.

       How Lexical Warnings interact with -w/$^W:

       1.   If none of the three command line flags (-w, -W or -X) that control warnings is used and neither $^W or
            the "warnings" pragma are used, then default warnings will be enabled and optional warnings disabled.
            This means that legacy code that doesn't attempt to control the warnings will work unchanged.

       2.   The -w flag just sets the global $^W variable as in 5.005 -- this means that any legacy code that cur-
            rently relies on manipulating $^W to control warning behavior will still work as is.

       3.   Apart from now being a boolean, the $^W variable operates in exactly the same horrible uncontrolled global
            way, except that it cannot disable/enable default warnings.

       4.   If a piece of code is under the control of the "warnings" pragma, both the $^W variable and the -w flag
            will be ignored for the scope of the lexical warning.

       5.   The only way to override a lexical warnings setting is with the -W or -X command line flags.

       The combined effect of 3 & 4 is that it will allow code which uses the "warnings" pragma to control the warning
       behavior of $^W-type code (using a "local $^W=0") if it really wants to, but not vice-versa.

       Category Hierarchy

       A hierarchy of "categories" have been defined to allow groups of warnings to be enabled/disabled in isolation.

       The current hierarchy is:

         all -+
              |
              +- closure
              |
              +- deprecated
              |
              +- exiting
              |
              +- glob
              |
              +- io -----------+
              |                |
              |                +- closed
              |                |
              |                +- exec
              |                |
              |                +- layer
              |                |
              |                +- newline
              |                |
              |                +- pipe
              |                |
              |                +- unopened
              |
              +- misc
              |
              +- numeric
              |
              +- once
              |
              +- overflow
              |
              +- pack
              |
              +- portable
              |
              +- recursion
              |
              +- redefine
              |
              +- regexp
              |
              +- severe -------+
              |                |
              |                +- debugging
              |                |
              |                +- inplace
              |                |
              |                +- internal
              |                |
              |                +- malloc
              |
              +- signal
              |
              +- substr
              |
              +- syntax -------+
              |                |
              |                +- ambiguous
              |                |
              |                +- bareword
              |                |
              |                +- digit
              |                |
              |                +- parenthesis
              |                |
              |                +- precedence
              |                |
              |                +- printf
              |                |
              |                +- prototype
              |                |
              |                +- qw
              |                |
              |                +- reserved
              |                |
              |                +- semicolon
              |
              +- taint
              |
              +- threads
              |
              +- uninitialized
              |
              +- unpack
              |
              +- untie
              |
              +- utf8
              |
              +- void
              |
              +- y2k

       Just like the "strict" pragma any of these categories can be combined

           use warnings qw(void redefine);
           no warnings qw(io syntax untie);

       Also like the "strict" pragma, if there is more than one instance of the "warnings" pragma in a given scope the
       cumulative effect is additive.

           use warnings qw(void); # only "void" warnings enabled
           ...
           use warnings qw(io);   # only "void" & "io" warnings enabled
           ...
           no warnings qw(void);  # only "io" warnings enabled

       To determine which category a specific warning has been assigned to see perldiag.

       Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a sub-category of the "syntax" category. It
       is now a top-level category in its own right.

       Fatal Warnings

       The presence of the word "FATAL" in the category list will escalate any warnings detected from the categories
       specified in the lexical scope into fatal errors. In the code below, the use of "time", "length" and "join" can
       all produce a "Useless use of xxx in void context" warning.

           use warnings;

           time;

           {
               use warnings FATAL => qw(void);
               length "abc";
           }

           join "", 1,2,3;

           print "done\n";

       When run it produces this output

           Useless use of time in void context at fatal line 3.
           Useless use of length in void context at fatal line 7.

       The scope where "length" is used has escalated the "void" warnings category into a fatal error, so the program
       terminates immediately it encounters the warning.

       To explicitly turn off a "FATAL" warning you just disable the warning it is associated with.  So, for example,
       to disable the "void" warning in the example above, either of these will do the trick:

           no warnings qw(void);
           no warnings FATAL => qw(void);

       If you want to downgrade a warning that has been escalated into a fatal error back to a normal warning, you can
       use the "NONFATAL" keyword. For example, the code below will promote all warnings into fatal errors, except for
       those in the "syntax" category.

           use warnings FATAL => 'all', NONFATAL => 'syntax';

       Reporting Warnings from a Module

       The "warnings" pragma provides a number of functions that are useful for module authors. These are used when
       you want to report a module-specific warning to a calling module has enabled warnings via the "warnings"
       pragma.

       Consider the module "MyMod::Abc" below.

           package MyMod::Abc;

           use warnings::register;

           sub open {
               my $path = shift;
               if ($path !~ m#^/#) {
                   warnings::warn("changing relative path to /var/abc")
                       if warnings::enabled();
                   $path = "/var/abc/$path";
               }
           }

           1;

       The call to "warnings::register" will create a new warnings category called "MyMod::abc", i.e. the new category
       name matches the current package name. The "open" function in the module will display a warning message if it
       gets given a relative path as a parameter. This warnings will only be displayed if the code that uses
       "MyMod::Abc" has actually enabled them with the "warnings" pragma like below.

           use MyMod::Abc;
           use warnings 'MyMod::Abc';
           ...
           abc::open("../fred.txt");

       It is also possible to test whether the pre-defined warnings categories are set in the calling module with the
       "warnings::enabled" function. Consider this snippet of code:

           package MyMod::Abc;

           sub open {
               warnings::warnif("deprecated",
                                "open is deprecated, use new instead");
               new(@_);
           }

           sub new
           ...
           1;

       The function "open" has been deprecated, so code has been included to display a warning message whenever the
       calling module has (at least) the "deprecated" warnings category enabled. Something like this, say.

           use warnings 'deprecated';
           use MyMod::Abc;
           ...
           MyMod::Abc::open($filename);

       Either the "warnings::warn" or "warnings::warnif" function should be used to actually display the warnings mes-
       sage. This is because they can make use of the feature that allows warnings to be escalated into fatal errors.
       So in this case

           use MyMod::Abc;
           use warnings FATAL => 'MyMod::Abc';
           ...
           MyMod::Abc::open('../fred.txt');

       the "warnings::warnif" function will detect this and die after displaying the warning message.

       The three warnings functions, "warnings::warn", "warnings::warnif" and "warnings::enabled" can optionally take
       an object reference in place of a category name. In this case the functions will use the class name of the
       object as the warnings category.

       Consider this example:

           package Original;

           no warnings;
           use warnings::register;

           sub new
           {
               my $class = shift;
               bless [], $class;
           }

           sub check
           {
               my $self = shift;
               my $value = shift;

               if ($value % 2 && warnings::enabled($self))
                 { warnings::warn($self, "Odd numbers are unsafe") }
           }

           sub doit
           {
               my $self = shift;
               my $value = shift;
               $self->check($value);
               # ...
           }

           1;

           package Derived;

           use warnings::register;
           use Original;
           our @ISA = qw( Original );
           sub new
           {
               my $class = shift;
               bless [], $class;
           }

           1;

       The code below makes use of both modules, but it only enables warnings from "Derived".

           use Original;
           use Derived;
           use warnings 'Derived';
           my $a = new Original;
           $a->doit(1);
           my $b = new Derived;
           $a->doit(1);

       When this code is run only the "Derived" object, $b, will generate a warning.

           Odd numbers are unsafe at main.pl line 7

       Notice also that the warning is reported at the line where the object is first used.

TODO
         perl5db.pl
           The debugger saves and restores C<$^W> at runtime. I haven't checked
           whether the debugger will still work with the lexical warnings
           patch applied.

         diagnostics.pm
           I *think* I've got diagnostics to work with the lexical warnings
           patch, but there were design decisions made in diagnostics to work
           around the limitations of C<$^W>. Now that those limitations are gone,
           the module should be revisited.

         document calling the warnings::* functions from XS

SEE ALSO
       warnings, perldiag.

AUTHOR
       Paul Marquess



perl v5.8.8                       2006-01-07                    PERLLEXWARN(1)