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

NAME
       perllexwarn - Perl Lexical Warnings

DESCRIPTION
       The "use warnings" pragma enables to control precisely what warnings are to be enabled in
       which parts of a Perl program. It's a more flexible alternative for both the command line
       flag -w and the equivalent Perl variable, $^W.

       This pragma works just like the "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 mandatory 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 yourself, 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 to 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 nor 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 currently 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
	      |
	      +- imprecision
	      |
	      +- misc
	      |
	      +- numeric
	      |
	      +- once
	      |
	      +- overflow
	      |
	      +- pack
	      |
	      +- portable
	      |
	      +- recursion
	      |
	      +- redefine
	      |
	      +- regexp
	      |
	      +- severe -------+
	      | 	       |
	      | 	       +- debugging
	      | 	       |
	      | 	       +- inplace
	      | 	       |
	      | 	       +- internal
	      | 	       |
	      | 	       +- malloc
	      |
	      +- signal
	      |
	      +- substr
	      |
	      +- syntax -------+
	      | 	       |
	      | 	       +- ambiguous
	      | 	       |
	      | 	       +- bareword
	      | 	       |
	      | 	       +- digit
	      | 	       |
	      | 	       +- illegalproto
	      | 	       |
	      | 	       +- parenthesis
	      | 	       |
	      | 	       +- precedence
	      | 	       |
	      | 	       +- printf
	      | 	       |
	      | 	       +- prototype
	      | 	       |
	      | 	       +- qw
	      | 	       |
	      | 	       +- reserved
	      | 	       |
	      | 	       +- semicolon
	      |
	      +- taint
	      |
	      +- threads
	      |
	      +- uninitialized
	      |
	      +- unpack
	      |
	      +- untie
	      |
	      +- utf8----------+
	      | 	       |
	      | 	       +- surrogate
	      | 	       |
	      | 	       +- non_unicode
	      | 	       |
	      | 	       +- nonchar
	      |
	      +- void

       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 message. 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 = Original->new();
	   $a->doit(1);
	   my $b = Derived->new();
	   $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.

       When registering new categories of warning, you can supply more names to
       warnings::register like this:

	   package MyModule;
	   use warnings::register qw(format precision);

	   ...

	   warnings::warnif('MyModule::format', '...');

SEE ALSO
       warnings, perldiag.

AUTHOR
       Paul Marquess

perl v5.16.3				    2013-03-04				   PERLLEXWARN(1)
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