MirBSD manpage: perllexwarn(1)

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


     perllexwarn - Perl Lexical Warnings


     The "use warnings" pragma is a replacement for both the com-
     mand 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 set-
     ting 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

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

     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

         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.

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     As its name suggests, if your code tripped a mandatory warn-
     ing, 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 warn-
     ings 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 dis-
     abled 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 noth-
     ing. 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 writ-

     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:

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

     The other big problem with $^W is the way you can inadver-
     tently 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;


             local ($^W) = 1;

     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 con-
     trol 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 dis-
          ables all warnings.

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     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 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 glo-
          bal way, except that it cannot disable/enable default

     4.   If a piece of code is under the control of the "warn-
          ings" 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:

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       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

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            +- 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

         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 cumu-
     lative effect is additive.

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         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 "depre-
     cated" 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;


             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 ter-
     minates 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 dis-
     able the "void" warning in the example above, either of
     these will do the trick:

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

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     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";


     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';

     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;

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         sub open {
                              "open is deprecated, use new instead");

         sub new

     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;

     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';

     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;

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         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;
             # ...


         package Derived;

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


     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;
         my $b = new Derived;

     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.

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         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.

         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


     warnings, perldiag.


     Paul Marquess

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