+ 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 C<length> is used has escalated the C<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';
+
+=head2 Reporting Warnings from a Module
+
+The C<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 C<warnings>
+pragma.
+
+Consider the module C<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 C<warnings::register> will create a new warnings category
+called "MyMod::abc", i.e. the new category name matches the current
+package name. The C<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 C<MyMod::Abc> has actually
+enabled them with the C<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 C<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 C<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 C<warnings::warn> or C<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 C<warnings::warnif> function will detect this and die after
+displaying the warning message.
+
+The three warnings functions, C<warnings::warn>, C<warnings::warnif>
+and C<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
+C<Derived>.
+
+ use Original ;
+ use Derived ;
+ use warnings 'Derived';
+ my $a = new Original ;
+ $a->doit(1) ;
+ my $b = new Derived ;
+ $a->doit(1) ;
+
+When this code is run only the C<Derived> object, C<$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.
+
+=head1 TODO
https://perl5.git.perl.org / perl5.git / blobdiff