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 that the pragma setting will not
+enclosing block. It also means that the pragma setting will not
leak across files (via C<use>, C<require> or C<do>). This allows
authors to independently define the degree of warning checks that will
be applied to their module.
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 ;
details of how this flag interacts with lexical warnings.
=item B<-W>
-
+
If the B<-W> flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
locally using C<no warnings> or C<$^W =0>. This includes all files that get
to control warning behavior will still work as is.
=item 3.
-
+
Apart from now being a boolean, the C<$^W> variable operates in exactly
the same horrible uncontrolled global way, except that it cannot
disable/enable default warnings.
=item 4.
-
+
If a piece of code is under the control of the C<warnings> pragma,
both the C<$^W> variable and the B<-w> flag will be ignored for the
scope of the lexical warning.
=item 5.
-
+
The only way to override a lexical warnings setting is with the B<-W>
or B<-X> command line flags.
=back
-The combined effect of 3 & 4 is that it will will allow code which uses
+The combined effect of 3 & 4 is that it will allow code which uses
the C<warnings> pragma to control the warning behavior of $^W-type
code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
=head2 Category Hierarchy
-
+
A hierarchy of "categories" have been defined to allow groups of warnings
to be enabled/disabled in isolation.
L<perldiag>.
=head2 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, there are 3 places where a
use warnings ;
-
+
$a = 1 if $a EQ $b ;
-
+
{
use warnings FATAL => qw(deprecated) ;
$a = 1 if $a EQ $b ;
}
-
+
$a = 1 if $a EQ $b ;
=head2 Reporting Warnings from a Module
-The C<warnings> pragma provides two functions, namely C<warnings::enabled>
-and C<warnings::warn>, that are useful for module authors. They are
-used when you want to report a module-specific warning, but only when
-the calling module has enabled warnings via the C<warnings> pragma.
+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<abc> below.
+Consider the module C<MyMod::Abc> below.
- package abc;
+ package MyMod::Abc;
- sub open
- {
- if (warnings::enabled("deprecated")) {
- warnings::warn("deprecated",
- "abc::open is deprecated. Use abc:new") ;
+ use warnings::register;
+
+ sub open {
+ my $path = shift ;
+ if (warnings::enabled() && $path !~ m#^/#) {
+ warnings::warn("changing relative path to /tmp/");
+ $path = "/tmp/$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(@_) ;
}
"deprecated" warnings category enabled. Something like this, say.
use warnings 'deprecated';
- use abc;
+ use MyMod::Abc;
...
- abc::open($filename) ;
+ MyMod::Abc::open($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
-If the calling module has escalated the "deprecated" warnings category
-into a fatal error like this:
-
- use warnings 'FATAL deprecated';
- use abc;
+ use MyMod::Abc;
+ use warnings FATAL => 'MyMod::Abc';
...
- abc::open($filename) ;
+ 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 ;
-then C<warnings::warn> will detect this and die after displaying the
-warning message.
+ 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
-
+
perl5db.pl
The debugger saves and restores C<$^W> at runtime. I haven't checked
whether the debugger will still work with the lexical warnings
around the limitations of C<$^W>. Now that those limitations are gone,
the module should be revisited.
+ document calling the warnings::* functions from XS
+
=head1 SEE ALSO
L<warnings>, L<perldiag>.
-
+
=head1 AUTHOR
-
+
Paul Marquess
projects / p5sagit/p5-mst-13.2.git / blobdiff