=head1 NAME
+X<warning, lexical> X<warnings> X<warning>
perllexwarn - Perl Lexical Warnings
=head1 DESCRIPTION
-The C<use warnings> pragma is a replacement for both the command line
-flag B<-w> and the equivalent Perl variable, C<$^W>.
+The C<use warnings> pragma enables to control precisely what warnings are
+to be enabled in which parts of a Perl program. It's a more flexible
+alternative for both the command line flag B<-w> and the equivalent Perl
+variable, C<$^W>.
-The pragma works just like the existing "strict" pragma.
+This pragma works just like the C<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 C<use>, C<require> or C<do>). This allows
All warnings are enabled in a block by either of these:
- use warnings ;
- use warnings 'all' ;
+ use warnings;
+ use warnings 'all';
Similarly all warnings are disabled in a block by either of these:
- no warnings ;
- no warnings 'all' ;
+ no warnings;
+ no warnings 'all';
For example, consider the code below:
- use warnings ;
- my @a ;
+ use warnings;
+ my @a;
{
- no warnings ;
- my $b = @a[0] ;
+ no warnings;
+ my $b = @a[0];
}
my $c = @a[0];
be reported for the C<$a> variable.
my $a = "2:" + 3;
- no warnings ;
+ no warnings;
my $b = "2:" + 3;
Note that neither the B<-w> flag or the C<$^W> can be used to
a block of code. You might expect this to be enough to do the trick:
{
- local ($^W) = 0 ;
- my $a =+ 2 ;
- my $b ; chop $b ;
+ local ($^W) = 0;
+ my $a =+ 2;
+ my $b; chop $b;
}
When this code is run with the B<-w> flag, a warning will be produced
-for the C<$a> line -- C<"Reversed += operator">.
+for the C<$a> line: C<"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 ;
+ my $a =+ 2;
+ my $b; chop $b;
}
The other big problem with C<$^W> is the way you can inadvertently
sub doit
{
- my $b ; chop $b ;
+ my $b; chop $b;
}
- doit() ;
+ doit();
{
- local ($^W) = 1 ;
+ local ($^W) = 1;
doit()
}
=over 5
=item B<-w>
+X<-w>
This is the existing flag. If the lexical warnings pragma is B<not>
used in any of you code, or any of the modules that you use, this flag
details of how this flag interacts with lexical warnings.
=item B<-W>
+X<-W>
If the B<-W> flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
Think of it as the Perl equivalent of the "lint" command.
=item B<-X>
+X<-X>
Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
=item 2.
-The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
+The B<-w> flag just sets the global C<$^W> variable as in 5.005. This
means that any legacy code that currently relies on manipulating C<$^W>
to control warning behavior will still work as is.
code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
=head2 Category Hierarchy
+X<warning, categories>
A hierarchy of "categories" have been defined to allow groups of warnings
to be enabled/disabled in isolation.
| |
| +- unopened
|
+ +- imprecision
+ |
+- misc
|
+- numeric
| |
| +- digit
| |
+ | +- illegalproto
+ | |
| +- parenthesis
| |
| +- precedence
|
+- taint
|
+ +- threads
+ |
+- uninitialized
|
+- unpack
|
+- untie
|
- +- utf8
+ +- utf8----------+
+ | |
+ | +- surrogate
+ | |
+ | +- non_unicode
+ | |
+ | +- nonchar
|
+- 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) ;
+ 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
C<warnings> pragma in a given scope the cumulative effect is additive.
- use warnings qw(void) ; # only "void" warnings enabled
+ use warnings qw(void); # only "void" warnings enabled
...
- use warnings qw(io) ; # only "void" & "io" warnings enabled
+ use warnings qw(io); # only "void" & "io" warnings enabled
...
- no warnings qw(void) ; # only "io" warnings enabled
+ no warnings qw(void); # only "io" warnings enabled
To determine which category a specific warning has been assigned to see
L<perldiag>.
=head2 Fatal Warnings
+X<warning, fatal>
The presence of the word "FATAL" in the category list will escalate any
warnings detected from the categories specified in the lexical scope
and C<join> can all produce a C<"Useless use of xxx in void context">
warning.
- use warnings ;
+ use warnings;
- time ;
+ time;
{
- use warnings FATAL => qw(void) ;
- length "abc" ;
+ use warnings FATAL => qw(void);
+ length "abc";
}
- join "", 1,2,3 ;
+ join "", 1,2,3;
- print "done\n" ;
+ print "done\n";
When run it produces this output
category into a fatal error, so the program terminates immediately it
encounters the warning.
-To explicitly disable 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:
+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
+X<warning, reporting> X<warning, registering>
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
use warnings::register;
sub open {
- my $path = shift ;
- if (warnings::enabled() && $path !~ m#^/#) {
- warnings::warn("changing relative path to /tmp/");
- $path = "/tmp/$path" ;
+ my $path = shift;
+ if ($path !~ m#^/#) {
+ warnings::warn("changing relative path to /var/abc")
+ if warnings::enabled();
+ $path = "/var/abc/$path";
}
}
- 1 ;
+ 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
+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
sub open {
warnings::warnif("deprecated",
- "open is deprecated, use new instead") ;
- new(@_) ;
+ "open is deprecated, use new instead");
+ new(@_);
}
sub new
...
- 1 ;
+ 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
use warnings 'deprecated';
use MyMod::Abc;
...
- MyMod::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
Consider this example:
- package Original ;
+ package Original;
- no warnings ;
- use warnings::register ;
+ no warnings;
+ use warnings::register;
sub new
{
- my $class = shift ;
- bless [], $class ;
+ my $class = shift;
+ bless [], $class;
}
sub check
{
- my $self = shift ;
- my $value = shift ;
+ 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) ;
+ my $self = shift;
+ my $value = shift;
+ $self->check($value);
# ...
}
- 1 ;
+ 1;
- package Derived ;
+ package Derived;
- use warnings::register ;
- use Original ;
- our @ISA = qw( Original ) ;
+ use warnings::register;
+ use Original;
+ our @ISA = qw( Original );
sub new
{
- my $class = shift ;
- bless [], $class ;
+ my $class = shift;
+ bless [], $class;
}
- 1 ;
+ 1;
The code below makes use of both modules, but it only enables warnings from
C<Derived>.
- use Original ;
- use Derived ;
+ use Original;
+ use Derived;
use warnings 'Derived';
- my $a = new Original ;
- $a->doit(1) ;
- my $b = new Derived ;
- $a->doit(1) ;
+ my $a = Original->new();
+ $a->doit(1);
+ my $b = Derived->new();
+ $a->doit(1);
When this code is run only the C<Derived> object, C<$b>, will generate
a warning.
Notice also that the warning is reported at the line where the object is first
used.
-=head1 TODO
+When registering new categories of warning, you can supply more names to
+warnings::register like this:
- perl5db.pl
- The debugger saves and restores C<$^W> at runtime. I haven't checked
- whether the debugger will still work with the lexical warnings
- patch applied.
+ package MyModule;
+ use warnings::register qw(format precision);
- diagnostics.pm
- I *think* I've got diagnostics to work with the lexical warnings
- patch, but there were design decisions made in diagnostics to work
- around the limitations of C<$^W>. Now that those limitations are gone,
- the module should be revisited.
+ ...
- document calling the warnings::* functions from XS
+ warnings::warnif('MyModule::format', '...');
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff