=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 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.
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 ;
- my $b ;
+ use warnings;
+ my @a;
{
- no warnings ;
- $b = 2 if $a EQ 3 ;
+ no warnings;
+ my $b = @a[0];
}
- $b = 1 if $a NE 3 ;
+ my $c = @a[0];
The code in the enclosing block has warnings enabled, but the inner
-block has them disabled. In this case that means that the use of the C<EQ>
-operator won't trip a C<"Use of EQ is deprecated"> warning, but the use of
-C<NE> will produce a C<"Use of NE is deprecated"> warning.
+block has them disabled. In this case that means the assignment to the
+scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
+warning, but the assignment to the scalar C<$b> will not.
=head2 Default Warnings and Optional Warnings
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 that way you can inadvertently
+The other big problem with C<$^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 B<-w> flag), the second call
to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
sub doit
{
- my $b ; chop $b ;
+ my $b; chop $b;
}
- doit() ;
+ doit();
{
- local ($^W) = 1 ;
+ local ($^W) = 1;
doit()
}
=over 5
=item B<-w>
+X<-w>
This is the existing flag. If the lexical warnings pragma is B<not>
used in any of you code, or any of the modules that you use, this flag
details of how this flag interacts with lexical warnings.
=item B<-W>
+X<-W>
If the B<-W> flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
Think of it as the Perl equivalent of the "lint" command.
=item B<-X>
+X<-X>
Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
=head2 Backward Compatibility
-If you are used with working with a version of Perl prior to the
+If you are used to working with a version of Perl prior to the
introduction of lexically scoped warnings, or have code that uses both
lexical warnings and C<$^W>, this section will describe how they interact.
=item 1.
If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
-control warnings is used and neither C<$^W> or the C<warnings> pragma
+control warnings is used and neither C<$^W> nor the C<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
=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.
=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
+X<warning, categories>
A hierarchy of "categories" have been defined to allow groups of warnings
to be enabled/disabled in isolation.
The current hierarchy is:
- all -+
- |
- +- chmod
- |
- +- closure
- |
- +- exiting
- |
- +- glob
- |
- +- io -----------+
- | |
- | +- closed
- | |
- | +- exec
- | |
- | +- newline
- | |
- | +- pipe
- | |
- | +- unopened
- |
- +- misc
- |
- +- numeric
- |
- +- once
- |
- +- overflow
- |
- +- pack
- |
- +- portable
- |
- +- recursion
- |
- +- redefine
- |
- +- regexp
- |
- +- severe -------+
- | |
- | +- debugging
- | |
- | +- inplace
- | |
- | +- internal
- | |
- | +- malloc
- |
- +- signal
- |
- +- substr
- |
- +- syntax -------+
- | |
- | +- ambiguous
- | |
- | +- bareword
- | |
- | +- deprecated
- | |
- | +- digit
- | |
- | +- parenthesis
- | |
- | +- precedence
- | |
- | +- printf
- | |
- | +- prototype
- | |
- | +- qw
- | |
- | +- reserved
- | |
- | +- semicolon
- |
- +- taint
- |
- +- umask
- |
- +- uninitialized
- |
- +- unpack
- |
- +- untie
- |
- +- utf8
- |
- +- void
- |
- +- y2k
+ all -+
+ |
+ +- closure
+ |
+ +- deprecated
+ |
+ +- exiting
+ |
+ +- experimental
+ |
+ +- glob
+ |
+ +- imprecision
+ |
+ +- io ------------+
+ | |
+ | +- closed
+ | |
+ | +- exec
+ | |
+ | +- layer
+ | |
+ | +- newline
+ | |
+ | +- pipe
+ | |
+ | +- unopened
+ |
+ +- 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 ----------+
+ | |
+ | +- non_unicode
+ | |
+ | +- nonchar
+ | |
+ | +- surrogate
+ |
+ +- void
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>.
+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.
+
+=head2 Individual Warning IDs
+
+The "experimental" warnings category, added in Perl 5.18,
+contains IDs for individual warnings, so that each warning can
+be turned on or off. Currently there is only one such warning,
+labelled "experimental:lexical_subs". You enable and disable
+it like this:
+
+ use warnings "experimental:lexical_subs";
+ no warnings "experimental:lexical_subs";
+
+The plan is to extend this convention to all warnings in a future release.
+
=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
-into fatal errors. In the code below, there are 3 places where a
-deprecated warning will be detected, the middle one will produce a
-fatal error.
-
+into fatal errors. In the code below, the use of C<time>, C<length>
+and C<join> can all produce a C<"Useless use of xxx in void context">
+warning.
- use warnings ;
+ use warnings;
- $a = 1 if $a EQ $b ;
+ time;
{
- use warnings FATAL => qw(deprecated) ;
- $a = 1 if $a EQ $b ;
+ use warnings FATAL => qw(void);
+ length "abc";
}
- $a = 1 if $a EQ $b ;
+ 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
+X<warning, reporting> X<warning, registering>
-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 $path = shift;
+ if ($path !~ m#^/#) {
+ warnings::warn("changing relative path to /var/abc")
+ if warnings::enabled();
+ $path = "/var/abc/$path";
}
- new(@_) ;
+ }
+
+ 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 ;
+ 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 abc;
+ use MyMod::Abc;
...
- abc::open($filename) ;
-
+ MyMod::Abc::open($filename);
-If the calling module has escalated the "deprecated" warnings category
-into a fatal error like this:
+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 warnings 'FATAL deprecated';
- use abc;
+ use MyMod::Abc;
+ use warnings FATAL => 'MyMod::Abc';
...
- abc::open($filename) ;
+ MyMod::Abc::open('../fred.txt');
+
+the C<warnings::warnif> function will detect this and die after
+displaying the warning message.
+
+The three warnings functions, C<warnings::warn>, C<warnings::warnif>
+and C<warnings::enabled> can optionally take an object reference in place
+of a category name. In this case the functions will use the class name
+of the object as the warnings category.
+
+Consider this example:
-then C<warnings::warn> will detect this and die after displaying the
-warning message.
+ package Original;
-=head1 TODO
+ no warnings;
+ use warnings::register;
- perl5db.pl
- The debugger saves and restores C<$^W> at runtime. I haven't checked
- whether the debugger will still work with the lexical warnings
- patch applied.
+ 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 = 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.
+
+ 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);
+
+ ...
- 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.
+ warnings::warnif('MyModule::format', '...');
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff