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 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 ;
For example, consider the code below:
use warnings ;
- my $a ;
- my $b ;
+ my @a ;
{
no warnings ;
- $b = 2 if $a EQ 3 ;
+ 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
my $a = "2:" + 3;
-though the result will be 5.
-
With the introduction of lexical warnings, mandatory warnings now become
I<default> warnings. The difference is that although the previously
mandatory warnings are still enabled by default, they can then be
subsequently enabled or disabled with the lexical warning pragma. For
-example, in the code below, an C<"integer overflow"> warning will only
+example, in the code below, an C<"isn't numeric"> warning will only
be reported for the C<$a> variable.
my $a = "2:" + 3;
my $b ; chop $b ;
}
-The other big problem with C<$^W> is that way you can inadvertently
+The other big problem with C<$^W> is the way you can inadvertently
change the warning setting in unexpected places in your code. For example,
when the code below is run (without the B<-w> flag), the second call
to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
details of how this flag interacts with lexical warnings.
=item B<-W>
-
+
If the B<-W> flag is used on the command line, it will enable all warnings
throughout the program regardless of whether warnings were disabled
locally using C<no warnings> or C<$^W =0>. This includes all files that get
lexical warnings and C<$^W>, this section will describe how they interact.
How Lexical Warnings interact with B<-w>/C<$^W>:
-
+
=over 5
=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<$^W> or lexical warnings are used,
-then default warnings will be enabled and optional warnings disabled.
+control warnings is used and neither C<$^W> or 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
will work unchanged.
to control warning behavior will still work as is.
=item 3.
-
+
Apart from now being a boolean, the C<$^W> variable operates in exactly
the same horrible uncontrolled global way, except that it cannot
disable/enable default warnings.
=item 4.
-
-If a piece of code is under the control of the lexical warning pragma,
+
+If a piece of code is under the control of the C<warnings> pragma,
both the C<$^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 lexical warnings pragma to control the warning behavior of $^W-type
+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.
-=head1 EXPERIMENTAL FEATURES
-
-The features described in this section are experimental, and so subject
-to change.
-
=head2 Category Hierarchy
-
-A B<tentative> hierarchy of "categories" have been defined to allow groups
-of warnings to be enabled/disabled in isolation. The current
-hierarchy is:
-
- all - +--- unsafe -------+--- taint
- | |
- | +--- substr
- | |
- | +--- signal
- | |
- | +--- closure
- | |
- | +--- overflow
- | |
- | +--- portable
- | |
- | +--- untie
- | |
- | +--- utf8
- |
- +--- io ---------+--- pipe
- | |
- | +--- unopened
- | |
- | +--- closed
- | |
- | +--- newline
- | |
- | +--- exec
- |
- +--- syntax ----+--- ambiguous
- | |
- | +--- semicolon
- | |
- | +--- precedence
- | |
- | +--- reserved
- | |
- | +--- digit
- | |
- | +--- parenthesis
- | |
- | +--- deprecated
- | |
- | +--- printf
- |
- +--- severe ----+--- inplace
- | |
- | +--- internal
- | |
- | +--- debugging
- |
- |--- uninitialized
- |
- +--- void
- |
- +--- recursion
- |
- +--- redefine
- |
- +--- numeric
- |
- +--- once
- |
- +--- misc
+A hierarchy of "categories" have been defined to allow groups of warnings
+to be enabled/disabled in isolation.
+
+The current hierarchy is:
+
+ all -+
+ |
+ +- assertions
+ |
+ +- 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
+ |
+ +- 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 combined
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 cumulative effect is additive.
+C<warnings> pragma in a given scope the cumulative effect is additive.
use warnings qw(void) ; # only "void" 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 Fatal Warnings
-
-The presence of the word "FATAL" in the category list will escalate any
-warnings from the category/categories specified that are detected 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.
+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 C<time>, C<length>
+and C<join> can all produce a C<"Useless use of xxx in void context">
+warning.
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 ;
-
-=head1 TODO
-
-The experimental features need bottomed out.
- perldiag.pod
- Need to add warning class information and notes on
- how to use the class info with the warnings pragma.
+ 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 /tmp/")
+ if warnings::enabled();
+ $path = "/tmp/$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(@_) ;
+ }
+
+ 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
perl5db.pl
The debugger saves and restores C<$^W> at runtime. I haven't checked
around the limitations of C<$^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<warnings>, L<perldiag>.
+
=head1 AUTHOR
-
+
Paul Marquess
https://perl5.git.perl.org / perl5.git / blobdiff