PERLLEXWARN(1) Perl Programmers Reference Guide PERLLEXWARN(1)
NAME
perllexwarn - Perl Lexical Warnings
DESCRIPTION
The "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 -w and the equivalent Perl
variable, $^W.
This pragma works just like the "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 "use", "require"
or "do"). This allows authors to independently define the degree of warning checks that will be applied to
their module.
By default, optional warnings are disabled, so any legacy code that 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;
no warnings 'all';
For example, consider the code below:
use warnings;
my @a;
{
no warnings;
my $b = @a[0];
}
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 the assignment to the scalar $c will trip the "Scalar value @a[0] better written as $a[0]" warning, but
the assignment to the scalar $b will not.
Default Warnings and Optional Warnings
Before the introduction of lexical warnings, Perl had two classes of warnings: mandatory and optional.
As its name suggests, if your code tripped a mandatory warning, you would get a warning whether you wanted it
or not. For example, the code below would always produce an "isn't numeric" warning about the "2:".
my $a = "2:" + 3;
With the introduction of lexical warnings, mandatory warnings now become 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 "isn't numeric"
warning will only be reported for the $a variable.
my $a = "2:" + 3;
no warnings;
my $b = "2:" + 3;
{
local ($^W) = 0;
my $a =+ 2;
my $b; chop $b;
}
When this code is run with the -w flag, a warning will be produced for the $a line: "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;
}
The other big problem with $^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 -w flag), the second call to "doit"
will trip a "Use of uninitialized value" warning, whereas the first will not.
sub doit
{
my $b; chop $b;
}
doit();
{
local ($^W) = 1;
doit()
}
This is a side-effect of $^W being dynamically scoped.
Lexical warnings get around these limitations by allowing finer control over where warnings can or can't be
tripped.
Controlling Warnings from the Command Line
There are three Command Line flags that can be used to control when warnings are (or aren't) produced:
-w This is the existing flag. If the lexical warnings pragma is not used in any of you code, or any of the
modules that you use, this flag will enable warnings everywhere. See "Backward Compatibility" for details
of how this flag interacts with lexical warnings.
-W If the -W flag is used on the command line, it will enable all warnings throughout the program regardless
of whether warnings were disabled locally using "no warnings" or "$^W =0". This includes all files that
get included via "use", "require" or "do". Think of it as the Perl equivalent of the "lint" command.
-X Does the exact opposite to the -W flag, i.e. it disables all warnings.
Backward Compatibility
If you are used to working with a version of Perl prior to the introduction of lexically scoped warnings, or
global way, except that it cannot disable/enable default warnings.
4. If a piece of code is under the control of the "warnings" pragma, both the $^W variable and the -w flag
will be ignored for the scope of the lexical warning.
5. The only way to override a lexical warnings setting is with the -W or -X command line flags.
The combined effect of 3 & 4 is that it will allow code which uses the "warnings" pragma to control the
warning behavior of $^W-type code (using a "local $^W=0") if it really wants to, but not vice-versa.
Category Hierarchy
A hierarchy of "categories" have been defined to allow groups of warnings to be enabled/disabled in isolation.
The current hierarchy is:
all -+
|
+- closure
|
+- deprecated
|
+- exiting
|
+- glob
|
+- io -----------+
| |
| +- closed
| |
| +- exec
| |
| +- layer
| |
| +- newline
| |
| +- pipe
| |
| +- unopened
|
+- imprecision
|
+- misc
|
+- numeric
|
+- once
|
+- overflow
|
+- pack
|
+- portable
|
+- recursion
| |
| +- malloc
|
+- signal
|
+- substr
|
+- syntax -------+
| |
| +- ambiguous
| |
| +- bareword
| |
| +- digit
| |
| +- illegalproto
| |
| +- parenthesis
| |
| +- precedence
| |
| +- printf
| |
| +- prototype
| |
| +- qw
| |
| +- reserved
| |
| +- semicolon
|
+- taint
|
+- threads
|
+- uninitialized
|
+- unpack
|
+- untie
|
+- utf8----------+
| |
| +- surrogate
| |
| +- non_unicode
| |
| +- nonchar
|
+- void
Just like the "strict" pragma any of these categories can be combined
use warnings qw(void redefine);
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.
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, the use of "time", "length" and "join"
can all produce a "Useless use of xxx in void context" warning.
use warnings;
time;
{
use warnings FATAL => qw(void);
length "abc";
}
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 "length" is used has escalated the "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';
Reporting Warnings from a Module
The "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 "warnings"
pragma.
Consider the module "MyMod::Abc" below.
package MyMod::Abc;
use warnings::register;
sub open {
message if it gets given a relative path as a parameter. This warnings will only be displayed if the code that
uses "MyMod::Abc" has actually enabled them with the "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
"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 "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 "warnings::warn" or "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 "warnings::warnif" function will detect this and die after displaying the warning message.
The three warnings functions, "warnings::warn", "warnings::warnif" and "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
}
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 "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 "Derived" object, $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);
...
warnings::warnif('MyModule::format', '...');
SEE ALSO
warnings, perldiag.
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