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Section: Perl Programmers Reference Guide (1)
Updated: 2009-02-12
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perllexwarn - Perl Lexical Warnings  


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 "isnt 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 "isnt numeric" warning will only be reported for the $a variable.

my $a = "2:" + 3; no warnings; my $b = "2:" + 3;

Note that neither the -w flag or the $^W can be used to disable/enable default warnings. They are still mandatory in this case.  

What's wrong with -w and $^W

Although very useful, the big problem with using -w on the command line to enable warnings is that it is all or nothing. Take the typical scenario when you are writing a Perl program. Parts of the code you will write yourself, but it's very likely that you will make use of pre-written Perl modules. If you use the -w flag in this case, you end up enabling warnings in pieces of code that you haven't written.

Similarly, using $^W to either disable or enable blocks of code is fundamentally flawed. For a start, say you want to disable warnings in 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; }

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:
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.
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.
Does the exact opposite to the -W flag, i.e. it disables all warnings.

Backward Compatibility

If you are used with working with a version of Perl prior to the introduction of lexically scoped warnings, or have code that uses both lexical warnings and $^W, this section will describe how they interact.

How Lexical Warnings interact with -w/$^W:

If none of the three command line flags (-w, -W or -X) that control warnings is used and neither $^W or the 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.
The -w flag just sets the global $^W variable as in 5.005 this means that any legacy code that currently relies on manipulating $^W to control warning behavior will still work as is.
Apart from now being a boolean, the $^W variable operates in exactly the same horrible uncontrolled global way, except that it cannot disable/enable default warnings.
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.
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 | +- 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

Just like the strict pragma any of these categories can be combined

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 warnings pragma in a given scope the cumulative effect is additive.

use warnings qw(void); # only "void" warnings enabled ... use warnings qw(io); # only "void" & "io" warnings enabled ... no warnings qw(void); # only "io" warnings enabled

To determine which category a specific warning has been assigned to see 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.  

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 { my $path = shift; if ($path !~ m#^/#) { warnings::warn("changing relative path to /var/abc") if warnings::enabled(); $path = "/var/abc/$path"; } } 1;

The call to warnings::register will create a new warnings category called MyMod::abc, i.e. the new category name matches the current package name. The 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 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 { 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 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.  


warnings, perldiag.  


Paul Marquess



Default Warnings and Optional Warnings
What's wrong with -w and $^W
Controlling Warnings from the Command Line
Backward Compatibility
Category Hierarchy
Fatal Warnings
Reporting Warnings from a Module

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