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;
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 $^W being dynamically scoped.
Lexical warnings get around these limitations by allowing finer control
over where warnings can or can't be tripped.
Controlling Warnings from the Command Line
There are three Command Line flags that can be used to control when
warnings are (or aren't) produced:
-w This is the existing flag. If the lexical warnings pragma is not
used in any of you code, or any of the modules that you use, this
flag will enable warnings everywhere. See "Backward Compatibility"
for details of how this flag interacts with lexical warnings.
-W If the -W flag is used on the command line, it will enable all
warnings throughout the program regardless of whether warnings
were disabled locally using "no warnings" or "$^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 have code that uses both
lexical warnings and $^W, this section will describe how they interact.
How Lexical Warnings interact with -w/$^W:
1. If none of the three command line flags (-w, -W or -X) that
control warnings is used and neither $^W nor 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.
2. 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.
3. 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.
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
|
+- 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----------+
| |
| +- surrogate
| |
| +- non_unicode
| |
| +- nonchar
|
+- 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.
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.
AUTHOR
Paul Marquess
perl v5.16.3 2013-03-04 PERLLEXWARN(1)
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