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 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:
- 1.
- 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.
- 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