PERLLEXWARN(1) Perl Programmers Reference Guide PERLLEXWARN(1)NAMEperllexwarn - Perl Lexical Warnings
DESCRIPTION
The "use warnings" pragma is a replacement for both the com-
mand line flag -w and the equivalent Perl variable, $^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 the pragma set-
ting 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.
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As its name suggests, if your code tripped a mandatory warn-
ing, 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 warn-
ings 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 dis-
abled 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 noth-
ing. 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 writ-
ten.
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:
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{
BEGIN { $^W = 0 }
my $a =+ 2;
my $b; chop $b;
}
The other big problem with $^W is the way you can inadver-
tently 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 $^W being dynamically scoped.
Lexical warnings get around these limitations by allowing
finer control over where warnings can or can't be tripped.
Controlling Warnings from the Command Line
There are three Command Line flags that can be used to con-
trol when warnings are (or aren't) produced:
-w This is the existing flag. If the lexical warnings
pragma is not used in any of you code, or any of the
modules that you use, this flag will enable warnings
everywhere. See "Backward Compatibility" for details of
how this flag interacts with lexical warnings.
-W If the -W flag is used on the command line, it will
enable all warnings throughout the program regardless
of whether warnings were disabled locally using "no
warnings" or "$^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 dis-
ables all warnings.
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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 glo-
bal way, except that it cannot disable/enable default
warnings.
4. If a piece of code is under the control of the "warn-
ings" 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:
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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
|
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+- 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
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 cumu-
lative effect is additive.
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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 "depre-
cated" 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 ter-
minates 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 dis-
able the "void" warning in the example above, either of
these will do the trick:
no warnings qw(void);
no warnings FATAL => qw(void);
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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;
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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;
}
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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 = new Original;
$a->doit(1);
my $b = new Derived;
$a->doit(1);
When this code is run only the "Derived" object, $b, will
generate a warning.
Odd numbers are unsafe at main.pl line 7
Notice also that the warning is reported at the line where
the object is first used.
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perl5db.pl
The debugger saves and restores C<$^W> at runtime. I haven't checked
whether the debugger will still work with the lexical warnings
patch applied.
diagnostics.pm
I *think* I've got diagnostics to work with the lexical warnings
patch, but there were design decisions made in diagnostics to work
around the limitations of C<$^W>. Now that those limitations are gone,
the module should be revisited.
document calling the warnings::* functions from XS
SEE ALSO
warnings, perldiag.
AUTHOR
Paul Marquess
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