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PERLLEXWARN(1)
NAME
perllexwarn - Perl Lexical Warnings
DESCRIPTION
The "use warnings" pragma is a replacement for both the command 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 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 "$^W" being dynamically scoped.
Lexical warnings get around these limitations by allowing finer control
over where warnings can or can't be tripped.
Controlling Warnings from the Command Line
There are three Command Line flags that can be used to control when
warnings are (or aren't) produced:
-w This is the existing flag. If the lexical warnings pragma is not used
in any of you code, or any of the modules that you use, this flag will
enable warnings everywhere. See the Backward Compatibility entry
elsewhere in this document for details of how this flag interacts with
lexical warnings.
-W If the -W flag is used on the command line, it will enable all
warnings throughout the program regardless of whether warnings were
disabled locally using "no warnings" or "$^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 -+
|
+- chmod
|
+- closure
|
+- exiting
|
+- glob
|
+- io -----------+
| |
| +- closed
| |
| +- exec
| |
| +- newline
| |
| +- pipe
| |
| +- unopened
|
+- misc
|
+- numeric
|
+- once
|
+- overflow
|
+- pack
|
+- portable
|
+- recursion
|
+- redefine
|
+- regexp
|
+- severe -------+
| |
| +- debugging
| |
| +- inplace
| |
| +- internal
| |
| +- malloc
|
+- signal
|
+- substr
|
+- syntax -------+
| |
| +- ambiguous
| |
| +- bareword
| |
| +- deprecated
| |
| +- digit
| |
| +- parenthesis
| |
| +- precedence
| |
| +- printf
| |
| +- prototype
| |
| +- qw
| |
| +- reserved
| |
| +- semicolon
|
+- taint
|
+- umask
|
+- 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 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 the
perldiag manpage.
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.
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 (warnings::enabled() && $path !~ m#^/#) {
warnings::warn("changing relative path to /tmp/");
$path = "/tmp/$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 = 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.
TODO
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
the warnings manpage, the perldiag manpage.
AUTHOR
Paul Marquess
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