PERLLEXWARN(1) Perl Programmers Reference Guide PERLLEXWARN(1)NAMEperllexwarn - 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 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.
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 $a = "2:" + 3;
With the introduction of lexical warnings, mandatory warn
ings now become default warnings. The difference is that
although the previously mandatory warnings are still
enabled by default, they can then be subsequently enabled
or disabled with the lexical warning pragma. For example,
in the code below, an ""isn't numeric"" warning will only
be reported for the "$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 "$^W"
Although very useful, the big problem with using -w on the
command line to enable warnings is that it is all or noth
ing. Take the typical scenario when you are writing a Perl
program. Parts of the code you will write yourself, but
it's very likely that you will make use of pre-written
Perl modules. If you use the -w flag in this case, you end
up enabling warnings in pieces of code that you haven't
written.
Similarly, using "$^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 inad
vertently 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 dis
ables 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 "$^W", this sec
tion will describe how they interact.
How Lexical Warnings interact with -w/"$^W":
1. If none of the three command line flags (-w, -W or
-X) that control warnings is used and neither "$^W"
or the "warnings" pragma are used, then default warn
ings will be enabled and optional warnings disabled.
This means that legacy code that doesn't attempt to
control the warnings will work unchanged.
2. The -w flag just sets the global "$^W" variable as in
5.005 -- this means that any legacy code that cur
rently relies on manipulating "$^W" to control warn
ing behavior will still work as is.
3. Apart from now being a boolean, the "$^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 "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:
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 speci
fied in the lexical scope into fatal errors. In the code
below, the use of "time", "length" and "join" can all pro
duce 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.
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 mod
ule 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 warn
ings 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 warn
ings 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", "warn
ings::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
2001-03-18 perl v5.6.1 PERLLEXWARN(1)
