=head1 DESCRIPTION
-The C<use warnings> pragma is a replacement for both the command line
-flag B<-w> and the equivalent Perl variable, C<$^W>.
+Perl v5.6.0 introduced lexical control over the handling of warnings by
+category. The C<warnings> pragma generally replaces the command line flag
+B<-w>. Documentation on the use of lexical warnings, once partly found in
+this document, is now found in the L<warnings> documentation.
-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 C<use>, C<require> or C<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 ;
- my $b ;
- {
- no warnings ;
- $b = 2 if $a EQ 3 ;
- }
- $b = 1 if $a NE 3 ;
-
-The code in the enclosing block has warnings enabled, but the inner
-block has them disabled. In this case that means that the use of the C<EQ>
-operator won't trip a C<"Use of EQ is deprecated"> warning, but the use of
-C<NE> will produce a C<"Use of NE is deprecated"> warning.
-
-=head2 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 C<"isn't numeric">
-warning about the "2:".
-
- my $a = "2:" + 3;
-
-With the introduction of lexical warnings, mandatory warnings now become
-I<default> warnings. The difference is that although the previously
-mandatory warnings are still enabled by default, they can then be
-subsequently enabled or disabled with the lexical warning pragma. For
-example, in the code below, an C<"isn't numeric"> warning will only
-be reported for the C<$a> variable.
-
- my $a = "2:" + 3;
- no warnings ;
- my $b = "2:" + 3;
-
-Note that neither the B<-w> flag or the C<$^W> can be used to
-disable/enable default warnings. They are still mandatory in this case.
-
-=head2 What's wrong with B<-w> and C<$^W>
-
-Although very useful, the big problem with using B<-w> on the command
-line to enable warnings is that it is all or nothing. Take the typical
-scenario when you are writing a Perl program. Parts of the code you
-will write yourself, but it's very likely that you will make use of
-pre-written Perl modules. If you use the B<-w> flag in this case, you
-end up enabling warnings in pieces of code that you haven't written.
-
-Similarly, using C<$^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 $b ;
- }
-
-When this code is run with the B<-w> flag, a warning will be produced
-for the C<$a> line -- C<"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 $b ;
- }
-
-The other big problem with C<$^W> is that way you can inadvertently
-change the warning setting in unexpected places in your code. For example,
-when the code below is run (without the B<-w> flag), the second call
-to C<doit> will trip a C<"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 C<$^W> being dynamically scoped.
-
-Lexical warnings get around these limitations by allowing finer control
-over where warnings can or can't be tripped.
-
-=head2 Controlling Warnings from the Command Line
-
-There are three Command Line flags that can be used to control when
-warnings are (or aren't) produced:
-
-=over 5
-
-=item B<-w>
-
-This is the existing flag. If the lexical warnings pragma is B<not>
-used in any of you code, or any of the modules that you use, this flag
-will enable warnings everywhere. See L<Backward Compatibility> for
-details of how this flag interacts with lexical warnings.
-
-=item B<-W>
-
-If the B<-W> flag is used on the command line, it will enable all warnings
-throughout the program regardless of whether warnings were disabled
-locally using C<no warnings> or C<$^W =0>. This includes all files that get
-included via C<use>, C<require> or C<do>.
-Think of it as the Perl equivalent of the "lint" command.
-
-=item B<-X>
-
-Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
-
-=back
-
-=head2 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 C<$^W>, this section will describe how they interact.
-
-How Lexical Warnings interact with B<-w>/C<$^W>:
-
-=over 5
-
-=item 1.
-
-If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
-control warnings is used and neither C<$^W> or the C<warnings> pragma
-are used, then default warnings will be enabled and optional warnings
-disabled.
-This means that legacy code that doesn't attempt to control the warnings
-will work unchanged.
-
-=item 2.
-
-The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
-means that any legacy code that currently relies on manipulating C<$^W>
-to control warning behavior will still work as is.
-
-=item 3.
-
-Apart from now being a boolean, the C<$^W> variable operates in exactly
-the same horrible uncontrolled global way, except that it cannot
-disable/enable default warnings.
-
-=item 4.
-
-If a piece of code is under the control of the C<warnings> pragma,
-both the C<$^W> variable and the B<-w> flag will be ignored for the
-scope of the lexical warning.
-
-=item 5.
-
-The only way to override a lexical warnings setting is with the B<-W>
-or B<-X> command line flags.
-
-=back
-
-The combined effect of 3 & 4 is that it will allow code which uses
-the C<warnings> pragma to control the warning behavior of $^W-type
-code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
-
-=head2 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
-C<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
-L<perldiag>.
-
-=head2 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, there are 3 places where a
-deprecated warning will be detected, the middle one will produce a
-fatal error.
-
-
- use warnings ;
-
- $a = 1 if $a EQ $b ;
-
- {
- use warnings FATAL => qw(deprecated) ;
- $a = 1 if $a EQ $b ;
- }
-
- $a = 1 if $a EQ $b ;
-
-=head2 Reporting Warnings from a Module
-
-The C<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 when the calling module has enabled warnings via the C<warnings>
-pragma.
-
-Consider the module C<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 C<warnings::register> will create a new warnings category
-called "MyMod::abc", i.e. the new category name matches the module
-name. The C<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 C<MyMod::Abc> has actually enabled
-them with the C<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 C<warnings::enabled> function. Consider
-this snippet of code:
-
- package MyMod::Abc;
-
- sub open {
- if (warnings::enabled("deprecated")) {
- warnings::warn("deprecated",
- "open is deprecated, use new instead") ;
- }
- new(@_) ;
- }
-
- sub new
- ...
- 1 ;
-
-The function C<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($filename) ;
-
-The C<warnings::warn> function should be used to actually display the
-warnings message. This is because they can make use of the feature that
-allows warnings to be escalated into fatal errors. So in this case
-
- use MyMod::Abc;
- use warnings FATAL => 'MyMod::Abc';
- ...
- MyMod::Abc::open('../fred.txt');
-
-the C<warnings::warn> function will detect this and die after
-displaying the warning message.
-
-=head1 TODO
-
- perl5db.pl
- The debugger saves and restores C<$^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.
-
-=head1 SEE ALSO
-
-L<warnings>, L<perldiag>.
-
-=head1 AUTHOR
-
-Paul Marquess
https://perl5.git.perl.org / perl5.git / blobdiff