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perllexwarn ()
  • >> perllexwarn (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perllexwarn (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perllexwarn - Perl Lexical Warnings


         The `use warnings' pragma is a replacement for both the
         command line flag -w and the equivalent Perl variable,
         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 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
         By default, optional warnings are disabled, so any legacy
         code that doesn't attempt to control the warnings will work
         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
             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 `EQ' operator won't trip a `"Use of EQ
         is deprecated"' warning, but the use of `NE' will produce a
         `"Use of NE is deprecated"' warning.
         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 `$^W'
         Although very useful, the big problem with using -w on the
         command line to enable warnings is that it is all or
         nothing. Take the typical scenario when you are writing a
         Perl program. Parts of the code you will write yourself, but
         it's very likely that you will make use of pre-written Perl
         modules. If you use the -w flag in this case, you end up
         enabling warnings in pieces of code that you haven't
         Similarly, using `$^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 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 -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 ;
         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
         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 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
             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,
         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 ;
         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 when the 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 module 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';
         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 {
                 if (warnings::enabled("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) ;
         The `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';
         the `warnings::warn' function will detect this and die after
         displaying the warning message.


             The debugger saves and restores C<$^W> at runtime. I haven't checked
             whether the debugger will still work with the lexical warnings
             patch applied.
             I *think* I've got diagnostics to work with the lexical warnings
             patch, but there were design decisions made in diagnostics to work
             around the limitations of C<$^W>. Now that those limitations are gone,
             the module should be revisited.


         the warnings manpage, the perldiag manpage.


         Paul Marquess

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