[14489] | 1 | =head1 NAME
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| 2 | X<warning, lexical> X<warnings> X<warning>
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| 3 |
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| 4 | perllexwarn - Perl Lexical Warnings
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| 5 |
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| 6 | =head1 DESCRIPTION
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| 7 |
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| 8 | The C<use warnings> pragma is a replacement for both the command line
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| 9 | flag B<-w> and the equivalent Perl variable, C<$^W>.
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| 10 |
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| 11 | The pragma works just like the existing "strict" pragma.
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| 12 | This means that the scope of the warning pragma is limited to the
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| 13 | enclosing block. It also means that the pragma setting will not
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| 14 | leak across files (via C<use>, C<require> or C<do>). This allows
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| 15 | authors to independently define the degree of warning checks that will
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| 16 | be applied to their module.
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| 17 |
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| 18 | By default, optional warnings are disabled, so any legacy code that
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| 19 | doesn't attempt to control the warnings will work unchanged.
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| 20 |
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| 21 | All warnings are enabled in a block by either of these:
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| 22 |
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| 23 | use warnings;
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| 24 | use warnings 'all';
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| 25 |
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| 26 | Similarly all warnings are disabled in a block by either of these:
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| 27 |
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| 28 | no warnings;
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| 29 | no warnings 'all';
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| 30 |
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| 31 | For example, consider the code below:
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| 32 |
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| 33 | use warnings;
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| 34 | my @a;
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| 35 | {
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| 36 | no warnings;
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| 37 | my $b = @a[0];
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| 38 | }
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| 39 | my $c = @a[0];
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| 40 |
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| 41 | The code in the enclosing block has warnings enabled, but the inner
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| 42 | block has them disabled. In this case that means the assignment to the
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| 43 | scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]">
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| 44 | warning, but the assignment to the scalar C<$b> will not.
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| 45 |
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| 46 | =head2 Default Warnings and Optional Warnings
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| 47 |
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| 48 | Before the introduction of lexical warnings, Perl had two classes of
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| 49 | warnings: mandatory and optional.
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| 50 |
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| 51 | As its name suggests, if your code tripped a mandatory warning, you
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| 52 | would get a warning whether you wanted it or not.
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| 53 | For example, the code below would always produce an C<"isn't numeric">
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| 54 | warning about the "2:".
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| 55 |
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| 56 | my $a = "2:" + 3;
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| 57 |
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| 58 | With the introduction of lexical warnings, mandatory warnings now become
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| 59 | I<default> warnings. The difference is that although the previously
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| 60 | mandatory warnings are still enabled by default, they can then be
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| 61 | subsequently enabled or disabled with the lexical warning pragma. For
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| 62 | example, in the code below, an C<"isn't numeric"> warning will only
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| 63 | be reported for the C<$a> variable.
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| 64 |
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| 65 | my $a = "2:" + 3;
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| 66 | no warnings;
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| 67 | my $b = "2:" + 3;
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| 68 |
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| 69 | Note that neither the B<-w> flag or the C<$^W> can be used to
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| 70 | disable/enable default warnings. They are still mandatory in this case.
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| 71 |
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| 72 | =head2 What's wrong with B<-w> and C<$^W>
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| 73 |
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| 74 | Although very useful, the big problem with using B<-w> on the command
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| 75 | line to enable warnings is that it is all or nothing. Take the typical
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| 76 | scenario when you are writing a Perl program. Parts of the code you
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| 77 | will write yourself, but it's very likely that you will make use of
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| 78 | pre-written Perl modules. If you use the B<-w> flag in this case, you
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| 79 | end up enabling warnings in pieces of code that you haven't written.
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| 80 |
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| 81 | Similarly, using C<$^W> to either disable or enable blocks of code is
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| 82 | fundamentally flawed. For a start, say you want to disable warnings in
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| 83 | a block of code. You might expect this to be enough to do the trick:
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| 84 |
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| 85 | {
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| 86 | local ($^W) = 0;
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| 87 | my $a =+ 2;
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| 88 | my $b; chop $b;
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| 89 | }
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| 90 |
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| 91 | When this code is run with the B<-w> flag, a warning will be produced
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| 92 | for the C<$a> line -- C<"Reversed += operator">.
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| 93 |
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| 94 | The problem is that Perl has both compile-time and run-time warnings. To
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| 95 | disable compile-time warnings you need to rewrite the code like this:
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| 96 |
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| 97 | {
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| 98 | BEGIN { $^W = 0 }
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| 99 | my $a =+ 2;
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| 100 | my $b; chop $b;
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| 101 | }
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| 102 |
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| 103 | The other big problem with C<$^W> is the way you can inadvertently
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| 104 | change the warning setting in unexpected places in your code. For example,
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| 105 | when the code below is run (without the B<-w> flag), the second call
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| 106 | to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas
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| 107 | the first will not.
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| 108 |
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| 109 | sub doit
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| 110 | {
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| 111 | my $b; chop $b;
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| 112 | }
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| 113 |
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| 114 | doit();
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| 115 |
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| 116 | {
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| 117 | local ($^W) = 1;
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| 118 | doit()
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| 119 | }
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| 120 |
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| 121 | This is a side-effect of C<$^W> being dynamically scoped.
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| 122 |
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| 123 | Lexical warnings get around these limitations by allowing finer control
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| 124 | over where warnings can or can't be tripped.
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| 125 |
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| 126 | =head2 Controlling Warnings from the Command Line
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| 127 |
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| 128 | There are three Command Line flags that can be used to control when
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| 129 | warnings are (or aren't) produced:
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| 130 |
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| 131 | =over 5
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| 132 |
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| 133 | =item B<-w>
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| 134 | X<-w>
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| 135 |
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| 136 | This is the existing flag. If the lexical warnings pragma is B<not>
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| 137 | used in any of you code, or any of the modules that you use, this flag
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| 138 | will enable warnings everywhere. See L<Backward Compatibility> for
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| 139 | details of how this flag interacts with lexical warnings.
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| 140 |
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| 141 | =item B<-W>
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| 142 | X<-W>
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| 143 |
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| 144 | If the B<-W> flag is used on the command line, it will enable all warnings
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| 145 | throughout the program regardless of whether warnings were disabled
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| 146 | locally using C<no warnings> or C<$^W =0>. This includes all files that get
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| 147 | included via C<use>, C<require> or C<do>.
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| 148 | Think of it as the Perl equivalent of the "lint" command.
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| 149 |
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| 150 | =item B<-X>
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| 151 | X<-X>
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| 152 |
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| 153 | Does the exact opposite to the B<-W> flag, i.e. it disables all warnings.
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| 154 |
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| 155 | =back
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| 156 |
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| 157 | =head2 Backward Compatibility
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| 158 |
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| 159 | If you are used with working with a version of Perl prior to the
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| 160 | introduction of lexically scoped warnings, or have code that uses both
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| 161 | lexical warnings and C<$^W>, this section will describe how they interact.
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| 162 |
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| 163 | How Lexical Warnings interact with B<-w>/C<$^W>:
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| 164 |
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| 165 | =over 5
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| 166 |
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| 167 | =item 1.
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| 168 |
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| 169 | If none of the three command line flags (B<-w>, B<-W> or B<-X>) that
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| 170 | control warnings is used and neither C<$^W> or the C<warnings> pragma
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| 171 | are used, then default warnings will be enabled and optional warnings
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| 172 | disabled.
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| 173 | This means that legacy code that doesn't attempt to control the warnings
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| 174 | will work unchanged.
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| 175 |
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| 176 | =item 2.
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| 177 |
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| 178 | The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this
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| 179 | means that any legacy code that currently relies on manipulating C<$^W>
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| 180 | to control warning behavior will still work as is.
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| 181 |
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| 182 | =item 3.
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| 183 |
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| 184 | Apart from now being a boolean, the C<$^W> variable operates in exactly
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| 185 | the same horrible uncontrolled global way, except that it cannot
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| 186 | disable/enable default warnings.
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| 187 |
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| 188 | =item 4.
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| 189 |
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| 190 | If a piece of code is under the control of the C<warnings> pragma,
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| 191 | both the C<$^W> variable and the B<-w> flag will be ignored for the
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| 192 | scope of the lexical warning.
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| 193 |
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| 194 | =item 5.
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| 195 |
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| 196 | The only way to override a lexical warnings setting is with the B<-W>
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| 197 | or B<-X> command line flags.
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| 198 |
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| 199 | =back
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| 200 |
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| 201 | The combined effect of 3 & 4 is that it will allow code which uses
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| 202 | the C<warnings> pragma to control the warning behavior of $^W-type
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| 203 | code (using a C<local $^W=0>) if it really wants to, but not vice-versa.
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| 204 |
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| 205 | =head2 Category Hierarchy
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| 206 | X<warning, categories>
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| 207 |
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| 208 | A hierarchy of "categories" have been defined to allow groups of warnings
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| 209 | to be enabled/disabled in isolation.
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| 210 |
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| 211 | The current hierarchy is:
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| 212 |
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| 213 | all -+
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| 214 | |
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| 215 | +- closure
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| 216 | |
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| 217 | +- deprecated
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| 218 | |
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| 219 | +- exiting
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| 220 | |
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| 221 | +- glob
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| 222 | |
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| 223 | +- io -----------+
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| 224 | | |
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| 225 | | +- closed
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| 226 | | |
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| 227 | | +- exec
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| 228 | | |
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| 229 | | +- layer
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| 230 | | |
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| 231 | | +- newline
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| 232 | | |
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| 233 | | +- pipe
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| 234 | | |
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| 235 | | +- unopened
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| 236 | |
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| 237 | +- misc
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| 238 | |
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| 239 | +- numeric
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| 240 | |
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| 241 | +- once
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| 242 | |
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| 243 | +- overflow
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| 244 | |
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| 245 | +- pack
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| 246 | |
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| 247 | +- portable
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| 248 | |
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| 249 | +- recursion
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| 250 | |
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| 251 | +- redefine
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| 252 | |
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| 253 | +- regexp
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| 254 | |
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| 255 | +- severe -------+
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| 256 | | |
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| 257 | | +- debugging
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| 258 | | |
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| 259 | | +- inplace
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| 260 | | |
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| 261 | | +- internal
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| 262 | | |
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| 263 | | +- malloc
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| 264 | |
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| 265 | +- signal
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| 266 | |
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| 267 | +- substr
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| 268 | |
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| 269 | +- syntax -------+
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| 270 | | |
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| 271 | | +- ambiguous
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| 272 | | |
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| 273 | | +- bareword
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| 274 | | |
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| 275 | | +- digit
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| 276 | | |
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| 277 | | +- parenthesis
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| 278 | | |
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| 279 | | +- precedence
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| 280 | | |
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| 281 | | +- printf
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| 282 | | |
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| 283 | | +- prototype
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| 284 | | |
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| 285 | | +- qw
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| 286 | | |
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| 287 | | +- reserved
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| 288 | | |
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| 289 | | +- semicolon
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| 290 | |
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| 291 | +- taint
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| 292 | |
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| 293 | +- threads
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| 294 | |
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| 295 | +- uninitialized
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| 296 | |
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| 297 | +- unpack
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| 298 | |
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| 299 | +- untie
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| 300 | |
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| 301 | +- utf8
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| 302 | |
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| 303 | +- void
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| 304 | |
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| 305 | +- y2k
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| 306 |
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| 307 | Just like the "strict" pragma any of these categories can be combined
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| 308 |
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| 309 | use warnings qw(void redefine);
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| 310 | no warnings qw(io syntax untie);
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| 311 |
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| 312 | Also like the "strict" pragma, if there is more than one instance of the
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| 313 | C<warnings> pragma in a given scope the cumulative effect is additive.
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| 314 |
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| 315 | use warnings qw(void); # only "void" warnings enabled
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| 316 | ...
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| 317 | use warnings qw(io); # only "void" & "io" warnings enabled
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| 318 | ...
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| 319 | no warnings qw(void); # only "io" warnings enabled
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| 320 |
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| 321 | To determine which category a specific warning has been assigned to see
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| 322 | L<perldiag>.
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| 323 |
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| 324 | Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a
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| 325 | sub-category of the "syntax" category. It is now a top-level category
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| 326 | in its own right.
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| 327 |
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| 328 |
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| 329 | =head2 Fatal Warnings
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| 330 | X<warning, fatal>
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| 331 |
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| 332 | The presence of the word "FATAL" in the category list will escalate any
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| 333 | warnings detected from the categories specified in the lexical scope
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| 334 | into fatal errors. In the code below, the use of C<time>, C<length>
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| 335 | and C<join> can all produce a C<"Useless use of xxx in void context">
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| 336 | warning.
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| 337 |
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| 338 | use warnings;
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| 339 |
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| 340 | time;
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| 341 |
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| 342 | {
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| 343 | use warnings FATAL => qw(void);
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| 344 | length "abc";
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| 345 | }
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| 346 |
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| 347 | join "", 1,2,3;
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| 348 |
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| 349 | print "done\n";
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| 350 |
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| 351 | When run it produces this output
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| 352 |
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| 353 | Useless use of time in void context at fatal line 3.
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| 354 | Useless use of length in void context at fatal line 7.
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| 355 |
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| 356 | The scope where C<length> is used has escalated the C<void> warnings
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| 357 | category into a fatal error, so the program terminates immediately it
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| 358 | encounters the warning.
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| 359 |
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| 360 | To explicitly turn off a "FATAL" warning you just disable the warning
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| 361 | it is associated with. So, for example, to disable the "void" warning
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| 362 | in the example above, either of these will do the trick:
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| 363 |
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| 364 | no warnings qw(void);
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| 365 | no warnings FATAL => qw(void);
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| 366 |
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| 367 | If you want to downgrade a warning that has been escalated into a fatal
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| 368 | error back to a normal warning, you can use the "NONFATAL" keyword. For
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| 369 | example, the code below will promote all warnings into fatal errors,
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| 370 | except for those in the "syntax" category.
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| 371 |
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| 372 | use warnings FATAL => 'all', NONFATAL => 'syntax';
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| 373 |
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| 374 | =head2 Reporting Warnings from a Module
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| 375 | X<warning, reporting> X<warning, registering>
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| 376 |
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| 377 | The C<warnings> pragma provides a number of functions that are useful for
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| 378 | module authors. These are used when you want to report a module-specific
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| 379 | warning to a calling module has enabled warnings via the C<warnings>
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| 380 | pragma.
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| 381 |
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| 382 | Consider the module C<MyMod::Abc> below.
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| 383 |
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| 384 | package MyMod::Abc;
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| 385 |
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| 386 | use warnings::register;
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| 387 |
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| 388 | sub open {
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| 389 | my $path = shift;
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| 390 | if ($path !~ m#^/#) {
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| 391 | warnings::warn("changing relative path to /var/abc")
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| 392 | if warnings::enabled();
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| 393 | $path = "/var/abc/$path";
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| 394 | }
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| 395 | }
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| 396 |
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| 397 | 1;
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| 398 |
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| 399 | The call to C<warnings::register> will create a new warnings category
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| 400 | called "MyMod::abc", i.e. the new category name matches the current
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| 401 | package name. The C<open> function in the module will display a warning
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| 402 | message if it gets given a relative path as a parameter. This warnings
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| 403 | will only be displayed if the code that uses C<MyMod::Abc> has actually
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| 404 | enabled them with the C<warnings> pragma like below.
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| 405 |
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| 406 | use MyMod::Abc;
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| 407 | use warnings 'MyMod::Abc';
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| 408 | ...
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| 409 | abc::open("../fred.txt");
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| 410 |
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| 411 | It is also possible to test whether the pre-defined warnings categories are
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| 412 | set in the calling module with the C<warnings::enabled> function. Consider
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| 413 | this snippet of code:
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| 414 |
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| 415 | package MyMod::Abc;
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| 416 |
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| 417 | sub open {
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| 418 | warnings::warnif("deprecated",
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| 419 | "open is deprecated, use new instead");
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| 420 | new(@_);
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| 421 | }
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| 422 |
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| 423 | sub new
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| 424 | ...
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| 425 | 1;
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| 426 |
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| 427 | The function C<open> has been deprecated, so code has been included to
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| 428 | display a warning message whenever the calling module has (at least) the
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| 429 | "deprecated" warnings category enabled. Something like this, say.
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| 430 |
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| 431 | use warnings 'deprecated';
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| 432 | use MyMod::Abc;
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| 433 | ...
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| 434 | MyMod::Abc::open($filename);
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| 435 |
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| 436 | Either the C<warnings::warn> or C<warnings::warnif> function should be
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| 437 | used to actually display the warnings message. This is because they can
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| 438 | make use of the feature that allows warnings to be escalated into fatal
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| 439 | errors. So in this case
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| 440 |
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| 441 | use MyMod::Abc;
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| 442 | use warnings FATAL => 'MyMod::Abc';
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| 443 | ...
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| 444 | MyMod::Abc::open('../fred.txt');
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| 445 |
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| 446 | the C<warnings::warnif> function will detect this and die after
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| 447 | displaying the warning message.
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| 448 |
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| 449 | The three warnings functions, C<warnings::warn>, C<warnings::warnif>
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| 450 | and C<warnings::enabled> can optionally take an object reference in place
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| 451 | of a category name. In this case the functions will use the class name
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| 452 | of the object as the warnings category.
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| 453 |
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| 454 | Consider this example:
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| 455 |
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| 456 | package Original;
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| 457 |
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| 458 | no warnings;
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| 459 | use warnings::register;
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| 460 |
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| 461 | sub new
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| 462 | {
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| 463 | my $class = shift;
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| 464 | bless [], $class;
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| 465 | }
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| 466 |
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| 467 | sub check
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| 468 | {
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| 469 | my $self = shift;
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| 470 | my $value = shift;
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| 471 |
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| 472 | if ($value % 2 && warnings::enabled($self))
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| 473 | { warnings::warn($self, "Odd numbers are unsafe") }
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| 474 | }
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| 475 |
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| 476 | sub doit
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| 477 | {
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| 478 | my $self = shift;
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| 479 | my $value = shift;
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| 480 | $self->check($value);
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| 481 | # ...
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| 482 | }
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| 483 |
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| 484 | 1;
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| 485 |
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| 486 | package Derived;
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| 487 |
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| 488 | use warnings::register;
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| 489 | use Original;
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| 490 | our @ISA = qw( Original );
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| 491 | sub new
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| 492 | {
|
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| 493 | my $class = shift;
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| 494 | bless [], $class;
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| 495 | }
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| 496 |
|
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| 497 |
|
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| 498 | 1;
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| 499 |
|
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| 500 | The code below makes use of both modules, but it only enables warnings from
|
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| 501 | C<Derived>.
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| 502 |
|
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| 503 | use Original;
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| 504 | use Derived;
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| 505 | use warnings 'Derived';
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| 506 | my $a = new Original;
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| 507 | $a->doit(1);
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| 508 | my $b = new Derived;
|
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| 509 | $a->doit(1);
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| 510 |
|
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| 511 | When this code is run only the C<Derived> object, C<$b>, will generate
|
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| 512 | a warning.
|
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| 513 |
|
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| 514 | Odd numbers are unsafe at main.pl line 7
|
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| 515 |
|
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| 516 | Notice also that the warning is reported at the line where the object is first
|
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| 517 | used.
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| 518 |
|
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| 519 | =head1 TODO
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| 520 |
|
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| 521 | perl5db.pl
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| 522 | The debugger saves and restores C<$^W> at runtime. I haven't checked
|
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| 523 | whether the debugger will still work with the lexical warnings
|
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| 524 | patch applied.
|
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| 525 |
|
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| 526 | diagnostics.pm
|
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| 527 | I *think* I've got diagnostics to work with the lexical warnings
|
---|
| 528 | patch, but there were design decisions made in diagnostics to work
|
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| 529 | around the limitations of C<$^W>. Now that those limitations are gone,
|
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| 530 | the module should be revisited.
|
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| 531 |
|
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| 532 | document calling the warnings::* functions from XS
|
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| 533 |
|
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| 534 | =head1 SEE ALSO
|
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| 535 |
|
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| 536 | L<warnings>, L<perldiag>.
|
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| 537 |
|
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| 538 | =head1 AUTHOR
|
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| 539 |
|
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| 540 | Paul Marquess
|
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