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