1 | =head1 NAME
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2 |
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3 | perlfaq7 - General Perl Language Issues ($Revision: 1.28 $, $Date: 2005/12/31 00:54:37 $)
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4 |
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5 | =head1 DESCRIPTION
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6 |
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7 | This section deals with general Perl language issues that don't
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8 | clearly fit into any of the other sections.
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9 |
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10 | =head2 Can I get a BNF/yacc/RE for the Perl language?
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11 |
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12 | There is no BNF, but you can paw your way through the yacc grammar in
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13 | perly.y in the source distribution if you're particularly brave. The
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14 | grammar relies on very smart tokenizing code, so be prepared to
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15 | venture into toke.c as well.
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16 |
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17 | In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF.
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18 | The work of parsing perl is distributed between yacc, the lexer, smoke
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19 | and mirrors."
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20 |
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21 | =head2 What are all these $@%&* punctuation signs, and how do I know when to use them?
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22 |
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23 | They are type specifiers, as detailed in L<perldata>:
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24 |
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25 | $ for scalar values (number, string or reference)
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26 | @ for arrays
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27 | % for hashes (associative arrays)
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28 | & for subroutines (aka functions, procedures, methods)
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29 | * for all types of that symbol name. In version 4 you used them like
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30 | pointers, but in modern perls you can just use references.
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31 |
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32 | There are couple of other symbols that you're likely to encounter that aren't
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33 | really type specifiers:
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34 |
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35 | <> are used for inputting a record from a filehandle.
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36 | \ takes a reference to something.
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37 |
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38 | Note that <FILE> is I<neither> the type specifier for files
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39 | nor the name of the handle. It is the C<< <> >> operator applied
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40 | to the handle FILE. It reads one line (well, record--see
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41 | L<perlvar/$E<sol>>) from the handle FILE in scalar context, or I<all> lines
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42 | in list context. When performing open, close, or any other operation
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43 | besides C<< <> >> on files, or even when talking about the handle, do
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44 | I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0,
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45 | 2)> and "copying from STDIN to FILE".
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46 |
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47 | =head2 Do I always/never have to quote my strings or use semicolons and commas?
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48 |
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49 | Normally, a bareword doesn't need to be quoted, but in most cases
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50 | probably should be (and must be under C<use strict>). But a hash key
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51 | consisting of a simple word (that isn't the name of a defined
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52 | subroutine) and the left-hand operand to the C<< => >> operator both
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53 | count as though they were quoted:
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54 |
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55 | This is like this
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56 | ------------ ---------------
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57 | $foo{line} $foo{'line'}
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58 | bar => stuff 'bar' => stuff
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59 |
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60 | The final semicolon in a block is optional, as is the final comma in a
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61 | list. Good style (see L<perlstyle>) says to put them in except for
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62 | one-liners:
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63 |
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64 | if ($whoops) { exit 1 }
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65 | @nums = (1, 2, 3);
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66 |
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67 | if ($whoops) {
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68 | exit 1;
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69 | }
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70 | @lines = (
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71 | "There Beren came from mountains cold",
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72 | "And lost he wandered under leaves",
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73 | );
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74 |
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75 | =head2 How do I skip some return values?
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76 |
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77 | One way is to treat the return values as a list and index into it:
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78 |
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79 | $dir = (getpwnam($user))[7];
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80 |
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81 | Another way is to use undef as an element on the left-hand-side:
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82 |
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83 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
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84 |
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85 | You can also use a list slice to select only the elements that
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86 | you need:
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87 |
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88 | ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
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89 |
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90 | =head2 How do I temporarily block warnings?
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91 |
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92 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma
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93 | allows fine control of what warning are produced.
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94 | See L<perllexwarn> for more details.
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95 |
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96 | {
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97 | no warnings; # temporarily turn off warnings
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98 | $a = $b + $c; # I know these might be undef
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99 | }
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100 |
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101 | Additionally, you can enable and disable categories of warnings.
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102 | You turn off the categories you want to ignore and you can still
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103 | get other categories of warnings. See L<perllexwarn> for the
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104 | complete details, including the category names and hierarchy.
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105 |
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106 | {
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107 | no warnings 'uninitialized';
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108 | $a = $b + $c;
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109 | }
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110 |
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111 | If you have an older version of Perl, the C<$^W> variable (documented
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112 | in L<perlvar>) controls runtime warnings for a block:
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113 |
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114 | {
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115 | local $^W = 0; # temporarily turn off warnings
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116 | $a = $b + $c; # I know these might be undef
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117 | }
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118 |
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119 | Note that like all the punctuation variables, you cannot currently
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120 | use my() on C<$^W>, only local().
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121 |
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122 | =head2 What's an extension?
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123 |
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124 | An extension is a way of calling compiled C code from Perl. Reading
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125 | L<perlxstut> is a good place to learn more about extensions.
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126 |
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127 | =head2 Why do Perl operators have different precedence than C operators?
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128 |
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129 | Actually, they don't. All C operators that Perl copies have the same
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130 | precedence in Perl as they do in C. The problem is with operators that C
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131 | doesn't have, especially functions that give a list context to everything
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132 | on their right, eg. print, chmod, exec, and so on. Such functions are
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133 | called "list operators" and appear as such in the precedence table in
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134 | L<perlop>.
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135 |
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136 | A common mistake is to write:
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137 |
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138 | unlink $file || die "snafu";
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139 |
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140 | This gets interpreted as:
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141 |
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142 | unlink ($file || die "snafu");
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143 |
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144 | To avoid this problem, either put in extra parentheses or use the
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145 | super low precedence C<or> operator:
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146 |
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147 | (unlink $file) || die "snafu";
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148 | unlink $file or die "snafu";
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149 |
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150 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>)
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151 | deliberately have precedence lower than that of list operators for
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152 | just such situations as the one above.
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153 |
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154 | Another operator with surprising precedence is exponentiation. It
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155 | binds more tightly even than unary minus, making C<-2**2> product a
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156 | negative not a positive four. It is also right-associating, meaning
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157 | that C<2**3**2> is two raised to the ninth power, not eight squared.
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158 |
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159 | Although it has the same precedence as in C, Perl's C<?:> operator
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160 | produces an lvalue. This assigns $x to either $a or $b, depending
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161 | on the trueness of $maybe:
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162 |
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163 | ($maybe ? $a : $b) = $x;
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164 |
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165 | =head2 How do I declare/create a structure?
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166 |
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167 | In general, you don't "declare" a structure. Just use a (probably
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168 | anonymous) hash reference. See L<perlref> and L<perldsc> for details.
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169 | Here's an example:
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170 |
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171 | $person = {}; # new anonymous hash
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172 | $person->{AGE} = 24; # set field AGE to 24
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173 | $person->{NAME} = "Nat"; # set field NAME to "Nat"
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174 |
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175 | If you're looking for something a bit more rigorous, try L<perltoot>.
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176 |
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177 | =head2 How do I create a module?
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178 |
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179 | (contributed by brian d foy)
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180 |
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181 | L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules
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182 | in all the gory details. L<perlnewmod> gives a brief
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183 | overview of the process along with a couple of suggestions
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184 | about style.
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185 |
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186 | If you need to include C code or C library interfaces in
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187 | your module, you'll need h2xs. h2xs will create the module
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188 | distribution structure and the initial interface files
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189 | you'll need. L<perlxs> and L<perlxstut> explain the details.
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190 |
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191 | If you don't need to use C code, other tools such as
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192 | ExtUtils::ModuleMaker and Module::Starter, can help you
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193 | create a skeleton module distribution.
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194 |
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195 | You may also want to see Sam Tregar's "Writing Perl Modules
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196 | for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
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197 | which is the best hands-on guide to creating module
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198 | distributions.
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199 |
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200 | =head2 How do I create a class?
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201 |
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202 | See L<perltoot> for an introduction to classes and objects, as well as
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203 | L<perlobj> and L<perlbot>.
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204 |
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205 | =head2 How can I tell if a variable is tainted?
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206 |
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207 | You can use the tainted() function of the Scalar::Util module, available
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208 | from CPAN (or included with Perl since release 5.8.0).
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209 | See also L<perlsec/"Laundering and Detecting Tainted Data">.
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210 |
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211 | =head2 What's a closure?
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212 |
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213 | Closures are documented in L<perlref>.
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214 |
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215 | I<Closure> is a computer science term with a precise but
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216 | hard-to-explain meaning. Closures are implemented in Perl as anonymous
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217 | subroutines with lasting references to lexical variables outside their
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218 | own scopes. These lexicals magically refer to the variables that were
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219 | around when the subroutine was defined (deep binding).
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220 |
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221 | Closures make sense in any programming language where you can have the
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222 | return value of a function be itself a function, as you can in Perl.
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223 | Note that some languages provide anonymous functions but are not
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224 | capable of providing proper closures: the Python language, for
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225 | example. For more information on closures, check out any textbook on
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226 | functional programming. Scheme is a language that not only supports
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227 | but encourages closures.
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228 |
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229 | Here's a classic function-generating function:
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230 |
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231 | sub add_function_generator {
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232 | return sub { shift() + shift() };
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233 | }
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234 |
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235 | $add_sub = add_function_generator();
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236 | $sum = $add_sub->(4,5); # $sum is 9 now.
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237 |
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238 | The closure works as a I<function template> with some customization
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239 | slots left out to be filled later. The anonymous subroutine returned
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240 | by add_function_generator() isn't technically a closure because it
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241 | refers to no lexicals outside its own scope.
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242 |
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243 | Contrast this with the following make_adder() function, in which the
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244 | returned anonymous function contains a reference to a lexical variable
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245 | outside the scope of that function itself. Such a reference requires
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246 | that Perl return a proper closure, thus locking in for all time the
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247 | value that the lexical had when the function was created.
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248 |
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249 | sub make_adder {
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250 | my $addpiece = shift;
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251 | return sub { shift() + $addpiece };
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252 | }
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253 |
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254 | $f1 = make_adder(20);
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255 | $f2 = make_adder(555);
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256 |
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257 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas
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258 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece
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259 | in the closure sticks around.
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260 |
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261 | Closures are often used for less esoteric purposes. For example, when
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262 | you want to pass in a bit of code into a function:
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263 |
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264 | my $line;
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265 | timeout( 30, sub { $line = <STDIN> } );
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266 |
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267 | If the code to execute had been passed in as a string,
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268 | C<< '$line = <STDIN>' >>, there would have been no way for the
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269 | hypothetical timeout() function to access the lexical variable
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270 | $line back in its caller's scope.
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271 |
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272 | =head2 What is variable suicide and how can I prevent it?
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273 |
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274 | This problem was fixed in perl 5.004_05, so preventing it means upgrading
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275 | your version of perl. ;)
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276 |
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277 | Variable suicide is when you (temporarily or permanently) lose the value
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278 | of a variable. It is caused by scoping through my() and local()
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279 | interacting with either closures or aliased foreach() iterator variables
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280 | and subroutine arguments. It used to be easy to inadvertently lose a
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281 | variable's value this way, but now it's much harder. Take this code:
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282 |
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283 | my $f = 'foo';
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284 | sub T {
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285 | while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
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286 | }
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287 | T;
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288 | print "Finally $f\n";
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289 |
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290 | If you are experiencing variable suicide, that C<my $f> in the subroutine
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291 | doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output
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292 | shows that inside the subroutine the value of C<$f> leaks through when it
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293 | shouldn't, as in this output:
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294 |
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295 | foobar
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296 | foobarbar
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297 | foobarbarbar
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298 | Finally foo
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299 |
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300 | The $f that has "bar" added to it three times should be a new C<$f>
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301 | C<my $f> should create a new lexical variable each time through the loop.
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302 | The expected output is:
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303 |
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304 | foobar
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305 | foobar
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306 | foobar
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307 | Finally foo
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308 |
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309 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?
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310 |
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311 | With the exception of regexes, you need to pass references to these
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312 | objects. See L<perlsub/"Pass by Reference"> for this particular
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313 | question, and L<perlref> for information on references.
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314 |
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315 | See "Passing Regexes", below, for information on passing regular
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316 | expressions.
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317 |
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318 | =over 4
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319 |
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320 | =item Passing Variables and Functions
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321 |
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322 | Regular variables and functions are quite easy to pass: just pass in a
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323 | reference to an existing or anonymous variable or function:
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324 |
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325 | func( \$some_scalar );
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326 |
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327 | func( \@some_array );
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328 | func( [ 1 .. 10 ] );
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329 |
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330 | func( \%some_hash );
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331 | func( { this => 10, that => 20 } );
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332 |
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333 | func( \&some_func );
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334 | func( sub { $_[0] ** $_[1] } );
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335 |
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336 | =item Passing Filehandles
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337 |
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338 | As of Perl 5.6, you can represent filehandles with scalar variables
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339 | which you treat as any other scalar.
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340 |
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341 | open my $fh, $filename or die "Cannot open $filename! $!";
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342 | func( $fh );
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343 |
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344 | sub func {
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345 | my $passed_fh = shift;
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346 |
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347 | my $line = <$fh>;
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348 | }
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349 |
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350 | Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations.
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351 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles">
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352 | and especially L<perlsub/"Pass by Reference"> for more information.
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353 |
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354 | =item Passing Regexes
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355 |
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356 | To pass regexes around, you'll need to be using a release of Perl
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357 | sufficiently recent as to support the C<qr//> construct, pass around
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358 | strings and use an exception-trapping eval, or else be very, very clever.
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359 |
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360 | Here's an example of how to pass in a string to be regex compared
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361 | using C<qr//>:
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362 |
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363 | sub compare($$) {
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364 | my ($val1, $regex) = @_;
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365 | my $retval = $val1 =~ /$regex/;
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366 | return $retval;
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367 | }
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368 | $match = compare("old McDonald", qr/d.*D/i);
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369 |
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370 | Notice how C<qr//> allows flags at the end. That pattern was compiled
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371 | at compile time, although it was executed later. The nifty C<qr//>
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372 | notation wasn't introduced until the 5.005 release. Before that, you
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373 | had to approach this problem much less intuitively. For example, here
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374 | it is again if you don't have C<qr//>:
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375 |
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376 | sub compare($$) {
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377 | my ($val1, $regex) = @_;
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378 | my $retval = eval { $val1 =~ /$regex/ };
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379 | die if $@;
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380 | return $retval;
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381 | }
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382 |
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383 | $match = compare("old McDonald", q/($?i)d.*D/);
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384 |
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385 | Make sure you never say something like this:
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386 |
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387 | return eval "\$val =~ /$regex/"; # WRONG
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388 |
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389 | or someone can sneak shell escapes into the regex due to the double
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390 | interpolation of the eval and the double-quoted string. For example:
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391 |
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392 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
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393 |
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394 | eval "\$string =~ /$pattern_of_evil/";
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395 |
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396 | Those preferring to be very, very clever might see the O'Reilly book,
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397 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's
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398 | Build_MatchMany_Function() is particularly interesting. A complete
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399 | citation of this book is given in L<perlfaq2>.
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400 |
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401 | =item Passing Methods
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402 |
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403 | To pass an object method into a subroutine, you can do this:
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404 |
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405 | call_a_lot(10, $some_obj, "methname")
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406 | sub call_a_lot {
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407 | my ($count, $widget, $trick) = @_;
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408 | for (my $i = 0; $i < $count; $i++) {
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409 | $widget->$trick();
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410 | }
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411 | }
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412 |
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413 | Or, you can use a closure to bundle up the object, its
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414 | method call, and arguments:
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415 |
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416 | my $whatnot = sub { $some_obj->obfuscate(@args) };
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417 | func($whatnot);
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418 | sub func {
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419 | my $code = shift;
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420 | &$code();
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421 | }
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422 |
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423 | You could also investigate the can() method in the UNIVERSAL class
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424 | (part of the standard perl distribution).
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425 |
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426 | =back
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427 |
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428 | =head2 How do I create a static variable?
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429 |
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430 | (contributed by brian d foy)
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431 |
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432 | Perl doesn't have "static" variables, which can only be accessed from
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433 | the function in which they are declared. You can get the same effect
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434 | with lexical variables, though.
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435 |
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436 | You can fake a static variable by using a lexical variable which goes
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437 | out of scope. In this example, you define the subroutine C<counter>, and
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438 | it uses the lexical variable C<$count>. Since you wrap this in a BEGIN
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439 | block, C<$count> is defined at compile-time, but also goes out of
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440 | scope at the end of the BEGIN block. The BEGIN block also ensures that
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441 | the subroutine and the value it uses is defined at compile-time so the
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442 | subroutine is ready to use just like any other subroutine, and you can
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443 | put this code in the same place as other subroutines in the program
|
---|
444 | text (i.e. at the end of the code, typically). The subroutine
|
---|
445 | C<counter> still has a reference to the data, and is the only way you
|
---|
446 | can access the value (and each time you do, you increment the value).
|
---|
447 | The data in chunk of memory defined by C<$count> is private to
|
---|
448 | C<counter>.
|
---|
449 |
|
---|
450 | BEGIN {
|
---|
451 | my $count = 1;
|
---|
452 | sub counter { $count++ }
|
---|
453 | }
|
---|
454 |
|
---|
455 | my $start = count();
|
---|
456 |
|
---|
457 | .... # code that calls count();
|
---|
458 |
|
---|
459 | my $end = count();
|
---|
460 |
|
---|
461 | In the previous example, you created a function-private variable
|
---|
462 | because only one function remembered its reference. You could define
|
---|
463 | multiple functions while the variable is in scope, and each function
|
---|
464 | can share the "private" variable. It's not really "static" because you
|
---|
465 | can access it outside the function while the lexical variable is in
|
---|
466 | scope, and even create references to it. In this example,
|
---|
467 | C<increment_count> and C<return_count> share the variable. One
|
---|
468 | function adds to the value and the other simply returns the value.
|
---|
469 | They can both access C<$count>, and since it has gone out of scope,
|
---|
470 | there is no other way to access it.
|
---|
471 |
|
---|
472 | BEGIN {
|
---|
473 | my $count = 1;
|
---|
474 | sub increment_count { $count++ }
|
---|
475 | sub return_count { $count }
|
---|
476 | }
|
---|
477 |
|
---|
478 | To declare a file-private variable, you still use a lexical variable.
|
---|
479 | A file is also a scope, so a lexical variable defined in the file
|
---|
480 | cannot be seen from any other file.
|
---|
481 |
|
---|
482 | See L<perlsub/"Persistent Private Variables"> for more information.
|
---|
483 | The discussion of closures in L<perlref> may help you even though we
|
---|
484 | did not use anonymous subroutines in this answer. See
|
---|
485 | L<perlsub/"Persistent Private Variables"> for details.
|
---|
486 |
|
---|
487 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()?
|
---|
488 |
|
---|
489 | C<local($x)> saves away the old value of the global variable C<$x>
|
---|
490 | and assigns a new value for the duration of the subroutine I<which is
|
---|
491 | visible in other functions called from that subroutine>. This is done
|
---|
492 | at run-time, so is called dynamic scoping. local() always affects global
|
---|
493 | variables, also called package variables or dynamic variables.
|
---|
494 |
|
---|
495 | C<my($x)> creates a new variable that is only visible in the current
|
---|
496 | subroutine. This is done at compile-time, so it is called lexical or
|
---|
497 | static scoping. my() always affects private variables, also called
|
---|
498 | lexical variables or (improperly) static(ly scoped) variables.
|
---|
499 |
|
---|
500 | For instance:
|
---|
501 |
|
---|
502 | sub visible {
|
---|
503 | print "var has value $var\n";
|
---|
504 | }
|
---|
505 |
|
---|
506 | sub dynamic {
|
---|
507 | local $var = 'local'; # new temporary value for the still-global
|
---|
508 | visible(); # variable called $var
|
---|
509 | }
|
---|
510 |
|
---|
511 | sub lexical {
|
---|
512 | my $var = 'private'; # new private variable, $var
|
---|
513 | visible(); # (invisible outside of sub scope)
|
---|
514 | }
|
---|
515 |
|
---|
516 | $var = 'global';
|
---|
517 |
|
---|
518 | visible(); # prints global
|
---|
519 | dynamic(); # prints local
|
---|
520 | lexical(); # prints global
|
---|
521 |
|
---|
522 | Notice how at no point does the value "private" get printed. That's
|
---|
523 | because $var only has that value within the block of the lexical()
|
---|
524 | function, and it is hidden from called subroutine.
|
---|
525 |
|
---|
526 | In summary, local() doesn't make what you think of as private, local
|
---|
527 | variables. It gives a global variable a temporary value. my() is
|
---|
528 | what you're looking for if you want private variables.
|
---|
529 |
|
---|
530 | See L<perlsub/"Private Variables via my()"> and
|
---|
531 | L<perlsub/"Temporary Values via local()"> for excruciating details.
|
---|
532 |
|
---|
533 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope?
|
---|
534 |
|
---|
535 | If you know your package, you can just mention it explicitly, as in
|
---|
536 | $Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var
|
---|
537 | in the current package, but rather the one in the "main" package, as
|
---|
538 | though you had written $main::var.
|
---|
539 |
|
---|
540 | use vars '$var';
|
---|
541 | local $var = "global";
|
---|
542 | my $var = "lexical";
|
---|
543 |
|
---|
544 | print "lexical is $var\n";
|
---|
545 | print "global is $main::var\n";
|
---|
546 |
|
---|
547 | Alternatively you can use the compiler directive our() to bring a
|
---|
548 | dynamic variable into the current lexical scope.
|
---|
549 |
|
---|
550 | require 5.006; # our() did not exist before 5.6
|
---|
551 | use vars '$var';
|
---|
552 |
|
---|
553 | local $var = "global";
|
---|
554 | my $var = "lexical";
|
---|
555 |
|
---|
556 | print "lexical is $var\n";
|
---|
557 |
|
---|
558 | {
|
---|
559 | our $var;
|
---|
560 | print "global is $var\n";
|
---|
561 | }
|
---|
562 |
|
---|
563 | =head2 What's the difference between deep and shallow binding?
|
---|
564 |
|
---|
565 | In deep binding, lexical variables mentioned in anonymous subroutines
|
---|
566 | are the same ones that were in scope when the subroutine was created.
|
---|
567 | In shallow binding, they are whichever variables with the same names
|
---|
568 | happen to be in scope when the subroutine is called. Perl always uses
|
---|
569 | deep binding of lexical variables (i.e., those created with my()).
|
---|
570 | However, dynamic variables (aka global, local, or package variables)
|
---|
571 | are effectively shallowly bound. Consider this just one more reason
|
---|
572 | not to use them. See the answer to L<"What's a closure?">.
|
---|
573 |
|
---|
574 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right?
|
---|
575 |
|
---|
576 | C<my()> and C<local()> give list context to the right hand side
|
---|
577 | of C<=>. The <FH> read operation, like so many of Perl's
|
---|
578 | functions and operators, can tell which context it was called in and
|
---|
579 | behaves appropriately. In general, the scalar() function can help.
|
---|
580 | This function does nothing to the data itself (contrary to popular myth)
|
---|
581 | but rather tells its argument to behave in whatever its scalar fashion is.
|
---|
582 | If that function doesn't have a defined scalar behavior, this of course
|
---|
583 | doesn't help you (such as with sort()).
|
---|
584 |
|
---|
585 | To enforce scalar context in this particular case, however, you need
|
---|
586 | merely omit the parentheses:
|
---|
587 |
|
---|
588 | local($foo) = <FILE>; # WRONG
|
---|
589 | local($foo) = scalar(<FILE>); # ok
|
---|
590 | local $foo = <FILE>; # right
|
---|
591 |
|
---|
592 | You should probably be using lexical variables anyway, although the
|
---|
593 | issue is the same here:
|
---|
594 |
|
---|
595 | my($foo) = <FILE>; # WRONG
|
---|
596 | my $foo = <FILE>; # right
|
---|
597 |
|
---|
598 | =head2 How do I redefine a builtin function, operator, or method?
|
---|
599 |
|
---|
600 | Why do you want to do that? :-)
|
---|
601 |
|
---|
602 | If you want to override a predefined function, such as open(),
|
---|
603 | then you'll have to import the new definition from a different
|
---|
604 | module. See L<perlsub/"Overriding Built-in Functions">. There's
|
---|
605 | also an example in L<perltoot/"Class::Template">.
|
---|
606 |
|
---|
607 | If you want to overload a Perl operator, such as C<+> or C<**>,
|
---|
608 | then you'll want to use the C<use overload> pragma, documented
|
---|
609 | in L<overload>.
|
---|
610 |
|
---|
611 | If you're talking about obscuring method calls in parent classes,
|
---|
612 | see L<perltoot/"Overridden Methods">.
|
---|
613 |
|
---|
614 | =head2 What's the difference between calling a function as &foo and foo()?
|
---|
615 |
|
---|
616 | When you call a function as C<&foo>, you allow that function access to
|
---|
617 | your current @_ values, and you bypass prototypes.
|
---|
618 | The function doesn't get an empty @_--it gets yours! While not
|
---|
619 | strictly speaking a bug (it's documented that way in L<perlsub>), it
|
---|
620 | would be hard to consider this a feature in most cases.
|
---|
621 |
|
---|
622 | When you call your function as C<&foo()>, then you I<do> get a new @_,
|
---|
623 | but prototyping is still circumvented.
|
---|
624 |
|
---|
625 | Normally, you want to call a function using C<foo()>. You may only
|
---|
626 | omit the parentheses if the function is already known to the compiler
|
---|
627 | because it already saw the definition (C<use> but not C<require>),
|
---|
628 | or via a forward reference or C<use subs> declaration. Even in this
|
---|
629 | case, you get a clean @_ without any of the old values leaking through
|
---|
630 | where they don't belong.
|
---|
631 |
|
---|
632 | =head2 How do I create a switch or case statement?
|
---|
633 |
|
---|
634 | This is explained in more depth in the L<perlsyn>. Briefly, there's
|
---|
635 | no official case statement, because of the variety of tests possible
|
---|
636 | in Perl (numeric comparison, string comparison, glob comparison,
|
---|
637 | regex matching, overloaded comparisons, ...).
|
---|
638 | Larry couldn't decide how best to do this, so he left it out, even
|
---|
639 | though it's been on the wish list since perl1.
|
---|
640 |
|
---|
641 | Starting from Perl 5.8 to get switch and case one can use the
|
---|
642 | Switch extension and say:
|
---|
643 |
|
---|
644 | use Switch;
|
---|
645 |
|
---|
646 | after which one has switch and case. It is not as fast as it could be
|
---|
647 | because it's not really part of the language (it's done using source
|
---|
648 | filters) but it is available, and it's very flexible.
|
---|
649 |
|
---|
650 | But if one wants to use pure Perl, the general answer is to write a
|
---|
651 | construct like this:
|
---|
652 |
|
---|
653 | for ($variable_to_test) {
|
---|
654 | if (/pat1/) { } # do something
|
---|
655 | elsif (/pat2/) { } # do something else
|
---|
656 | elsif (/pat3/) { } # do something else
|
---|
657 | else { } # default
|
---|
658 | }
|
---|
659 |
|
---|
660 | Here's a simple example of a switch based on pattern matching, this
|
---|
661 | time lined up in a way to make it look more like a switch statement.
|
---|
662 | We'll do a multiway conditional based on the type of reference stored
|
---|
663 | in $whatchamacallit:
|
---|
664 |
|
---|
665 | SWITCH: for (ref $whatchamacallit) {
|
---|
666 |
|
---|
667 | /^$/ && die "not a reference";
|
---|
668 |
|
---|
669 | /SCALAR/ && do {
|
---|
670 | print_scalar($$ref);
|
---|
671 | last SWITCH;
|
---|
672 | };
|
---|
673 |
|
---|
674 | /ARRAY/ && do {
|
---|
675 | print_array(@$ref);
|
---|
676 | last SWITCH;
|
---|
677 | };
|
---|
678 |
|
---|
679 | /HASH/ && do {
|
---|
680 | print_hash(%$ref);
|
---|
681 | last SWITCH;
|
---|
682 | };
|
---|
683 |
|
---|
684 | /CODE/ && do {
|
---|
685 | warn "can't print function ref";
|
---|
686 | last SWITCH;
|
---|
687 | };
|
---|
688 |
|
---|
689 | # DEFAULT
|
---|
690 |
|
---|
691 | warn "User defined type skipped";
|
---|
692 |
|
---|
693 | }
|
---|
694 |
|
---|
695 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other
|
---|
696 | examples in this style.
|
---|
697 |
|
---|
698 | Sometimes you should change the positions of the constant and the variable.
|
---|
699 | For example, let's say you wanted to test which of many answers you were
|
---|
700 | given, but in a case-insensitive way that also allows abbreviations.
|
---|
701 | You can use the following technique if the strings all start with
|
---|
702 | different characters or if you want to arrange the matches so that
|
---|
703 | one takes precedence over another, as C<"SEND"> has precedence over
|
---|
704 | C<"STOP"> here:
|
---|
705 |
|
---|
706 | chomp($answer = <>);
|
---|
707 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
|
---|
708 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
|
---|
709 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
|
---|
710 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
|
---|
711 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
|
---|
712 |
|
---|
713 | A totally different approach is to create a hash of function references.
|
---|
714 |
|
---|
715 | my %commands = (
|
---|
716 | "happy" => \&joy,
|
---|
717 | "sad", => \&sullen,
|
---|
718 | "done" => sub { die "See ya!" },
|
---|
719 | "mad" => \&angry,
|
---|
720 | );
|
---|
721 |
|
---|
722 | print "How are you? ";
|
---|
723 | chomp($string = <STDIN>);
|
---|
724 | if ($commands{$string}) {
|
---|
725 | $commands{$string}->();
|
---|
726 | } else {
|
---|
727 | print "No such command: $string\n";
|
---|
728 | }
|
---|
729 |
|
---|
730 | =head2 How can I catch accesses to undefined variables, functions, or methods?
|
---|
731 |
|
---|
732 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and
|
---|
733 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to
|
---|
734 | undefined functions and methods.
|
---|
735 |
|
---|
736 | When it comes to undefined variables that would trigger a warning
|
---|
737 | under C<use warnings>, you can promote the warning to an error.
|
---|
738 |
|
---|
739 | use warnings FATAL => qw(uninitialized);
|
---|
740 |
|
---|
741 | =head2 Why can't a method included in this same file be found?
|
---|
742 |
|
---|
743 | Some possible reasons: your inheritance is getting confused, you've
|
---|
744 | misspelled the method name, or the object is of the wrong type. Check
|
---|
745 | out L<perltoot> for details about any of the above cases. You may
|
---|
746 | also use C<print ref($object)> to find out the class C<$object> was
|
---|
747 | blessed into.
|
---|
748 |
|
---|
749 | Another possible reason for problems is because you've used the
|
---|
750 | indirect object syntax (eg, C<find Guru "Samy">) on a class name
|
---|
751 | before Perl has seen that such a package exists. It's wisest to make
|
---|
752 | sure your packages are all defined before you start using them, which
|
---|
753 | will be taken care of if you use the C<use> statement instead of
|
---|
754 | C<require>. If not, make sure to use arrow notation (eg.,
|
---|
755 | C<< Guru->find("Samy") >>) instead. Object notation is explained in
|
---|
756 | L<perlobj>.
|
---|
757 |
|
---|
758 | Make sure to read about creating modules in L<perlmod> and
|
---|
759 | the perils of indirect objects in L<perlobj/"Method Invocation">.
|
---|
760 |
|
---|
761 | =head2 How can I find out my current package?
|
---|
762 |
|
---|
763 | If you're just a random program, you can do this to find
|
---|
764 | out what the currently compiled package is:
|
---|
765 |
|
---|
766 | my $packname = __PACKAGE__;
|
---|
767 |
|
---|
768 | But, if you're a method and you want to print an error message
|
---|
769 | that includes the kind of object you were called on (which is
|
---|
770 | not necessarily the same as the one in which you were compiled):
|
---|
771 |
|
---|
772 | sub amethod {
|
---|
773 | my $self = shift;
|
---|
774 | my $class = ref($self) || $self;
|
---|
775 | warn "called me from a $class object";
|
---|
776 | }
|
---|
777 |
|
---|
778 | =head2 How can I comment out a large block of perl code?
|
---|
779 |
|
---|
780 | You can use embedded POD to discard it. Enclose the blocks you want
|
---|
781 | to comment out in POD markers. The <=begin> directive marks a section
|
---|
782 | for a specific formatter. Use the C<comment> format, which no formatter
|
---|
783 | should claim to understand (by policy). Mark the end of the block
|
---|
784 | with <=end>.
|
---|
785 |
|
---|
786 | # program is here
|
---|
787 |
|
---|
788 | =begin comment
|
---|
789 |
|
---|
790 | all of this stuff
|
---|
791 |
|
---|
792 | here will be ignored
|
---|
793 | by everyone
|
---|
794 |
|
---|
795 | =end comment
|
---|
796 |
|
---|
797 | =cut
|
---|
798 |
|
---|
799 | # program continues
|
---|
800 |
|
---|
801 | The pod directives cannot go just anywhere. You must put a
|
---|
802 | pod directive where the parser is expecting a new statement,
|
---|
803 | not just in the middle of an expression or some other
|
---|
804 | arbitrary grammar production.
|
---|
805 |
|
---|
806 | See L<perlpod> for more details.
|
---|
807 |
|
---|
808 | =head2 How do I clear a package?
|
---|
809 |
|
---|
810 | Use this code, provided by Mark-Jason Dominus:
|
---|
811 |
|
---|
812 | sub scrub_package {
|
---|
813 | no strict 'refs';
|
---|
814 | my $pack = shift;
|
---|
815 | die "Shouldn't delete main package"
|
---|
816 | if $pack eq "" || $pack eq "main";
|
---|
817 | my $stash = *{$pack . '::'}{HASH};
|
---|
818 | my $name;
|
---|
819 | foreach $name (keys %$stash) {
|
---|
820 | my $fullname = $pack . '::' . $name;
|
---|
821 | # Get rid of everything with that name.
|
---|
822 | undef $$fullname;
|
---|
823 | undef @$fullname;
|
---|
824 | undef %$fullname;
|
---|
825 | undef &$fullname;
|
---|
826 | undef *$fullname;
|
---|
827 | }
|
---|
828 | }
|
---|
829 |
|
---|
830 | Or, if you're using a recent release of Perl, you can
|
---|
831 | just use the Symbol::delete_package() function instead.
|
---|
832 |
|
---|
833 | =head2 How can I use a variable as a variable name?
|
---|
834 |
|
---|
835 | Beginners often think they want to have a variable contain the name
|
---|
836 | of a variable.
|
---|
837 |
|
---|
838 | $fred = 23;
|
---|
839 | $varname = "fred";
|
---|
840 | ++$$varname; # $fred now 24
|
---|
841 |
|
---|
842 | This works I<sometimes>, but it is a very bad idea for two reasons.
|
---|
843 |
|
---|
844 | The first reason is that this technique I<only works on global
|
---|
845 | variables>. That means that if $fred is a lexical variable created
|
---|
846 | with my() in the above example, the code wouldn't work at all: you'd
|
---|
847 | accidentally access the global and skip right over the private lexical
|
---|
848 | altogether. Global variables are bad because they can easily collide
|
---|
849 | accidentally and in general make for non-scalable and confusing code.
|
---|
850 |
|
---|
851 | Symbolic references are forbidden under the C<use strict> pragma.
|
---|
852 | They are not true references and consequently are not reference counted
|
---|
853 | or garbage collected.
|
---|
854 |
|
---|
855 | The other reason why using a variable to hold the name of another
|
---|
856 | variable is a bad idea is that the question often stems from a lack of
|
---|
857 | understanding of Perl data structures, particularly hashes. By using
|
---|
858 | symbolic references, you are just using the package's symbol-table hash
|
---|
859 | (like C<%main::>) instead of a user-defined hash. The solution is to
|
---|
860 | use your own hash or a real reference instead.
|
---|
861 |
|
---|
862 | $USER_VARS{"fred"} = 23;
|
---|
863 | $varname = "fred";
|
---|
864 | $USER_VARS{$varname}++; # not $$varname++
|
---|
865 |
|
---|
866 | There we're using the %USER_VARS hash instead of symbolic references.
|
---|
867 | Sometimes this comes up in reading strings from the user with variable
|
---|
868 | references and wanting to expand them to the values of your perl
|
---|
869 | program's variables. This is also a bad idea because it conflates the
|
---|
870 | program-addressable namespace and the user-addressable one. Instead of
|
---|
871 | reading a string and expanding it to the actual contents of your program's
|
---|
872 | own variables:
|
---|
873 |
|
---|
874 | $str = 'this has a $fred and $barney in it';
|
---|
875 | $str =~ s/(\$\w+)/$1/eeg; # need double eval
|
---|
876 |
|
---|
877 | it would be better to keep a hash around like %USER_VARS and have
|
---|
878 | variable references actually refer to entries in that hash:
|
---|
879 |
|
---|
880 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
|
---|
881 |
|
---|
882 | That's faster, cleaner, and safer than the previous approach. Of course,
|
---|
883 | you don't need to use a dollar sign. You could use your own scheme to
|
---|
884 | make it less confusing, like bracketed percent symbols, etc.
|
---|
885 |
|
---|
886 | $str = 'this has a %fred% and %barney% in it';
|
---|
887 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
|
---|
888 |
|
---|
889 | Another reason that folks sometimes think they want a variable to
|
---|
890 | contain the name of a variable is because they don't know how to build
|
---|
891 | proper data structures using hashes. For example, let's say they
|
---|
892 | wanted two hashes in their program: %fred and %barney, and that they
|
---|
893 | wanted to use another scalar variable to refer to those by name.
|
---|
894 |
|
---|
895 | $name = "fred";
|
---|
896 | $$name{WIFE} = "wilma"; # set %fred
|
---|
897 |
|
---|
898 | $name = "barney";
|
---|
899 | $$name{WIFE} = "betty"; # set %barney
|
---|
900 |
|
---|
901 | This is still a symbolic reference, and is still saddled with the
|
---|
902 | problems enumerated above. It would be far better to write:
|
---|
903 |
|
---|
904 | $folks{"fred"}{WIFE} = "wilma";
|
---|
905 | $folks{"barney"}{WIFE} = "betty";
|
---|
906 |
|
---|
907 | And just use a multilevel hash to start with.
|
---|
908 |
|
---|
909 | The only times that you absolutely I<must> use symbolic references are
|
---|
910 | when you really must refer to the symbol table. This may be because it's
|
---|
911 | something that can't take a real reference to, such as a format name.
|
---|
912 | Doing so may also be important for method calls, since these always go
|
---|
913 | through the symbol table for resolution.
|
---|
914 |
|
---|
915 | In those cases, you would turn off C<strict 'refs'> temporarily so you
|
---|
916 | can play around with the symbol table. For example:
|
---|
917 |
|
---|
918 | @colors = qw(red blue green yellow orange purple violet);
|
---|
919 | for my $name (@colors) {
|
---|
920 | no strict 'refs'; # renege for the block
|
---|
921 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
|
---|
922 | }
|
---|
923 |
|
---|
924 | All those functions (red(), blue(), green(), etc.) appear to be separate,
|
---|
925 | but the real code in the closure actually was compiled only once.
|
---|
926 |
|
---|
927 | So, sometimes you might want to use symbolic references to directly
|
---|
928 | manipulate the symbol table. This doesn't matter for formats, handles, and
|
---|
929 | subroutines, because they are always global--you can't use my() on them.
|
---|
930 | For scalars, arrays, and hashes, though--and usually for subroutines--
|
---|
931 | you probably only want to use hard references.
|
---|
932 |
|
---|
933 | =head2 What does "bad interpreter" mean?
|
---|
934 |
|
---|
935 | (contributed by brian d foy)
|
---|
936 |
|
---|
937 | The "bad interpreter" message comes from the shell, not perl. The
|
---|
938 | actual message may vary depending on your platform, shell, and locale
|
---|
939 | settings.
|
---|
940 |
|
---|
941 | If you see "bad interpreter - no such file or directory", the first
|
---|
942 | line in your perl script (the "shebang" line) does not contain the
|
---|
943 | right path to perl (or any other program capable of running scripts).
|
---|
944 | Sometimes this happens when you move the script from one machine to
|
---|
945 | another and each machine has a different path to perl---/usr/bin/perl
|
---|
946 | versus /usr/local/bin/perl for instance. It may also indicate
|
---|
947 | that the source machine has CRLF line terminators and the
|
---|
948 | destination machine has LF only: the shell tries to find
|
---|
949 | /usr/bin/perl<CR>, but can't.
|
---|
950 |
|
---|
951 | If you see "bad interpreter: Permission denied", you need to make your
|
---|
952 | script executable.
|
---|
953 |
|
---|
954 | In either case, you should still be able to run the scripts with perl
|
---|
955 | explicitly:
|
---|
956 |
|
---|
957 | % perl script.pl
|
---|
958 |
|
---|
959 | If you get a message like "perl: command not found", perl is not in
|
---|
960 | your PATH, which might also mean that the location of perl is not
|
---|
961 | where you expect it so you need to adjust your shebang line.
|
---|
962 |
|
---|
963 | =head1 AUTHOR AND COPYRIGHT
|
---|
964 |
|
---|
965 | Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington, and
|
---|
966 | other authors as noted. All rights reserved.
|
---|
967 |
|
---|
968 | This documentation is free; you can redistribute it and/or modify it
|
---|
969 | under the same terms as Perl itself.
|
---|
970 |
|
---|
971 | Irrespective of its distribution, all code examples in this file
|
---|
972 | are hereby placed into the public domain. You are permitted and
|
---|
973 | encouraged to use this code in your own programs for fun
|
---|
974 | or for profit as you see fit. A simple comment in the code giving
|
---|
975 | credit would be courteous but is not required.
|
---|
976 |
|
---|