1 | =head1 NAME |
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2 | |
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3 | perlfaq7 - Perl Language Issues ($Revision: 1.1.1.2 $, $Date: 2000-04-07 20:44:21 $) |
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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 | A couple of others that you're likely to encounter that aren't |
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33 | really type specifiers are: |
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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/$/>) 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 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 | =head2 How do I temporarily block warnings? |
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86 | |
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87 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma |
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88 | allows fine control of what warning are produced. |
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89 | See L<perllexwarn> for more details. |
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90 | |
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91 | { |
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92 | no warnings; # temporarily turn off warnings |
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93 | $a = $b + $c; # I know these might be undef |
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94 | } |
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95 | |
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96 | If you have an older version of Perl, the C<$^W> variable (documented |
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97 | in L<perlvar>) controls runtime warnings for a block: |
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98 | |
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99 | { |
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100 | local $^W = 0; # temporarily turn off warnings |
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101 | $a = $b + $c; # I know these might be undef |
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102 | } |
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103 | |
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104 | Note that like all the punctuation variables, you cannot currently |
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105 | use my() on C<$^W>, only local(). |
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106 | |
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107 | =head2 What's an extension? |
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108 | |
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109 | A way of calling compiled C code from Perl. Reading L<perlxstut> |
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110 | is a good place to learn more about extensions. |
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111 | |
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112 | =head2 Why do Perl operators have different precedence than C operators? |
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113 | |
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114 | Actually, they don't. All C operators that Perl copies have the same |
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115 | precedence in Perl as they do in C. The problem is with operators that C |
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116 | doesn't have, especially functions that give a list context to everything |
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117 | on their right, eg print, chmod, exec, and so on. Such functions are |
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118 | called "list operators" and appear as such in the precedence table in |
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119 | L<perlop>. |
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120 | |
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121 | A common mistake is to write: |
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122 | |
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123 | unlink $file || die "snafu"; |
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124 | |
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125 | This gets interpreted as: |
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126 | |
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127 | unlink ($file || die "snafu"); |
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128 | |
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129 | To avoid this problem, either put in extra parentheses or use the |
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130 | super low precedence C<or> operator: |
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131 | |
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132 | (unlink $file) || die "snafu"; |
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133 | unlink $file or die "snafu"; |
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134 | |
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135 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
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136 | deliberately have precedence lower than that of list operators for |
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137 | just such situations as the one above. |
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138 | |
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139 | Another operator with surprising precedence is exponentiation. It |
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140 | binds more tightly even than unary minus, making C<-2**2> product a |
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141 | negative not a positive four. It is also right-associating, meaning |
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142 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
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143 | |
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144 | Although it has the same precedence as in C, Perl's C<?:> operator |
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145 | produces an lvalue. This assigns $x to either $a or $b, depending |
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146 | on the trueness of $maybe: |
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147 | |
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148 | ($maybe ? $a : $b) = $x; |
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149 | |
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150 | =head2 How do I declare/create a structure? |
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151 | |
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152 | In general, you don't "declare" a structure. Just use a (probably |
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153 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
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154 | Here's an example: |
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155 | |
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156 | $person = {}; # new anonymous hash |
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157 | $person->{AGE} = 24; # set field AGE to 24 |
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158 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
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159 | |
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160 | If you're looking for something a bit more rigorous, try L<perltoot>. |
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161 | |
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162 | =head2 How do I create a module? |
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163 | |
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164 | A module is a package that lives in a file of the same name. For |
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165 | example, the Hello::There module would live in Hello/There.pm. For |
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166 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If |
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167 | you're writing a C or mixed-language module with both C and Perl, then |
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168 | you should study L<perlxstut>. |
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169 | |
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170 | Here's a convenient template you might wish you use when starting your |
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171 | own module. Make sure to change the names appropriately. |
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172 | |
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173 | package Some::Module; # assumes Some/Module.pm |
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174 | |
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175 | use strict; |
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176 | use warnings; |
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177 | |
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178 | BEGIN { |
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179 | use Exporter (); |
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180 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
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181 | |
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182 | ## set the version for version checking; uncomment to use |
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183 | ## $VERSION = 1.00; |
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184 | |
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185 | # if using RCS/CVS, this next line may be preferred, |
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186 | # but beware two-digit versions. |
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187 | $VERSION = do{my@r=q$Revision: 1.1.1.2 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; |
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188 | |
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189 | @ISA = qw(Exporter); |
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190 | @EXPORT = qw(&func1 &func2 &func3); |
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191 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
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192 | |
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193 | # your exported package globals go here, |
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194 | # as well as any optionally exported functions |
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195 | @EXPORT_OK = qw($Var1 %Hashit); |
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196 | } |
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197 | our @EXPORT_OK; |
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198 | |
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199 | # non-exported package globals go here |
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200 | our @more; |
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201 | our $stuff; |
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202 | |
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203 | # initialize package globals, first exported ones |
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204 | $Var1 = ''; |
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205 | %Hashit = (); |
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206 | |
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207 | # then the others (which are still accessible as $Some::Module::stuff) |
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208 | $stuff = ''; |
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209 | @more = (); |
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210 | |
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211 | # all file-scoped lexicals must be created before |
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212 | # the functions below that use them. |
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213 | |
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214 | # file-private lexicals go here |
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215 | my $priv_var = ''; |
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216 | my %secret_hash = (); |
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217 | |
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218 | # here's a file-private function as a closure, |
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219 | # callable as &$priv_func; it cannot be prototyped. |
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220 | my $priv_func = sub { |
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221 | # stuff goes here. |
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222 | }; |
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223 | |
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224 | # make all your functions, whether exported or not; |
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225 | # remember to put something interesting in the {} stubs |
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226 | sub func1 {} # no prototype |
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227 | sub func2() {} # proto'd void |
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228 | sub func3($$) {} # proto'd to 2 scalars |
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229 | |
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230 | # this one isn't exported, but could be called! |
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231 | sub func4(\%) {} # proto'd to 1 hash ref |
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232 | |
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233 | END { } # module clean-up code here (global destructor) |
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234 | |
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235 | 1; # modules must return true |
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236 | |
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237 | The h2xs program will create stubs for all the important stuff for you: |
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238 | |
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239 | % h2xs -XA -n My::Module |
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240 | |
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241 | =head2 How do I create a class? |
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242 | |
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243 | See L<perltoot> for an introduction to classes and objects, as well as |
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244 | L<perlobj> and L<perlbot>. |
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245 | |
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246 | =head2 How can I tell if a variable is tainted? |
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247 | |
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248 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
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249 | example (which doesn't use any system calls, because the kill() |
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250 | is given no processes to signal): |
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251 | |
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252 | sub is_tainted { |
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253 | return ! eval { join('',@_), kill 0; 1; }; |
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254 | } |
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255 | |
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256 | This is not C<-w> clean, however. There is no C<-w> clean way to |
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257 | detect taintedness - take this as a hint that you should untaint |
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258 | all possibly-tainted data. |
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259 | |
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260 | =head2 What's a closure? |
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261 | |
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262 | Closures are documented in L<perlref>. |
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263 | |
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264 | I<Closure> is a computer science term with a precise but |
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265 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
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266 | subroutines with lasting references to lexical variables outside their |
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267 | own scopes. These lexicals magically refer to the variables that were |
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268 | around when the subroutine was defined (deep binding). |
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269 | |
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270 | Closures make sense in any programming language where you can have the |
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271 | return value of a function be itself a function, as you can in Perl. |
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272 | Note that some languages provide anonymous functions but are not |
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273 | capable of providing proper closures; the Python language, for |
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274 | example. For more information on closures, check out any textbook on |
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275 | functional programming. Scheme is a language that not only supports |
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276 | but encourages closures. |
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277 | |
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278 | Here's a classic function-generating function: |
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279 | |
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280 | sub add_function_generator { |
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281 | return sub { shift + shift }; |
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282 | } |
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283 | |
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284 | $add_sub = add_function_generator(); |
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285 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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286 | |
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287 | The closure works as a I<function template> with some customization |
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288 | slots left out to be filled later. The anonymous subroutine returned |
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289 | by add_function_generator() isn't technically a closure because it |
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290 | refers to no lexicals outside its own scope. |
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291 | |
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292 | Contrast this with the following make_adder() function, in which the |
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293 | returned anonymous function contains a reference to a lexical variable |
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294 | outside the scope of that function itself. Such a reference requires |
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295 | that Perl return a proper closure, thus locking in for all time the |
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296 | value that the lexical had when the function was created. |
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297 | |
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298 | sub make_adder { |
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299 | my $addpiece = shift; |
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300 | return sub { shift + $addpiece }; |
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301 | } |
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302 | |
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303 | $f1 = make_adder(20); |
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304 | $f2 = make_adder(555); |
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305 | |
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306 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
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307 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
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308 | in the closure sticks around. |
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309 | |
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310 | Closures are often used for less esoteric purposes. For example, when |
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311 | you want to pass in a bit of code into a function: |
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312 | |
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313 | my $line; |
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314 | timeout( 30, sub { $line = <STDIN> } ); |
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315 | |
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316 | If the code to execute had been passed in as a string, |
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317 | C<< '$line = <STDIN>' >>, there would have been no way for the |
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318 | hypothetical timeout() function to access the lexical variable |
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319 | $line back in its caller's scope. |
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320 | |
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321 | =head2 What is variable suicide and how can I prevent it? |
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322 | |
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323 | Variable suicide is when you (temporarily or permanently) lose the |
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324 | value of a variable. It is caused by scoping through my() and local() |
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325 | interacting with either closures or aliased foreach() iterator |
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326 | variables and subroutine arguments. It used to be easy to |
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327 | inadvertently lose a variable's value this way, but now it's much |
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328 | harder. Take this code: |
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329 | |
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330 | my $f = "foo"; |
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331 | sub T { |
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332 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
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333 | } |
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334 | T; |
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335 | print "Finally $f\n"; |
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336 | |
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337 | The $f that has "bar" added to it three times should be a new C<$f> |
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338 | (C<my $f> should create a new local variable each time through the loop). |
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339 | It isn't, however. This was a bug, now fixed in the latest releases |
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340 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
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341 | |
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342 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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343 | |
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344 | With the exception of regexes, you need to pass references to these |
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345 | objects. See L<perlsub/"Pass by Reference"> for this particular |
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346 | question, and L<perlref> for information on references. |
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347 | |
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348 | =over 4 |
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349 | |
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350 | =item Passing Variables and Functions |
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351 | |
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352 | Regular variables and functions are quite easy: just pass in a |
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353 | reference to an existing or anonymous variable or function: |
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354 | |
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355 | func( \$some_scalar ); |
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356 | |
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357 | func( \@some_array ); |
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358 | func( [ 1 .. 10 ] ); |
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359 | |
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360 | func( \%some_hash ); |
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361 | func( { this => 10, that => 20 } ); |
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362 | |
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363 | func( \&some_func ); |
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364 | func( sub { $_[0] ** $_[1] } ); |
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365 | |
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366 | =item Passing Filehandles |
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367 | |
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368 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
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369 | These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles"> |
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370 | and especially L<perlsub/"Pass by Reference"> for more information. |
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371 | |
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372 | Here's an excerpt: |
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373 | |
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374 | If you're passing around filehandles, you could usually just use the bare |
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375 | typeglob, like *STDOUT, but typeglobs references would be better because |
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376 | they'll still work properly under C<use strict 'refs'>. For example: |
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377 | |
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378 | splutter(\*STDOUT); |
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379 | sub splutter { |
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380 | my $fh = shift; |
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381 | print $fh "her um well a hmmm\n"; |
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382 | } |
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383 | |
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384 | $rec = get_rec(\*STDIN); |
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385 | sub get_rec { |
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386 | my $fh = shift; |
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387 | return scalar <$fh>; |
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388 | } |
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389 | |
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390 | If you're planning on generating new filehandles, you could do this: |
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391 | |
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392 | sub openit { |
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393 | my $name = shift; |
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394 | local *FH; |
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395 | return open (FH, $path) ? *FH : undef; |
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396 | } |
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397 | $fh = openit('< /etc/motd'); |
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398 | print <$fh>; |
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399 | |
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400 | =item Passing Regexes |
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401 | |
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402 | To pass regexes around, you'll need to be using a release of Perl |
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403 | sufficiently recent as to support the C<qr//> construct, pass around |
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404 | strings and use an exception-trapping eval, or else be very, very clever. |
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405 | |
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406 | Here's an example of how to pass in a string to be regex compared |
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407 | using C<qr//>: |
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408 | |
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409 | sub compare($$) { |
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410 | my ($val1, $regex) = @_; |
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411 | my $retval = $val1 =~ /$regex/; |
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412 | return $retval; |
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413 | } |
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414 | $match = compare("old McDonald", qr/d.*D/i); |
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415 | |
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416 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
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417 | at compile time, although it was executed later. The nifty C<qr//> |
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418 | notation wasn't introduced until the 5.005 release. Before that, you |
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419 | had to approach this problem much less intuitively. For example, here |
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420 | it is again if you don't have C<qr//>: |
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421 | |
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422 | sub compare($$) { |
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423 | my ($val1, $regex) = @_; |
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424 | my $retval = eval { $val1 =~ /$regex/ }; |
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425 | die if $@; |
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426 | return $retval; |
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427 | } |
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428 | |
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429 | $match = compare("old McDonald", q/($?i)d.*D/); |
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430 | |
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431 | Make sure you never say something like this: |
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432 | |
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433 | return eval "\$val =~ /$regex/"; # WRONG |
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434 | |
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435 | or someone can sneak shell escapes into the regex due to the double |
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436 | interpolation of the eval and the double-quoted string. For example: |
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437 | |
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438 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
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439 | |
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440 | eval "\$string =~ /$pattern_of_evil/"; |
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441 | |
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442 | Those preferring to be very, very clever might see the O'Reilly book, |
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443 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
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444 | Build_MatchMany_Function() is particularly interesting. A complete |
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445 | citation of this book is given in L<perlfaq2>. |
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446 | |
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447 | =item Passing Methods |
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448 | |
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449 | To pass an object method into a subroutine, you can do this: |
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450 | |
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451 | call_a_lot(10, $some_obj, "methname") |
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452 | sub call_a_lot { |
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453 | my ($count, $widget, $trick) = @_; |
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454 | for (my $i = 0; $i < $count; $i++) { |
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455 | $widget->$trick(); |
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456 | } |
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457 | } |
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458 | |
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459 | Or you can use a closure to bundle up the object and its method call |
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460 | and arguments: |
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461 | |
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462 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
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463 | func($whatnot); |
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464 | sub func { |
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465 | my $code = shift; |
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466 | &$code(); |
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467 | } |
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468 | |
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469 | You could also investigate the can() method in the UNIVERSAL class |
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470 | (part of the standard perl distribution). |
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471 | |
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472 | =back |
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473 | |
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474 | =head2 How do I create a static variable? |
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475 | |
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476 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
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477 | other languages could be either a function-private variable (visible |
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478 | only within a single function, retaining its value between calls to |
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479 | that function), or a file-private variable (visible only to functions |
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480 | within the file it was declared in) in Perl. |
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481 | |
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482 | Here's code to implement a function-private variable: |
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483 | |
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484 | BEGIN { |
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485 | my $counter = 42; |
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486 | sub prev_counter { return --$counter } |
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487 | sub next_counter { return $counter++ } |
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488 | } |
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489 | |
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490 | Now prev_counter() and next_counter() share a private variable $counter |
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491 | that was initialized at compile time. |
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492 | |
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493 | To declare a file-private variable, you'll still use a my(), putting |
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494 | it at the outer scope level at the top of the file. Assume this is in |
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495 | file Pax.pm: |
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496 | |
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497 | package Pax; |
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498 | my $started = scalar(localtime(time())); |
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499 | |
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500 | sub begun { return $started } |
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501 | |
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502 | When C<use Pax> or C<require Pax> loads this module, the variable will |
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503 | be initialized. It won't get garbage-collected the way most variables |
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504 | going out of scope do, because the begun() function cares about it, |
---|
505 | but no one else can get it. It is not called $Pax::started because |
---|
506 | its scope is unrelated to the package. It's scoped to the file. You |
---|
507 | could conceivably have several packages in that same file all |
---|
508 | accessing the same private variable, but another file with the same |
---|
509 | package couldn't get to it. |
---|
510 | |
---|
511 | See L<perlsub/"Persistent Private Variables"> for details. |
---|
512 | |
---|
513 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
---|
514 | |
---|
515 | C<local($x)> saves away the old value of the global variable C<$x>, |
---|
516 | and assigns a new value for the duration of the subroutine, I<which is |
---|
517 | visible in other functions called from that subroutine>. This is done |
---|
518 | at run-time, so is called dynamic scoping. local() always affects global |
---|
519 | variables, also called package variables or dynamic variables. |
---|
520 | |
---|
521 | C<my($x)> creates a new variable that is only visible in the current |
---|
522 | subroutine. This is done at compile-time, so is called lexical or |
---|
523 | static scoping. my() always affects private variables, also called |
---|
524 | lexical variables or (improperly) static(ly scoped) variables. |
---|
525 | |
---|
526 | For instance: |
---|
527 | |
---|
528 | sub visible { |
---|
529 | print "var has value $var\n"; |
---|
530 | } |
---|
531 | |
---|
532 | sub dynamic { |
---|
533 | local $var = 'local'; # new temporary value for the still-global |
---|
534 | visible(); # variable called $var |
---|
535 | } |
---|
536 | |
---|
537 | sub lexical { |
---|
538 | my $var = 'private'; # new private variable, $var |
---|
539 | visible(); # (invisible outside of sub scope) |
---|
540 | } |
---|
541 | |
---|
542 | $var = 'global'; |
---|
543 | |
---|
544 | visible(); # prints global |
---|
545 | dynamic(); # prints local |
---|
546 | lexical(); # prints global |
---|
547 | |
---|
548 | Notice how at no point does the value "private" get printed. That's |
---|
549 | because $var only has that value within the block of the lexical() |
---|
550 | function, and it is hidden from called subroutine. |
---|
551 | |
---|
552 | In summary, local() doesn't make what you think of as private, local |
---|
553 | variables. It gives a global variable a temporary value. my() is |
---|
554 | what you're looking for if you want private variables. |
---|
555 | |
---|
556 | See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary |
---|
557 | Values via local()"> for excruciating details. |
---|
558 | |
---|
559 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
---|
560 | |
---|
561 | You can do this via symbolic references, provided you haven't set |
---|
562 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
---|
563 | |
---|
564 | local $var = "global"; |
---|
565 | my $var = "lexical"; |
---|
566 | |
---|
567 | print "lexical is $var\n"; |
---|
568 | |
---|
569 | no strict 'refs'; |
---|
570 | print "global is ${'var'}\n"; |
---|
571 | |
---|
572 | If you know your package, you can just mention it explicitly, as in |
---|
573 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
---|
574 | $var in the current package, but rather the one in the C<main> |
---|
575 | package, as though you had written $main::var. Specifying the package |
---|
576 | directly makes you hard-code its name, but it executes faster and |
---|
577 | avoids running afoul of C<use strict "refs">. |
---|
578 | |
---|
579 | =head2 What's the difference between deep and shallow binding? |
---|
580 | |
---|
581 | In deep binding, lexical variables mentioned in anonymous subroutines |
---|
582 | are the same ones that were in scope when the subroutine was created. |
---|
583 | In shallow binding, they are whichever variables with the same names |
---|
584 | happen to be in scope when the subroutine is called. Perl always uses |
---|
585 | deep binding of lexical variables (i.e., those created with my()). |
---|
586 | However, dynamic variables (aka global, local, or package variables) |
---|
587 | are effectively shallowly bound. Consider this just one more reason |
---|
588 | not to use them. See the answer to L<"What's a closure?">. |
---|
589 | |
---|
590 | =head2 Why doesn't "my($foo) = <FILE>;" work right? |
---|
591 | |
---|
592 | C<my()> and C<local()> give list context to the right hand side |
---|
593 | of C<=>. The <FH> read operation, like so many of Perl's |
---|
594 | functions and operators, can tell which context it was called in and |
---|
595 | behaves appropriately. In general, the scalar() function can help. |
---|
596 | This function does nothing to the data itself (contrary to popular myth) |
---|
597 | but rather tells its argument to behave in whatever its scalar fashion is. |
---|
598 | If that function doesn't have a defined scalar behavior, this of course |
---|
599 | doesn't help you (such as with sort()). |
---|
600 | |
---|
601 | To enforce scalar context in this particular case, however, you need |
---|
602 | merely omit the parentheses: |
---|
603 | |
---|
604 | local($foo) = <FILE>; # WRONG |
---|
605 | local($foo) = scalar(<FILE>); # ok |
---|
606 | local $foo = <FILE>; # right |
---|
607 | |
---|
608 | You should probably be using lexical variables anyway, although the |
---|
609 | issue is the same here: |
---|
610 | |
---|
611 | my($foo) = <FILE>; # WRONG |
---|
612 | my $foo = <FILE>; # right |
---|
613 | |
---|
614 | =head2 How do I redefine a builtin function, operator, or method? |
---|
615 | |
---|
616 | Why do you want to do that? :-) |
---|
617 | |
---|
618 | If you want to override a predefined function, such as open(), |
---|
619 | then you'll have to import the new definition from a different |
---|
620 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
---|
621 | also an example in L<perltoot/"Class::Template">. |
---|
622 | |
---|
623 | If you want to overload a Perl operator, such as C<+> or C<**>, |
---|
624 | then you'll want to use the C<use overload> pragma, documented |
---|
625 | in L<overload>. |
---|
626 | |
---|
627 | If you're talking about obscuring method calls in parent classes, |
---|
628 | see L<perltoot/"Overridden Methods">. |
---|
629 | |
---|
630 | =head2 What's the difference between calling a function as &foo and foo()? |
---|
631 | |
---|
632 | When you call a function as C<&foo>, you allow that function access to |
---|
633 | your current @_ values, and you by-pass prototypes. That means that |
---|
634 | the function doesn't get an empty @_, it gets yours! While not |
---|
635 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
---|
636 | would be hard to consider this a feature in most cases. |
---|
637 | |
---|
638 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
---|
639 | but prototyping is still circumvented. |
---|
640 | |
---|
641 | Normally, you want to call a function using C<foo()>. You may only |
---|
642 | omit the parentheses if the function is already known to the compiler |
---|
643 | because it already saw the definition (C<use> but not C<require>), |
---|
644 | or via a forward reference or C<use subs> declaration. Even in this |
---|
645 | case, you get a clean @_ without any of the old values leaking through |
---|
646 | where they don't belong. |
---|
647 | |
---|
648 | =head2 How do I create a switch or case statement? |
---|
649 | |
---|
650 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
---|
651 | no official case statement, because of the variety of tests possible |
---|
652 | in Perl (numeric comparison, string comparison, glob comparison, |
---|
653 | regex matching, overloaded comparisons, ...). Larry couldn't decide |
---|
654 | how best to do this, so he left it out, even though it's been on the |
---|
655 | wish list since perl1. |
---|
656 | |
---|
657 | The general answer is to write a construct like this: |
---|
658 | |
---|
659 | for ($variable_to_test) { |
---|
660 | if (/pat1/) { } # do something |
---|
661 | elsif (/pat2/) { } # do something else |
---|
662 | elsif (/pat3/) { } # do something else |
---|
663 | else { } # default |
---|
664 | } |
---|
665 | |
---|
666 | Here's a simple example of a switch based on pattern matching, this |
---|
667 | time lined up in a way to make it look more like a switch statement. |
---|
668 | We'll do a multi-way conditional based on the type of reference stored |
---|
669 | in $whatchamacallit: |
---|
670 | |
---|
671 | SWITCH: for (ref $whatchamacallit) { |
---|
672 | |
---|
673 | /^$/ && die "not a reference"; |
---|
674 | |
---|
675 | /SCALAR/ && do { |
---|
676 | print_scalar($$ref); |
---|
677 | last SWITCH; |
---|
678 | }; |
---|
679 | |
---|
680 | /ARRAY/ && do { |
---|
681 | print_array(@$ref); |
---|
682 | last SWITCH; |
---|
683 | }; |
---|
684 | |
---|
685 | /HASH/ && do { |
---|
686 | print_hash(%$ref); |
---|
687 | last SWITCH; |
---|
688 | }; |
---|
689 | |
---|
690 | /CODE/ && do { |
---|
691 | warn "can't print function ref"; |
---|
692 | last SWITCH; |
---|
693 | }; |
---|
694 | |
---|
695 | # DEFAULT |
---|
696 | |
---|
697 | warn "User defined type skipped"; |
---|
698 | |
---|
699 | } |
---|
700 | |
---|
701 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
---|
702 | examples in this style. |
---|
703 | |
---|
704 | Sometimes you should change the positions of the constant and the variable. |
---|
705 | For example, let's say you wanted to test which of many answers you were |
---|
706 | given, but in a case-insensitive way that also allows abbreviations. |
---|
707 | You can use the following technique if the strings all start with |
---|
708 | different characters, or if you want to arrange the matches so that |
---|
709 | one takes precedence over another, as C<"SEND"> has precedence over |
---|
710 | C<"STOP"> here: |
---|
711 | |
---|
712 | chomp($answer = <>); |
---|
713 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
---|
714 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
---|
715 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
---|
716 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
---|
717 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
---|
718 | |
---|
719 | A totally different approach is to create a hash of function references. |
---|
720 | |
---|
721 | my %commands = ( |
---|
722 | "happy" => \&joy, |
---|
723 | "sad", => \&sullen, |
---|
724 | "done" => sub { die "See ya!" }, |
---|
725 | "mad" => \&angry, |
---|
726 | ); |
---|
727 | |
---|
728 | print "How are you? "; |
---|
729 | chomp($string = <STDIN>); |
---|
730 | if ($commands{$string}) { |
---|
731 | $commands{$string}->(); |
---|
732 | } else { |
---|
733 | print "No such command: $string\n"; |
---|
734 | } |
---|
735 | |
---|
736 | =head2 How can I catch accesses to undefined variables/functions/methods? |
---|
737 | |
---|
738 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
---|
739 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
---|
740 | undefined functions and methods. |
---|
741 | |
---|
742 | When it comes to undefined variables that would trigger a warning |
---|
743 | under C<-w>, you can use a handler to trap the pseudo-signal |
---|
744 | C<__WARN__> like this: |
---|
745 | |
---|
746 | $SIG{__WARN__} = sub { |
---|
747 | |
---|
748 | for ( $_[0] ) { # voici un switch statement |
---|
749 | |
---|
750 | /Use of uninitialized value/ && do { |
---|
751 | # promote warning to a fatal |
---|
752 | die $_; |
---|
753 | }; |
---|
754 | |
---|
755 | # other warning cases to catch could go here; |
---|
756 | |
---|
757 | warn $_; |
---|
758 | } |
---|
759 | |
---|
760 | }; |
---|
761 | |
---|
762 | =head2 Why can't a method included in this same file be found? |
---|
763 | |
---|
764 | Some possible reasons: your inheritance is getting confused, you've |
---|
765 | misspelled the method name, or the object is of the wrong type. Check |
---|
766 | out L<perltoot> for details on these. You may also use C<print |
---|
767 | ref($object)> to find out the class C<$object> was blessed into. |
---|
768 | |
---|
769 | Another possible reason for problems is because you've used the |
---|
770 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
---|
771 | before Perl has seen that such a package exists. It's wisest to make |
---|
772 | sure your packages are all defined before you start using them, which |
---|
773 | will be taken care of if you use the C<use> statement instead of |
---|
774 | C<require>. If not, make sure to use arrow notation (eg, |
---|
775 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
---|
776 | L<perlobj>. |
---|
777 | |
---|
778 | Make sure to read about creating modules in L<perlmod> and |
---|
779 | the perils of indirect objects in L<perlobj/"WARNING">. |
---|
780 | |
---|
781 | =head2 How can I find out my current package? |
---|
782 | |
---|
783 | If you're just a random program, you can do this to find |
---|
784 | out what the currently compiled package is: |
---|
785 | |
---|
786 | my $packname = __PACKAGE__; |
---|
787 | |
---|
788 | But if you're a method and you want to print an error message |
---|
789 | that includes the kind of object you were called on (which is |
---|
790 | not necessarily the same as the one in which you were compiled): |
---|
791 | |
---|
792 | sub amethod { |
---|
793 | my $self = shift; |
---|
794 | my $class = ref($self) || $self; |
---|
795 | warn "called me from a $class object"; |
---|
796 | } |
---|
797 | |
---|
798 | =head2 How can I comment out a large block of perl code? |
---|
799 | |
---|
800 | Use embedded POD to discard it: |
---|
801 | |
---|
802 | # program is here |
---|
803 | |
---|
804 | =for nobody |
---|
805 | This paragraph is commented out |
---|
806 | |
---|
807 | # program continues |
---|
808 | |
---|
809 | =begin comment text |
---|
810 | |
---|
811 | all of this stuff |
---|
812 | |
---|
813 | here will be ignored |
---|
814 | by everyone |
---|
815 | |
---|
816 | =end comment text |
---|
817 | |
---|
818 | =cut |
---|
819 | |
---|
820 | This can't go just anywhere. You have to put a pod directive where |
---|
821 | the parser is expecting a new statement, not just in the middle |
---|
822 | of an expression or some other arbitrary yacc grammar production. |
---|
823 | |
---|
824 | =head2 How do I clear a package? |
---|
825 | |
---|
826 | Use this code, provided by Mark-Jason Dominus: |
---|
827 | |
---|
828 | sub scrub_package { |
---|
829 | no strict 'refs'; |
---|
830 | my $pack = shift; |
---|
831 | die "Shouldn't delete main package" |
---|
832 | if $pack eq "" || $pack eq "main"; |
---|
833 | my $stash = *{$pack . '::'}{HASH}; |
---|
834 | my $name; |
---|
835 | foreach $name (keys %$stash) { |
---|
836 | my $fullname = $pack . '::' . $name; |
---|
837 | # Get rid of everything with that name. |
---|
838 | undef $$fullname; |
---|
839 | undef @$fullname; |
---|
840 | undef %$fullname; |
---|
841 | undef &$fullname; |
---|
842 | undef *$fullname; |
---|
843 | } |
---|
844 | } |
---|
845 | |
---|
846 | Or, if you're using a recent release of Perl, you can |
---|
847 | just use the Symbol::delete_package() function instead. |
---|
848 | |
---|
849 | =head2 How can I use a variable as a variable name? |
---|
850 | |
---|
851 | Beginners often think they want to have a variable contain the name |
---|
852 | of a variable. |
---|
853 | |
---|
854 | $fred = 23; |
---|
855 | $varname = "fred"; |
---|
856 | ++$$varname; # $fred now 24 |
---|
857 | |
---|
858 | This works I<sometimes>, but it is a very bad idea for two reasons. |
---|
859 | |
---|
860 | The first reason is that they I<only work on global variables>. |
---|
861 | That means above that if $fred is a lexical variable created with my(), |
---|
862 | that the code won't work at all: you'll accidentally access the global |
---|
863 | and skip right over the private lexical altogether. Global variables |
---|
864 | are bad because they can easily collide accidentally and in general make |
---|
865 | for non-scalable and confusing code. |
---|
866 | |
---|
867 | Symbolic references are forbidden under the C<use strict> pragma. |
---|
868 | They are not true references and consequently are not reference counted |
---|
869 | or garbage collected. |
---|
870 | |
---|
871 | The other reason why using a variable to hold the name of another |
---|
872 | variable a bad idea is that the question often stems from a lack of |
---|
873 | understanding of Perl data structures, particularly hashes. By using |
---|
874 | symbolic references, you are just using the package's symbol-table hash |
---|
875 | (like C<%main::>) instead of a user-defined hash. The solution is to |
---|
876 | use your own hash or a real reference instead. |
---|
877 | |
---|
878 | $fred = 23; |
---|
879 | $varname = "fred"; |
---|
880 | $USER_VARS{$varname}++; # not $$varname++ |
---|
881 | |
---|
882 | There we're using the %USER_VARS hash instead of symbolic references. |
---|
883 | Sometimes this comes up in reading strings from the user with variable |
---|
884 | references and wanting to expand them to the values of your perl |
---|
885 | program's variables. This is also a bad idea because it conflates the |
---|
886 | program-addressable namespace and the user-addressable one. Instead of |
---|
887 | reading a string and expanding it to the actual contents of your program's |
---|
888 | own variables: |
---|
889 | |
---|
890 | $str = 'this has a $fred and $barney in it'; |
---|
891 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
---|
892 | |
---|
893 | Instead, it would be better to keep a hash around like %USER_VARS and have |
---|
894 | variable references actually refer to entries in that hash: |
---|
895 | |
---|
896 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
---|
897 | |
---|
898 | That's faster, cleaner, and safer than the previous approach. Of course, |
---|
899 | you don't need to use a dollar sign. You could use your own scheme to |
---|
900 | make it less confusing, like bracketed percent symbols, etc. |
---|
901 | |
---|
902 | $str = 'this has a %fred% and %barney% in it'; |
---|
903 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
---|
904 | |
---|
905 | Another reason that folks sometimes think they want a variable to contain |
---|
906 | the name of a variable is because they don't know how to build proper |
---|
907 | data structures using hashes. For example, let's say they wanted two |
---|
908 | hashes in their program: %fred and %barney, and to use another scalar |
---|
909 | variable to refer to those by name. |
---|
910 | |
---|
911 | $name = "fred"; |
---|
912 | $$name{WIFE} = "wilma"; # set %fred |
---|
913 | |
---|
914 | $name = "barney"; |
---|
915 | $$name{WIFE} = "betty"; # set %barney |
---|
916 | |
---|
917 | This is still a symbolic reference, and is still saddled with the |
---|
918 | problems enumerated above. It would be far better to write: |
---|
919 | |
---|
920 | $folks{"fred"}{WIFE} = "wilma"; |
---|
921 | $folks{"barney"}{WIFE} = "betty"; |
---|
922 | |
---|
923 | And just use a multilevel hash to start with. |
---|
924 | |
---|
925 | The only times that you absolutely I<must> use symbolic references are |
---|
926 | when you really must refer to the symbol table. This may be because it's |
---|
927 | something that can't take a real reference to, such as a format name. |
---|
928 | Doing so may also be important for method calls, since these always go |
---|
929 | through the symbol table for resolution. |
---|
930 | |
---|
931 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
---|
932 | can play around with the symbol table. For example: |
---|
933 | |
---|
934 | @colors = qw(red blue green yellow orange purple violet); |
---|
935 | for my $name (@colors) { |
---|
936 | no strict 'refs'; # renege for the block |
---|
937 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
---|
938 | } |
---|
939 | |
---|
940 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
---|
941 | but the real code in the closure actually was compiled only once. |
---|
942 | |
---|
943 | So, sometimes you might want to use symbolic references to directly |
---|
944 | manipulate the symbol table. This doesn't matter for formats, handles, and |
---|
945 | subroutines, because they are always global -- you can't use my() on them. |
---|
946 | But for scalars, arrays, and hashes -- and usually for subroutines -- |
---|
947 | you probably want to use hard references only. |
---|
948 | |
---|
949 | =head1 AUTHOR AND COPYRIGHT |
---|
950 | |
---|
951 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
---|
952 | All rights reserved. |
---|
953 | |
---|
954 | When included as part of the Standard Version of Perl, or as part of |
---|
955 | its complete documentation whether printed or otherwise, this work |
---|
956 | may be distributed only under the terms of Perl's Artistic License. |
---|
957 | Any distribution of this file or derivatives thereof I<outside> |
---|
958 | of that package require that special arrangements be made with |
---|
959 | copyright holder. |
---|
960 | |
---|
961 | Irrespective of its distribution, all code examples in this file |
---|
962 | are hereby placed into the public domain. You are permitted and |
---|
963 | encouraged to use this code in your own programs for fun |
---|
964 | or for profit as you see fit. A simple comment in the code giving |
---|
965 | credit would be courteous but is not required. |
---|