1 |
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2 | # Call.pm
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3 | #
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4 | # Copyright (c) 1995-2001 Paul Marquess. All rights reserved.
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5 | #
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6 | # This program is free software; you can redistribute it and/or
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7 | # modify it under the same terms as Perl itself.
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8 |
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9 | package Filter::Util::Call ;
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10 |
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11 | require 5.002 ;
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12 | require DynaLoader;
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13 | require Exporter;
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14 | use Carp ;
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15 | use strict;
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16 | use warnings;
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17 | use vars qw($VERSION @ISA @EXPORT) ;
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18 |
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19 | @ISA = qw(Exporter DynaLoader);
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20 | @EXPORT = qw( filter_add filter_del filter_read filter_read_exact) ;
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21 | $VERSION = "1.0601" ;
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22 |
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23 | sub filter_read_exact($)
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24 | {
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25 | my ($size) = @_ ;
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26 | my ($left) = $size ;
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27 | my ($status) ;
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28 |
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29 | croak ("filter_read_exact: size parameter must be > 0")
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30 | unless $size > 0 ;
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31 |
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32 | # try to read a block which is exactly $size bytes long
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33 | while ($left and ($status = filter_read($left)) > 0) {
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34 | $left = $size - length $_ ;
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35 | }
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36 |
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37 | # EOF with pending data is a special case
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38 | return 1 if $status == 0 and length $_ ;
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39 |
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40 | return $status ;
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41 | }
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42 |
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43 | sub filter_add($)
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44 | {
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45 | my($obj) = @_ ;
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46 |
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47 | # Did we get a code reference?
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48 | my $coderef = (ref $obj eq 'CODE') ;
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49 |
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50 | # If the parameter isn't already a reference, make it one.
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51 | $obj = \$obj unless ref $obj ;
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52 |
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53 | $obj = bless ($obj, (caller)[0]) unless $coderef ;
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54 |
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55 | # finish off the installation of the filter in C.
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56 | Filter::Util::Call::real_import($obj, (caller)[0], $coderef) ;
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57 | }
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58 |
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59 | bootstrap Filter::Util::Call ;
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60 |
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61 | 1;
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62 | __END__
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63 |
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64 | =head1 NAME
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65 |
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66 | Filter::Util::Call - Perl Source Filter Utility Module
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67 |
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68 | =head1 SYNOPSIS
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69 |
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70 | use Filter::Util::Call ;
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71 |
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72 | =head1 DESCRIPTION
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73 |
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74 | This module provides you with the framework to write I<Source Filters>
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75 | in Perl.
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76 |
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77 | An alternate interface to Filter::Util::Call is now available. See
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78 | L<Filter::Simple> for more details.
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79 |
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80 | A I<Perl Source Filter> is implemented as a Perl module. The structure
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81 | of the module can take one of two broadly similar formats. To
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82 | distinguish between them, the first will be referred to as I<method
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83 | filter> and the second as I<closure filter>.
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84 |
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85 | Here is a skeleton for the I<method filter>:
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86 |
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87 | package MyFilter ;
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88 |
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89 | use Filter::Util::Call ;
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90 |
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91 | sub import
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92 | {
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93 | my($type, @arguments) = @_ ;
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94 | filter_add([]) ;
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95 | }
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96 |
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97 | sub filter
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98 | {
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99 | my($self) = @_ ;
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100 | my($status) ;
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101 |
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102 | $status = filter_read() ;
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103 | $status ;
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104 | }
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105 |
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106 | 1 ;
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107 |
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108 | and this is the equivalent skeleton for the I<closure filter>:
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109 |
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110 | package MyFilter ;
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111 |
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112 | use Filter::Util::Call ;
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113 |
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114 | sub import
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115 | {
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116 | my($type, @arguments) = @_ ;
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117 |
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118 | filter_add(
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119 | sub
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120 | {
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121 | my($status) ;
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122 | $status = filter_read() ;
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123 | $status ;
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124 | } )
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125 | }
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126 |
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127 | 1 ;
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128 |
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129 | To make use of either of the two filter modules above, place the line
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130 | below in a Perl source file.
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131 |
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132 | use MyFilter;
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133 |
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134 | In fact, the skeleton modules shown above are fully functional I<Source
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135 | Filters>, albeit fairly useless ones. All they does is filter the
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136 | source stream without modifying it at all.
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137 |
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138 | As you can see both modules have a broadly similar structure. They both
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139 | make use of the C<Filter::Util::Call> module and both have an C<import>
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140 | method. The difference between them is that the I<method filter>
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141 | requires a I<filter> method, whereas the I<closure filter> gets the
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142 | equivalent of a I<filter> method with the anonymous sub passed to
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143 | I<filter_add>.
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144 |
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145 | To make proper use of the I<closure filter> shown above you need to
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146 | have a good understanding of the concept of a I<closure>. See
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147 | L<perlref> for more details on the mechanics of I<closures>.
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148 |
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149 | =head2 B<use Filter::Util::Call>
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150 |
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151 | The following functions are exported by C<Filter::Util::Call>:
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152 |
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153 | filter_add()
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154 | filter_read()
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155 | filter_read_exact()
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156 | filter_del()
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157 |
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158 | =head2 B<import()>
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159 |
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160 | The C<import> method is used to create an instance of the filter. It is
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161 | called indirectly by Perl when it encounters the C<use MyFilter> line
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162 | in a source file (See L<perlfunc/import> for more details on
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163 | C<import>).
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164 |
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165 | It will always have at least one parameter automatically passed by Perl
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166 | - this corresponds to the name of the package. In the example above it
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167 | will be C<"MyFilter">.
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168 |
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169 | Apart from the first parameter, import can accept an optional list of
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170 | parameters. These can be used to pass parameters to the filter. For
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171 | example:
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172 |
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173 | use MyFilter qw(a b c) ;
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174 |
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175 | will result in the C<@_> array having the following values:
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176 |
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177 | @_ [0] => "MyFilter"
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178 | @_ [1] => "a"
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179 | @_ [2] => "b"
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180 | @_ [3] => "c"
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181 |
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182 | Before terminating, the C<import> function must explicitly install the
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183 | filter by calling C<filter_add>.
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184 |
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185 | B<filter_add()>
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186 |
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187 | The function, C<filter_add>, actually installs the filter. It takes one
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188 | parameter which should be a reference. The kind of reference used will
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189 | dictate which of the two filter types will be used.
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190 |
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191 | If a CODE reference is used then a I<closure filter> will be assumed.
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192 |
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193 | If a CODE reference is not used, a I<method filter> will be assumed.
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194 | In a I<method filter>, the reference can be used to store context
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195 | information. The reference will be I<blessed> into the package by
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196 | C<filter_add>.
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197 |
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198 | See the filters at the end of this documents for examples of using
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199 | context information using both I<method filters> and I<closure
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200 | filters>.
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201 |
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202 | =head2 B<filter() and anonymous sub>
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203 |
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204 | Both the C<filter> method used with a I<method filter> and the
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205 | anonymous sub used with a I<closure filter> is where the main
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206 | processing for the filter is done.
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207 |
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208 | The big difference between the two types of filter is that the I<method
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209 | filter> uses the object passed to the method to store any context data,
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210 | whereas the I<closure filter> uses the lexical variables that are
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211 | maintained by the closure.
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212 |
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213 | Note that the single parameter passed to the I<method filter>,
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214 | C<$self>, is the same reference that was passed to C<filter_add>
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215 | blessed into the filter's package. See the example filters later on for
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216 | details of using C<$self>.
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217 |
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218 | Here is a list of the common features of the anonymous sub and the
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219 | C<filter()> method.
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220 |
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221 | =over 5
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222 |
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223 | =item B<$_>
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224 |
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225 | Although C<$_> doesn't actually appear explicitly in the sample filters
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226 | above, it is implicitly used in a number of places.
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227 |
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228 | Firstly, when either C<filter> or the anonymous sub are called, a local
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229 | copy of C<$_> will automatically be created. It will always contain the
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230 | empty string at this point.
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231 |
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232 | Next, both C<filter_read> and C<filter_read_exact> will append any
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233 | source data that is read to the end of C<$_>.
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234 |
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235 | Finally, when C<filter> or the anonymous sub are finished processing,
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236 | they are expected to return the filtered source using C<$_>.
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237 |
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238 | This implicit use of C<$_> greatly simplifies the filter.
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239 |
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240 | =item B<$status>
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241 |
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242 | The status value that is returned by the user's C<filter> method or
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243 | anonymous sub and the C<filter_read> and C<read_exact> functions take
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244 | the same set of values, namely:
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245 |
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246 | < 0 Error
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247 | = 0 EOF
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248 | > 0 OK
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249 |
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250 | =item B<filter_read> and B<filter_read_exact>
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251 |
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252 | These functions are used by the filter to obtain either a line or block
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253 | from the next filter in the chain or the actual source file if there
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254 | aren't any other filters.
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255 |
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256 | The function C<filter_read> takes two forms:
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257 |
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258 | $status = filter_read() ;
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259 | $status = filter_read($size) ;
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260 |
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261 | The first form is used to request a I<line>, the second requests a
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262 | I<block>.
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263 |
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264 | In line mode, C<filter_read> will append the next source line to the
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265 | end of the C<$_> scalar.
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266 |
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267 | In block mode, C<filter_read> will append a block of data which is <=
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268 | C<$size> to the end of the C<$_> scalar. It is important to emphasise
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269 | the that C<filter_read> will not necessarily read a block which is
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270 | I<precisely> C<$size> bytes.
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271 |
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272 | If you need to be able to read a block which has an exact size, you can
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273 | use the function C<filter_read_exact>. It works identically to
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274 | C<filter_read> in block mode, except it will try to read a block which
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275 | is exactly C<$size> bytes in length. The only circumstances when it
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276 | will not return a block which is C<$size> bytes long is on EOF or
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277 | error.
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278 |
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279 | It is I<very> important to check the value of C<$status> after I<every>
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280 | call to C<filter_read> or C<filter_read_exact>.
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281 |
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282 | =item B<filter_del>
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283 |
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284 | The function, C<filter_del>, is used to disable the current filter. It
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285 | does not affect the running of the filter. All it does is tell Perl not
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286 | to call filter any more.
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287 |
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288 | See L<Example 4: Using filter_del> for details.
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289 |
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290 | =back
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291 |
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292 | =head1 EXAMPLES
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293 |
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294 | Here are a few examples which illustrate the key concepts - as such
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295 | most of them are of little practical use.
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296 |
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297 | The C<examples> sub-directory has copies of all these filters
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298 | implemented both as I<method filters> and as I<closure filters>.
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299 |
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300 | =head2 Example 1: A simple filter.
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301 |
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302 | Below is a I<method filter> which is hard-wired to replace all
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303 | occurrences of the string C<"Joe"> to C<"Jim">. Not particularly
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304 | Useful, but it is the first example and I wanted to keep it simple.
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305 |
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306 | package Joe2Jim ;
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307 |
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308 | use Filter::Util::Call ;
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309 |
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310 | sub import
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311 | {
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312 | my($type) = @_ ;
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313 |
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314 | filter_add(bless []) ;
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315 | }
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316 |
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317 | sub filter
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318 | {
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319 | my($self) = @_ ;
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320 | my($status) ;
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321 |
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322 | s/Joe/Jim/g
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323 | if ($status = filter_read()) > 0 ;
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324 | $status ;
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325 | }
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326 |
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327 | 1 ;
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328 |
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329 | Here is an example of using the filter:
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330 |
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331 | use Joe2Jim ;
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332 | print "Where is Joe?\n" ;
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333 |
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334 | And this is what the script above will print:
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335 |
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336 | Where is Jim?
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337 |
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338 | =head2 Example 2: Using the context
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339 |
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340 | The previous example was not particularly useful. To make it more
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341 | general purpose we will make use of the context data and allow any
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342 | arbitrary I<from> and I<to> strings to be used. This time we will use a
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343 | I<closure filter>. To reflect its enhanced role, the filter is called
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344 | C<Subst>.
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345 |
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346 | package Subst ;
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347 |
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348 | use Filter::Util::Call ;
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349 | use Carp ;
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350 |
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351 | sub import
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352 | {
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353 | croak("usage: use Subst qw(from to)")
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354 | unless @_ == 3 ;
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355 | my ($self, $from, $to) = @_ ;
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356 | filter_add(
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357 | sub
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358 | {
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359 | my ($status) ;
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360 | s/$from/$to/
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361 | if ($status = filter_read()) > 0 ;
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362 | $status ;
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363 | })
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364 | }
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365 | 1 ;
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366 |
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367 | and is used like this:
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368 |
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369 | use Subst qw(Joe Jim) ;
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370 | print "Where is Joe?\n" ;
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371 |
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372 |
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373 | =head2 Example 3: Using the context within the filter
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374 |
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375 | Here is a filter which a variation of the C<Joe2Jim> filter. As well as
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376 | substituting all occurrences of C<"Joe"> to C<"Jim"> it keeps a count
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377 | of the number of substitutions made in the context object.
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378 |
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379 | Once EOF is detected (C<$status> is zero) the filter will insert an
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380 | extra line into the source stream. When this extra line is executed it
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381 | will print a count of the number of substitutions actually made.
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382 | Note that C<$status> is set to C<1> in this case.
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383 |
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384 | package Count ;
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385 |
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386 | use Filter::Util::Call ;
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387 |
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388 | sub filter
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389 | {
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390 | my ($self) = @_ ;
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391 | my ($status) ;
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392 |
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393 | if (($status = filter_read()) > 0 ) {
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394 | s/Joe/Jim/g ;
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395 | ++ $$self ;
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396 | }
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397 | elsif ($$self >= 0) { # EOF
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398 | $_ = "print q[Made ${$self} substitutions\n]" ;
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399 | $status = 1 ;
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400 | $$self = -1 ;
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401 | }
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402 |
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403 | $status ;
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404 | }
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405 |
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406 | sub import
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407 | {
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408 | my ($self) = @_ ;
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409 | my ($count) = 0 ;
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410 | filter_add(\$count) ;
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411 | }
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412 |
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413 | 1 ;
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414 |
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415 | Here is a script which uses it:
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416 |
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417 | use Count ;
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418 | print "Hello Joe\n" ;
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419 | print "Where is Joe\n" ;
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420 |
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421 | Outputs:
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422 |
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423 | Hello Jim
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424 | Where is Jim
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425 | Made 2 substitutions
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426 |
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427 | =head2 Example 4: Using filter_del
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428 |
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429 | Another variation on a theme. This time we will modify the C<Subst>
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430 | filter to allow a starting and stopping pattern to be specified as well
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431 | as the I<from> and I<to> patterns. If you know the I<vi> editor, it is
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432 | the equivalent of this command:
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433 |
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434 | :/start/,/stop/s/from/to/
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435 |
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436 | When used as a filter we want to invoke it like this:
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437 |
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438 | use NewSubst qw(start stop from to) ;
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439 |
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440 | Here is the module.
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441 |
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442 | package NewSubst ;
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443 |
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444 | use Filter::Util::Call ;
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445 | use Carp ;
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446 |
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447 | sub import
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448 | {
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449 | my ($self, $start, $stop, $from, $to) = @_ ;
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450 | my ($found) = 0 ;
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451 | croak("usage: use Subst qw(start stop from to)")
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452 | unless @_ == 5 ;
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453 |
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454 | filter_add(
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455 | sub
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456 | {
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457 | my ($status) ;
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458 |
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459 | if (($status = filter_read()) > 0) {
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460 |
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461 | $found = 1
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462 | if $found == 0 and /$start/ ;
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463 |
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464 | if ($found) {
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465 | s/$from/$to/ ;
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466 | filter_del() if /$stop/ ;
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467 | }
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468 |
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469 | }
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470 | $status ;
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471 | } )
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472 |
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473 | }
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474 |
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475 | 1 ;
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476 |
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477 | =head1 Filter::Simple
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478 |
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479 | If you intend using the Filter::Call functionality, I would strongly
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480 | recommend that you check out Damian Conway's excellent Filter::Simple
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481 | module. Damian's module provides a much cleaner interface than
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482 | Filter::Util::Call. Although it doesn't allow the fine control that
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483 | Filter::Util::Call does, it should be adequate for the majority of
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484 | applications. It's available at
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485 |
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486 | http://www.cpan.org/modules/by-author/Damian_Conway/Filter-Simple.tar.gz
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487 | http://www.csse.monash.edu.au/~damian/CPAN/Filter-Simple.tar.gz
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488 |
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489 | =head1 AUTHOR
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490 |
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491 | Paul Marquess
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492 |
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493 | =head1 DATE
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494 |
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495 | 26th January 1996
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496 |
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497 | =cut
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498 |
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