1 package Switch; |
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2 |
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3 use strict; |
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4 use vars qw($VERSION); |
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5 use Carp; |
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6 |
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7 use if $] >= 5.011, 'deprecate'; |
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8 |
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9 $VERSION = '2.16'; |
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10 |
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11 |
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12 # LOAD FILTERING MODULE... |
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13 use Filter::Util::Call; |
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14 |
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15 sub __(); |
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16 |
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17 # CATCH ATTEMPTS TO CALL case OUTSIDE THE SCOPE OF ANY switch |
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18 |
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19 $::_S_W_I_T_C_H = sub { croak "case/when statement not in switch/given block" }; |
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20 |
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21 my $offset; |
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22 my $fallthrough; |
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23 my ($Perl5, $Perl6) = (0,0); |
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24 |
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25 sub import |
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26 { |
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27 $fallthrough = grep /\bfallthrough\b/, @_; |
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28 $offset = (caller)[2]+1; |
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29 filter_add({}) unless @_>1 && $_[1] eq 'noimport'; |
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30 my $pkg = caller; |
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31 no strict 'refs'; |
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32 for ( qw( on_defined on_exists ) ) |
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33 { |
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34 *{"${pkg}::$_"} = \&$_; |
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35 } |
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36 *{"${pkg}::__"} = \&__ if grep /__/, @_; |
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37 $Perl6 = 1 if grep(/Perl\s*6/i, @_); |
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38 $Perl5 = 1 if grep(/Perl\s*5/i, @_) || !grep(/Perl\s*6/i, @_); |
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39 1; |
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40 } |
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41 |
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42 sub unimport |
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43 { |
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44 filter_del() |
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45 } |
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46 |
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47 sub filter |
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48 { |
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49 my($self) = @_ ; |
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50 local $Switch::file = (caller)[1]; |
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51 |
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52 my $status = 1; |
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53 $status = filter_read(1_000_000); |
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54 return $status if $status<0; |
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55 $_ = filter_blocks($_,$offset); |
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56 $_ = "# line $offset\n" . $_ if $offset; undef $offset; |
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57 return $status; |
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58 } |
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59 |
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60 use Text::Balanced ':ALL'; |
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61 |
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62 sub line |
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63 { |
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64 my ($pretext,$offset) = @_; |
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65 ($pretext=~tr/\n/\n/)+($offset||0); |
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66 } |
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67 |
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68 sub is_block |
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69 { |
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70 local $SIG{__WARN__}=sub{die$@}; |
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71 local $^W=1; |
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72 my $ishash = defined eval 'my $hr='.$_[0]; |
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73 undef $@; |
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74 return !$ishash; |
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75 } |
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76 |
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77 my $pod_or_DATA = qr/ ^=[A-Za-z] .*? ^=cut (?![A-Za-z]) .*? $ |
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78 | ^__(DATA|END)__\n.* |
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79 /smx; |
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80 |
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81 my $casecounter = 1; |
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82 sub filter_blocks |
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83 { |
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84 my ($source, $line) = @_; |
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85 return $source unless $Perl5 && $source =~ /case|switch/ |
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86 || $Perl6 && $source =~ /when|given|default/; |
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87 pos $source = 0; |
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88 my $text = ""; |
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89 component: while (pos $source < length $source) |
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90 { |
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91 if ($source =~ m/(\G\s*use\s+Switch\b)/gc) |
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92 { |
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93 $text .= q{use Switch 'noimport'}; |
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94 next component; |
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95 } |
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96 my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0); |
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97 if (defined $pos[0]) |
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98 { |
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99 my $pre = substr($source,$pos[0],$pos[1]); # matched prefix |
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100 my $iEol; |
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101 if( substr($source,$pos[4],$pos[5]) eq '/' && # 1st delimiter |
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102 substr($source,$pos[2],$pos[3]) eq '' && # no op like 'm' |
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103 index( substr($source,$pos[16],$pos[17]), 'x' ) == -1 && # no //x |
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104 ($iEol = index( $source, "\n", $pos[4] )) > 0 && |
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105 $iEol < $pos[8] ){ # embedded newlines |
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106 # If this is a pattern, it isn't compatible with Switch. Backup past 1st '/'. |
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107 pos( $source ) = $pos[6]; |
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108 $text .= $pre . substr($source,$pos[2],$pos[6]-$pos[2]); |
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109 } else { |
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110 $text .= $pre . substr($source,$pos[2],$pos[18]-$pos[2]); |
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111 } |
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112 next component; |
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113 } |
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114 if ($source =~ m/(\G\s*$pod_or_DATA)/gc) { |
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115 $text .= $1; |
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116 next component; |
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117 } |
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118 @pos = Text::Balanced::_match_variable(\$source,qr/\s*/); |
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119 if (defined $pos[0]) |
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120 { |
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121 $text .= " " if $pos[0] < $pos[2]; |
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122 $text .= substr($source,$pos[0],$pos[4]-$pos[0]); |
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123 next component; |
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124 } |
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125 |
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126 if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc |
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127 || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc |
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128 || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc) |
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129 { |
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130 my $keyword = $3; |
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131 my $arg = $4; |
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132 $text .= $1.$2.'S_W_I_T_C_H: while (1) '; |
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133 unless ($arg) { |
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134 @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef) |
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135 or do { |
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136 die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
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137 }; |
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138 $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
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139 } |
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140 $arg =~ s {^\s*[(]\s*%} { ( \\\%} || |
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141 $arg =~ s {^\s*[(]\s*m\b} { ( qr} || |
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142 $arg =~ s {^\s*[(]\s*/} { ( qr/} || |
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143 $arg =~ s {^\s*[(]\s*qw} { ( \\qw}; |
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144 @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef) |
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145 or do { |
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146 die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0, pos $source), $line), "\n"; |
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147 }; |
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148 my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
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149 $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/; |
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150 $text .= $code . 'continue {last}'; |
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151 next component; |
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152 } |
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153 elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc |
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154 || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc |
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155 || $Perl6 && $source =~ m/\G(\s*)(default\b)(?=\s*\{)/gc) |
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156 { |
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157 my $keyword = $2; |
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158 $text .= $1 . ($keyword eq "default" |
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159 ? "if (1)" |
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160 : "if (Switch::case"); |
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161 |
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162 if ($keyword eq "default") { |
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163 # Nothing to do |
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164 } |
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165 elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) { |
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166 my $code = substr($source,$pos[0],$pos[4]-$pos[0]); |
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167 $text .= " " if $pos[0] < $pos[2]; |
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168 $text .= "sub " if is_block $code; |
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169 $text .= filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")"; |
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170 } |
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171 elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) { |
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172 my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
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173 $code =~ s {^\s*[(]\s*%} { ( \\\%} || |
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174 $code =~ s {^\s*[(]\s*m\b} { ( qr} || |
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175 $code =~ s {^\s*[(]\s*/} { ( qr/} || |
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176 $code =~ s {^\s*[(]\s*qw} { ( \\qw}; |
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177 $text .= " " if $pos[0] < $pos[2]; |
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178 $text .= "$code)"; |
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179 } |
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180 elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) { |
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181 my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
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182 $code =~ s {^\s*%} { \%} || |
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183 $code =~ s {^\s*@} { \@}; |
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184 $text .= " " if $pos[0] < $pos[2]; |
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185 $text .= "$code)"; |
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186 } |
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187 elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) { |
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188 my $code = substr($source,$pos[2],$pos[18]-$pos[2]); |
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189 $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line)); |
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190 $code =~ s {^\s*m} { qr} || |
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191 $code =~ s {^\s*/} { qr/} || |
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192 $code =~ s {^\s*qw} { \\qw}; |
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193 $text .= " " if $pos[0] < $pos[2]; |
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194 $text .= "$code)"; |
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195 } |
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196 elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc |
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197 || $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) { |
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198 my $code = filter_blocks($1,line(substr($source,0,pos $source),$line)); |
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199 $text .= ' \\' if $2 eq '%'; |
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200 $text .= " $code)"; |
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201 } |
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202 else { |
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203 die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"; |
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204 } |
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205 |
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206 die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n" |
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207 unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc; |
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208 |
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209 do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)} |
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210 or do { |
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211 if ($source =~ m/\G\s*(?=([};]|\Z))/gc) { |
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212 $casecounter++; |
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213 next component; |
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214 } |
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215 die "Bad $keyword statement (problem in the code block?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n"; |
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216 }; |
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217 my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line)); |
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218 $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/ |
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219 unless $fallthrough; |
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220 $text .= "{ while (1) $code continue { goto C_A_S_E_$casecounter } last S_W_I_T_C_H; C_A_S_E_$casecounter: }"; |
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221 $casecounter++; |
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222 next component; |
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223 } |
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224 |
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225 $source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc; |
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226 $text .= $1; |
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227 } |
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228 $text; |
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229 } |
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230 |
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231 |
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232 |
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233 sub in |
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234 { |
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235 my ($x,$y) = @_; |
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236 my @numy; |
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237 for my $nextx ( @$x ) |
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238 { |
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239 my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0; |
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240 for my $j ( 0..$#$y ) |
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241 { |
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242 my $nexty = $y->[$j]; |
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243 push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0 |
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244 if @numy <= $j; |
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245 return 1 if $numx && $numy[$j] && $nextx==$nexty |
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246 || $nextx eq $nexty; |
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247 |
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248 } |
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249 } |
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250 return ""; |
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251 } |
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252 |
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253 sub on_exists |
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254 { |
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255 my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
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256 [ keys %$ref ] |
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257 } |
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258 |
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259 sub on_defined |
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260 { |
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261 my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ }; |
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262 [ grep { defined $ref->{$_} } keys %$ref ] |
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263 } |
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264 |
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265 sub switch(;$) |
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266 { |
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267 my ($s_val) = @_ ? $_[0] : $_; |
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268 my $s_ref = ref $s_val; |
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269 |
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270 if ($s_ref eq 'CODE') |
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271 { |
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272 $::_S_W_I_T_C_H = |
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273 sub { my $c_val = $_[0]; |
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274 return $s_val == $c_val if ref $c_val eq 'CODE'; |
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275 return $s_val->(@$c_val) if ref $c_val eq 'ARRAY'; |
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276 return $s_val->($c_val); |
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277 }; |
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278 } |
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279 elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR |
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280 { |
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281 $::_S_W_I_T_C_H = |
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282 sub { my $c_val = $_[0]; |
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283 my $c_ref = ref $c_val; |
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284 return $s_val == $c_val if $c_ref eq "" |
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285 && defined $c_val |
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286 && (~$c_val&$c_val) eq 0; |
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287 return $s_val eq $c_val if $c_ref eq ""; |
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288 return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
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289 return $c_val->($s_val) if $c_ref eq 'CODE'; |
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290 return $c_val->call($s_val) if $c_ref eq 'Switch'; |
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291 return scalar $s_val=~/$c_val/ |
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292 if $c_ref eq 'Regexp'; |
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293 return scalar $c_val->{$s_val} |
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294 if $c_ref eq 'HASH'; |
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295 return; |
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296 }; |
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297 } |
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298 elsif ($s_ref eq "") # STRING SCALAR |
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299 { |
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300 $::_S_W_I_T_C_H = |
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301 sub { my $c_val = $_[0]; |
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302 my $c_ref = ref $c_val; |
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303 return $s_val eq $c_val if $c_ref eq ""; |
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304 return in([$s_val],$c_val) if $c_ref eq 'ARRAY'; |
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305 return $c_val->($s_val) if $c_ref eq 'CODE'; |
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306 return $c_val->call($s_val) if $c_ref eq 'Switch'; |
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307 return scalar $s_val=~/$c_val/ |
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308 if $c_ref eq 'Regexp'; |
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309 return scalar $c_val->{$s_val} |
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310 if $c_ref eq 'HASH'; |
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311 return; |
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312 }; |
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313 } |
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314 elsif ($s_ref eq 'ARRAY') |
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315 { |
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316 $::_S_W_I_T_C_H = |
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317 sub { my $c_val = $_[0]; |
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318 my $c_ref = ref $c_val; |
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319 return in($s_val,[$c_val]) if $c_ref eq ""; |
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320 return in($s_val,$c_val) if $c_ref eq 'ARRAY'; |
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321 return $c_val->(@$s_val) if $c_ref eq 'CODE'; |
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322 return $c_val->call(@$s_val) |
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323 if $c_ref eq 'Switch'; |
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324 return scalar grep {$_=~/$c_val/} @$s_val |
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325 if $c_ref eq 'Regexp'; |
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326 return scalar grep {$c_val->{$_}} @$s_val |
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327 if $c_ref eq 'HASH'; |
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328 return; |
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329 }; |
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330 } |
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331 elsif ($s_ref eq 'Regexp') |
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332 { |
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333 $::_S_W_I_T_C_H = |
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334 sub { my $c_val = $_[0]; |
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335 my $c_ref = ref $c_val; |
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336 return $c_val=~/s_val/ if $c_ref eq ""; |
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337 return scalar grep {$_=~/s_val/} @$c_val |
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338 if $c_ref eq 'ARRAY'; |
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339 return $c_val->($s_val) if $c_ref eq 'CODE'; |
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340 return $c_val->call($s_val) if $c_ref eq 'Switch'; |
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341 return $s_val eq $c_val if $c_ref eq 'Regexp'; |
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342 return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val |
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343 if $c_ref eq 'HASH'; |
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344 return; |
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345 }; |
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346 } |
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347 elsif ($s_ref eq 'HASH') |
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348 { |
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349 $::_S_W_I_T_C_H = |
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350 sub { my $c_val = $_[0]; |
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351 my $c_ref = ref $c_val; |
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352 return $s_val->{$c_val} if $c_ref eq ""; |
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353 return scalar grep {$s_val->{$_}} @$c_val |
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354 if $c_ref eq 'ARRAY'; |
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355 return $c_val->($s_val) if $c_ref eq 'CODE'; |
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356 return $c_val->call($s_val) if $c_ref eq 'Switch'; |
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357 return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val |
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358 if $c_ref eq 'Regexp'; |
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359 return $s_val==$c_val if $c_ref eq 'HASH'; |
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360 return; |
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361 }; |
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362 } |
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363 elsif ($s_ref eq 'Switch') |
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364 { |
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365 $::_S_W_I_T_C_H = |
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366 sub { my $c_val = $_[0]; |
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367 return $s_val == $c_val if ref $c_val eq 'Switch'; |
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368 return $s_val->call(@$c_val) |
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369 if ref $c_val eq 'ARRAY'; |
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370 return $s_val->call($c_val); |
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371 }; |
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372 } |
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373 else |
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374 { |
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375 croak "Cannot switch on $s_ref"; |
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376 } |
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377 return 1; |
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378 } |
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379 |
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380 sub case($) { local $SIG{__WARN__} = \&carp; |
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381 $::_S_W_I_T_C_H->(@_); } |
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382 |
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383 # IMPLEMENT __ |
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384 |
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385 my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} }; |
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386 |
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387 sub __() { $placeholder } |
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388 |
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389 sub __arg($) |
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390 { |
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391 my $index = $_[0]+1; |
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392 bless { arity=>0, impl=>sub{$_[$index]} }; |
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393 } |
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394 |
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395 sub hosub(&@) |
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396 { |
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397 # WRITE THIS |
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398 } |
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399 |
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400 sub call |
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401 { |
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402 my ($self,@args) = @_; |
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403 return $self->{impl}->(0,@args); |
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404 } |
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405 |
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406 sub meta_bop(&) |
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407 { |
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408 my ($op) = @_; |
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409 sub |
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410 { |
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411 my ($left, $right, $reversed) = @_; |
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412 ($right,$left) = @_ if $reversed; |
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413 |
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414 my $rop = ref $right eq 'Switch' |
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415 ? $right |
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416 : bless { arity=>0, impl=>sub{$right} }; |
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417 |
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418 my $lop = ref $left eq 'Switch' |
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419 ? $left |
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420 : bless { arity=>0, impl=>sub{$left} }; |
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421 |
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422 my $arity = $lop->{arity} + $rop->{arity}; |
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423 |
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424 return bless { |
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425 arity => $arity, |
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426 impl => sub { my $start = shift; |
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427 return $op->($lop->{impl}->($start,@_), |
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428 $rop->{impl}->($start+$lop->{arity},@_)); |
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429 } |
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430 }; |
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431 }; |
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432 } |
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433 |
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434 sub meta_uop(&) |
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435 { |
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436 my ($op) = @_; |
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437 sub |
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438 { |
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439 my ($left) = @_; |
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440 |
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441 my $lop = ref $left eq 'Switch' |
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442 ? $left |
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443 : bless { arity=>0, impl=>sub{$left} }; |
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444 |
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445 my $arity = $lop->{arity}; |
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446 |
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447 return bless { |
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448 arity => $arity, |
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449 impl => sub { $op->($lop->{impl}->(@_)) } |
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450 }; |
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451 }; |
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452 } |
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453 |
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454 |
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455 use overload |
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456 "+" => meta_bop {$_[0] + $_[1]}, |
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457 "-" => meta_bop {$_[0] - $_[1]}, |
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458 "*" => meta_bop {$_[0] * $_[1]}, |
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459 "/" => meta_bop {$_[0] / $_[1]}, |
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460 "%" => meta_bop {$_[0] % $_[1]}, |
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461 "**" => meta_bop {$_[0] ** $_[1]}, |
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462 "<<" => meta_bop {$_[0] << $_[1]}, |
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463 ">>" => meta_bop {$_[0] >> $_[1]}, |
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464 "x" => meta_bop {$_[0] x $_[1]}, |
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465 "." => meta_bop {$_[0] . $_[1]}, |
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466 "<" => meta_bop {$_[0] < $_[1]}, |
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467 "<=" => meta_bop {$_[0] <= $_[1]}, |
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468 ">" => meta_bop {$_[0] > $_[1]}, |
|
469 ">=" => meta_bop {$_[0] >= $_[1]}, |
|
470 "==" => meta_bop {$_[0] == $_[1]}, |
|
471 "!=" => meta_bop {$_[0] != $_[1]}, |
|
472 "<=>" => meta_bop {$_[0] <=> $_[1]}, |
|
473 "lt" => meta_bop {$_[0] lt $_[1]}, |
|
474 "le" => meta_bop {$_[0] le $_[1]}, |
|
475 "gt" => meta_bop {$_[0] gt $_[1]}, |
|
476 "ge" => meta_bop {$_[0] ge $_[1]}, |
|
477 "eq" => meta_bop {$_[0] eq $_[1]}, |
|
478 "ne" => meta_bop {$_[0] ne $_[1]}, |
|
479 "cmp" => meta_bop {$_[0] cmp $_[1]}, |
|
480 "\&" => meta_bop {$_[0] & $_[1]}, |
|
481 "^" => meta_bop {$_[0] ^ $_[1]}, |
|
482 "|" => meta_bop {$_[0] | $_[1]}, |
|
483 "atan2" => meta_bop {atan2 $_[0], $_[1]}, |
|
484 |
|
485 "neg" => meta_uop {-$_[0]}, |
|
486 "!" => meta_uop {!$_[0]}, |
|
487 "~" => meta_uop {~$_[0]}, |
|
488 "cos" => meta_uop {cos $_[0]}, |
|
489 "sin" => meta_uop {sin $_[0]}, |
|
490 "exp" => meta_uop {exp $_[0]}, |
|
491 "abs" => meta_uop {abs $_[0]}, |
|
492 "log" => meta_uop {log $_[0]}, |
|
493 "sqrt" => meta_uop {sqrt $_[0]}, |
|
494 "bool" => sub { croak "Can't use && or || in expression containing __" }, |
|
495 |
|
496 # "&()" => sub { $_[0]->{impl} }, |
|
497 |
|
498 # "||" => meta_bop {$_[0] || $_[1]}, |
|
499 # "&&" => meta_bop {$_[0] && $_[1]}, |
|
500 # fallback => 1, |
|
501 ; |
|
502 1; |
|
503 |
|
504 __END__ |
|
505 |
|
506 |
|
507 =head1 NAME |
|
508 |
|
509 Switch - A switch statement for Perl |
|
510 |
|
511 =head1 SYNOPSIS |
|
512 |
|
513 use Switch; |
|
514 |
|
515 switch ($val) { |
|
516 case 1 { print "number 1" } |
|
517 case "a" { print "string a" } |
|
518 case [1..10,42] { print "number in list" } |
|
519 case (\@array) { print "number in list" } |
|
520 case /\w+/ { print "pattern" } |
|
521 case qr/\w+/ { print "pattern" } |
|
522 case (\%hash) { print "entry in hash" } |
|
523 case (\&sub) { print "arg to subroutine" } |
|
524 else { print "previous case not true" } |
|
525 } |
|
526 |
|
527 =head1 BACKGROUND |
|
528 |
|
529 [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys |
|
530 and wherefores of this control structure] |
|
531 |
|
532 In seeking to devise a "Swiss Army" case mechanism suitable for Perl, |
|
533 it is useful to generalize this notion of distributed conditional |
|
534 testing as far as possible. Specifically, the concept of "matching" |
|
535 between the switch value and the various case values need not be |
|
536 restricted to numeric (or string or referential) equality, as it is in other |
|
537 languages. Indeed, as Table 1 illustrates, Perl |
|
538 offers at least eighteen different ways in which two values could |
|
539 generate a match. |
|
540 |
|
541 Table 1: Matching a switch value ($s) with a case value ($c) |
|
542 |
|
543 Switch Case Type of Match Implied Matching Code |
|
544 Value Value |
|
545 ====== ===== ===================== ============= |
|
546 |
|
547 number same numeric or referential match if $s == $c; |
|
548 or ref equality |
|
549 |
|
550 object method result of method call match if $s->$c(); |
|
551 ref name match if defined $s->$c(); |
|
552 or ref |
|
553 |
|
554 other other string equality match if $s eq $c; |
|
555 non-ref non-ref |
|
556 scalar scalar |
|
557 |
|
558 string regexp pattern match match if $s =~ /$c/; |
|
559 |
|
560 array scalar array entry existence match if 0<=$c && $c<@$s; |
|
561 ref array entry definition match if defined $s->[$c]; |
|
562 array entry truth match if $s->[$c]; |
|
563 |
|
564 array array array intersection match if intersects(@$s, @$c); |
|
565 ref ref (apply this table to |
|
566 all pairs of elements |
|
567 $s->[$i] and |
|
568 $c->[$j]) |
|
569 |
|
570 array regexp array grep match if grep /$c/, @$s; |
|
571 ref |
|
572 |
|
573 hash scalar hash entry existence match if exists $s->{$c}; |
|
574 ref hash entry definition match if defined $s->{$c}; |
|
575 hash entry truth match if $s->{$c}; |
|
576 |
|
577 hash regexp hash grep match if grep /$c/, keys %$s; |
|
578 ref |
|
579 |
|
580 sub scalar return value defn match if defined $s->($c); |
|
581 ref return value truth match if $s->($c); |
|
582 |
|
583 sub array return value defn match if defined $s->(@$c); |
|
584 ref ref return value truth match if $s->(@$c); |
|
585 |
|
586 |
|
587 In reality, Table 1 covers 31 alternatives, because only the equality and |
|
588 intersection tests are commutative; in all other cases, the roles of |
|
589 the C<$s> and C<$c> variables could be reversed to produce a |
|
590 different test. For example, instead of testing a single hash for |
|
591 the existence of a series of keys (C<match if exists $s-E<gt>{$c}>), |
|
592 one could test for the existence of a single key in a series of hashes |
|
593 (C<match if exists $c-E<gt>{$s}>). |
|
594 |
|
595 =head1 DESCRIPTION |
|
596 |
|
597 The Switch.pm module implements a generalized case mechanism that covers |
|
598 most (but not all) of the numerous possible combinations of switch and case |
|
599 values described above. |
|
600 |
|
601 The module augments the standard Perl syntax with two new control |
|
602 statements: C<switch> and C<case>. The C<switch> statement takes a |
|
603 single scalar argument of any type, specified in parentheses. |
|
604 C<switch> stores this value as the |
|
605 current switch value in a (localized) control variable. |
|
606 The value is followed by a block which may contain one or more |
|
607 Perl statements (including the C<case> statement described below). |
|
608 The block is unconditionally executed once the switch value has |
|
609 been cached. |
|
610 |
|
611 A C<case> statement takes a single scalar argument (in mandatory |
|
612 parentheses if it's a variable; otherwise the parens are optional) and |
|
613 selects the appropriate type of matching between that argument and the |
|
614 current switch value. The type of matching used is determined by the |
|
615 respective types of the switch value and the C<case> argument, as |
|
616 specified in Table 1. If the match is successful, the mandatory |
|
617 block associated with the C<case> statement is executed. |
|
618 |
|
619 In most other respects, the C<case> statement is semantically identical |
|
620 to an C<if> statement. For example, it can be followed by an C<else> |
|
621 clause, and can be used as a postfix statement qualifier. |
|
622 |
|
623 However, when a C<case> block has been executed control is automatically |
|
624 transferred to the statement after the immediately enclosing C<switch> |
|
625 block, rather than to the next statement within the block. In other |
|
626 words, the success of any C<case> statement prevents other cases in the |
|
627 same scope from executing. But see L<"Allowing fall-through"> below. |
|
628 |
|
629 Together these two new statements provide a fully generalized case |
|
630 mechanism: |
|
631 |
|
632 use Switch; |
|
633 |
|
634 # AND LATER... |
|
635 |
|
636 %special = ( woohoo => 1, d'oh => 1 ); |
|
637 |
|
638 while (<>) { |
|
639 chomp; |
|
640 switch ($_) { |
|
641 case (%special) { print "homer\n"; } # if $special{$_} |
|
642 case /[a-z]/i { print "alpha\n"; } # if $_ =~ /a-z/i |
|
643 case [1..9] { print "small num\n"; } # if $_ in [1..9] |
|
644 case { $_[0] >= 10 } { print "big num\n"; } # if $_ >= 10 |
|
645 print "must be punctuation\n" case /\W/; # if $_ ~= /\W/ |
|
646 } |
|
647 } |
|
648 |
|
649 Note that C<switch>es can be nested within C<case> (or any other) blocks, |
|
650 and a series of C<case> statements can try different types of matches |
|
651 -- hash membership, pattern match, array intersection, simple equality, |
|
652 etc. -- against the same switch value. |
|
653 |
|
654 The use of intersection tests against an array reference is particularly |
|
655 useful for aggregating integral cases: |
|
656 |
|
657 sub classify_digit |
|
658 { |
|
659 switch ($_[0]) { case 0 { return 'zero' } |
|
660 case [2,4,6,8] { return 'even' } |
|
661 case [1,3,5,7,9] { return 'odd' } |
|
662 case /[A-F]/i { return 'hex' } |
|
663 } |
|
664 } |
|
665 |
|
666 |
|
667 =head2 Allowing fall-through |
|
668 |
|
669 Fall-though (trying another case after one has already succeeded) |
|
670 is usually a Bad Idea in a switch statement. However, this |
|
671 is Perl, not a police state, so there I<is> a way to do it, if you must. |
|
672 |
|
673 If a C<case> block executes an untargeted C<next>, control is |
|
674 immediately transferred to the statement I<after> the C<case> statement |
|
675 (i.e. usually another case), rather than out of the surrounding |
|
676 C<switch> block. |
|
677 |
|
678 For example: |
|
679 |
|
680 switch ($val) { |
|
681 case 1 { handle_num_1(); next } # and try next case... |
|
682 case "1" { handle_str_1(); next } # and try next case... |
|
683 case [0..9] { handle_num_any(); } # and we're done |
|
684 case /\d/ { handle_dig_any(); next } # and try next case... |
|
685 case /.*/ { handle_str_any(); next } # and try next case... |
|
686 } |
|
687 |
|
688 If $val held the number C<1>, the above C<switch> block would call the |
|
689 first three C<handle_...> subroutines, jumping to the next case test |
|
690 each time it encountered a C<next>. After the third C<case> block |
|
691 was executed, control would jump to the end of the enclosing |
|
692 C<switch> block. |
|
693 |
|
694 On the other hand, if $val held C<10>, then only the last two C<handle_...> |
|
695 subroutines would be called. |
|
696 |
|
697 Note that this mechanism allows the notion of I<conditional fall-through>. |
|
698 For example: |
|
699 |
|
700 switch ($val) { |
|
701 case [0..9] { handle_num_any(); next if $val < 7; } |
|
702 case /\d/ { handle_dig_any(); } |
|
703 } |
|
704 |
|
705 If an untargeted C<last> statement is executed in a case block, this |
|
706 immediately transfers control out of the enclosing C<switch> block |
|
707 (in other words, there is an implicit C<last> at the end of each |
|
708 normal C<case> block). Thus the previous example could also have been |
|
709 written: |
|
710 |
|
711 switch ($val) { |
|
712 case [0..9] { handle_num_any(); last if $val >= 7; next; } |
|
713 case /\d/ { handle_dig_any(); } |
|
714 } |
|
715 |
|
716 |
|
717 =head2 Automating fall-through |
|
718 |
|
719 In situations where case fall-through should be the norm, rather than an |
|
720 exception, an endless succession of terminal C<next>s is tedious and ugly. |
|
721 Hence, it is possible to reverse the default behaviour by specifying |
|
722 the string "fallthrough" when importing the module. For example, the |
|
723 following code is equivalent to the first example in L<"Allowing fall-through">: |
|
724 |
|
725 use Switch 'fallthrough'; |
|
726 |
|
727 switch ($val) { |
|
728 case 1 { handle_num_1(); } |
|
729 case "1" { handle_str_1(); } |
|
730 case [0..9] { handle_num_any(); last } |
|
731 case /\d/ { handle_dig_any(); } |
|
732 case /.*/ { handle_str_any(); } |
|
733 } |
|
734 |
|
735 Note the explicit use of a C<last> to preserve the non-fall-through |
|
736 behaviour of the third case. |
|
737 |
|
738 |
|
739 |
|
740 =head2 Alternative syntax |
|
741 |
|
742 Perl 6 will provide a built-in switch statement with essentially the |
|
743 same semantics as those offered by Switch.pm, but with a different |
|
744 pair of keywords. In Perl 6 C<switch> will be spelled C<given>, and |
|
745 C<case> will be pronounced C<when>. In addition, the C<when> statement |
|
746 will not require switch or case values to be parenthesized. |
|
747 |
|
748 This future syntax is also (largely) available via the Switch.pm module, by |
|
749 importing it with the argument C<"Perl6">. For example: |
|
750 |
|
751 use Switch 'Perl6'; |
|
752 |
|
753 given ($val) { |
|
754 when 1 { handle_num_1(); } |
|
755 when ($str1) { handle_str_1(); } |
|
756 when [0..9] { handle_num_any(); last } |
|
757 when /\d/ { handle_dig_any(); } |
|
758 when /.*/ { handle_str_any(); } |
|
759 default { handle anything else; } |
|
760 } |
|
761 |
|
762 Note that scalars still need to be parenthesized, since they would be |
|
763 ambiguous in Perl 5. |
|
764 |
|
765 Note too that you can mix and match both syntaxes by importing the module |
|
766 with: |
|
767 |
|
768 use Switch 'Perl5', 'Perl6'; |
|
769 |
|
770 |
|
771 =head2 Higher-order Operations |
|
772 |
|
773 One situation in which C<switch> and C<case> do not provide a good |
|
774 substitute for a cascaded C<if>, is where a switch value needs to |
|
775 be tested against a series of conditions. For example: |
|
776 |
|
777 sub beverage { |
|
778 switch (shift) { |
|
779 case { $_[0] < 10 } { return 'milk' } |
|
780 case { $_[0] < 20 } { return 'coke' } |
|
781 case { $_[0] < 30 } { return 'beer' } |
|
782 case { $_[0] < 40 } { return 'wine' } |
|
783 case { $_[0] < 50 } { return 'malt' } |
|
784 case { $_[0] < 60 } { return 'Moet' } |
|
785 else { return 'milk' } |
|
786 } |
|
787 } |
|
788 |
|
789 (This is equivalent to writing C<case (sub { $_[0] < 10 })>, etc.; C<$_[0]> |
|
790 is the argument to the anonymous subroutine.) |
|
791 |
|
792 The need to specify each condition as a subroutine block is tiresome. To |
|
793 overcome this, when importing Switch.pm, a special "placeholder" |
|
794 subroutine named C<__> [sic] may also be imported. This subroutine |
|
795 converts (almost) any expression in which it appears to a reference to a |
|
796 higher-order function. That is, the expression: |
|
797 |
|
798 use Switch '__'; |
|
799 |
|
800 __ < 2 |
|
801 |
|
802 is equivalent to: |
|
803 |
|
804 sub { $_[0] < 2 } |
|
805 |
|
806 With C<__>, the previous ugly case statements can be rewritten: |
|
807 |
|
808 case __ < 10 { return 'milk' } |
|
809 case __ < 20 { return 'coke' } |
|
810 case __ < 30 { return 'beer' } |
|
811 case __ < 40 { return 'wine' } |
|
812 case __ < 50 { return 'malt' } |
|
813 case __ < 60 { return 'Moet' } |
|
814 else { return 'milk' } |
|
815 |
|
816 The C<__> subroutine makes extensive use of operator overloading to |
|
817 perform its magic. All operations involving __ are overloaded to |
|
818 produce an anonymous subroutine that implements a lazy version |
|
819 of the original operation. |
|
820 |
|
821 The only problem is that operator overloading does not allow the |
|
822 boolean operators C<&&> and C<||> to be overloaded. So a case statement |
|
823 like this: |
|
824 |
|
825 case 0 <= __ && __ < 10 { return 'digit' } |
|
826 |
|
827 doesn't act as expected, because when it is |
|
828 executed, it constructs two higher order subroutines |
|
829 and then treats the two resulting references as arguments to C<&&>: |
|
830 |
|
831 sub { 0 <= $_[0] } && sub { $_[0] < 10 } |
|
832 |
|
833 This boolean expression is inevitably true, since both references are |
|
834 non-false. Fortunately, the overloaded C<'bool'> operator catches this |
|
835 situation and flags it as an error. |
|
836 |
|
837 =head1 DEPENDENCIES |
|
838 |
|
839 The module is implemented using Filter::Util::Call and Text::Balanced |
|
840 and requires both these modules to be installed. |
|
841 |
|
842 =head1 AUTHOR |
|
843 |
|
844 Damian Conway (damian@conway.org). This module is now maintained by Rafael |
|
845 Garcia-Suarez (rgarciasuarez@gmail.com) and more generally by the Perl 5 |
|
846 Porters (perl5-porters@perl.org), as part of the Perl core. |
|
847 |
|
848 =head1 BUGS |
|
849 |
|
850 There are undoubtedly serious bugs lurking somewhere in code this funky :-) |
|
851 Bug reports and other feedback are most welcome. |
|
852 |
|
853 =head1 LIMITATIONS |
|
854 |
|
855 Due to the heuristic nature of Switch.pm's source parsing, the presence of |
|
856 regexes with embedded newlines that are specified with raw C</.../> |
|
857 delimiters and don't have a modifier C<//x> are indistinguishable from |
|
858 code chunks beginning with the division operator C</>. As a workaround |
|
859 you must use C<m/.../> or C<m?...?> for such patterns. Also, the presence |
|
860 of regexes specified with raw C<?...?> delimiters may cause mysterious |
|
861 errors. The workaround is to use C<m?...?> instead. |
|
862 |
|
863 Due to the way source filters work in Perl, you can't use Switch inside |
|
864 an string C<eval>. |
|
865 |
|
866 If your source file is longer then 1 million characters and you have a |
|
867 switch statement that crosses the 1 million (or 2 million, etc.) |
|
868 character boundary you will get mysterious errors. The workaround is to |
|
869 use smaller source files. |
|
870 |
|
871 =head1 COPYRIGHT |
|
872 |
|
873 Copyright (c) 1997-2008, Damian Conway. All Rights Reserved. |
|
874 This module is free software. It may be used, redistributed |
|
875 and/or modified under the same terms as Perl itself. |
|