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 | $VERSION = '2.10_01';
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8 |
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9 |
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10 | # LOAD FILTERING MODULE...
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11 | use Filter::Util::Call;
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12 |
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13 | sub __();
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14 |
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15 | # CATCH ATTEMPTS TO CALL case OUTSIDE THE SCOPE OF ANY switch
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16 |
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17 | $::_S_W_I_T_C_H = sub { croak "case/when statement not in switch/given block" };
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18 |
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19 | my $offset;
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20 | my $fallthrough;
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21 | my ($Perl5, $Perl6) = (0,0);
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22 |
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23 | sub import
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24 | {
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25 | $fallthrough = grep /\bfallthrough\b/, @_;
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26 | $offset = (caller)[2]+1;
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27 | filter_add({}) unless @_>1 && $_[1] eq 'noimport';
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28 | my $pkg = caller;
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29 | no strict 'refs';
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30 | for ( qw( on_defined on_exists ) )
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31 | {
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32 | *{"${pkg}::$_"} = \&$_;
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33 | }
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34 | *{"${pkg}::__"} = \&__ if grep /__/, @_;
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35 | $Perl6 = 1 if grep(/Perl\s*6/i, @_);
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36 | $Perl5 = 1 if grep(/Perl\s*5/i, @_) || !grep(/Perl\s*6/i, @_);
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37 | 1;
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38 | }
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39 |
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40 | sub unimport
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41 | {
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42 | filter_del()
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43 | }
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44 |
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45 | sub filter
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46 | {
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47 | my($self) = @_ ;
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48 | local $Switch::file = (caller)[1];
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49 |
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50 | my $status = 1;
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51 | $status = filter_read(1_000_000);
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52 | return $status if $status<0;
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53 | $_ = filter_blocks($_,$offset);
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54 | $_ = "# line $offset\n" . $_ if $offset; undef $offset;
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55 | return $status;
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56 | }
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57 |
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58 | use Text::Balanced ':ALL';
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59 |
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60 | sub line
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61 | {
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62 | my ($pretext,$offset) = @_;
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63 | ($pretext=~tr/\n/\n/)+($offset||0);
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64 | }
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65 |
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66 | sub is_block
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67 | {
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68 | local $SIG{__WARN__}=sub{die$@};
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69 | local $^W=1;
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70 | my $ishash = defined eval 'my $hr='.$_[0];
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71 | undef $@;
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72 | return !$ishash;
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73 | }
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74 |
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75 |
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76 | my $EOP = qr/\n\n|\Z/;
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77 | my $CUT = qr/\n=cut.*$EOP/;
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78 | my $pod_or_DATA = qr/ ^=(?:head[1-4]|item) .*? $CUT
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79 | | ^=pod .*? $CUT
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80 | | ^=for .*? $EOP
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81 | | ^=begin \s* (\S+) .*? \n=end \s* \1 .*? $EOP
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82 | | ^__(DATA|END)__\n.*
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83 | /smx;
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84 |
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85 | my $casecounter = 1;
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86 | sub filter_blocks
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87 | {
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88 | my ($source, $line) = @_;
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89 | return $source unless $Perl5 && $source =~ /case|switch/
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90 | || $Perl6 && $source =~ /when|given|default/;
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91 | pos $source = 0;
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92 | my $text = "";
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93 | component: while (pos $source < length $source)
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94 | {
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95 | if ($source =~ m/(\G\s*use\s+Switch\b)/gc)
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96 | {
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97 | $text .= q{use Switch 'noimport'};
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98 | next component;
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99 | }
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100 | my @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0);
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101 | if (defined $pos[0])
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102 | {
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103 | my $pre = substr($source,$pos[0],$pos[1]); # matched prefix
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104 | $text .= $pre . substr($source,$pos[2],$pos[18]-$pos[2]);
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105 | next component;
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106 | }
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107 | if ($source =~ m/\G\s*($pod_or_DATA)/gc) {
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108 | next component;
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109 | }
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110 | @pos = Text::Balanced::_match_variable(\$source,qr/\s*/);
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111 | if (defined $pos[0])
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112 | {
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113 | $text .= " " if $pos[0] < $pos[2];
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114 | $text .= substr($source,$pos[0],$pos[4]-$pos[0]);
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115 | next component;
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116 | }
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117 |
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118 | if ($Perl5 && $source =~ m/\G(\n*)(\s*)(switch)\b(?=\s*[(])/gc
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119 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(?=\s*[(])/gc
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120 | || $Perl6 && $source =~ m/\G(\n*)(\s*)(given)\b(.*)(?=\{)/gc)
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121 | {
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122 | my $keyword = $3;
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123 | my $arg = $4;
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124 | $text .= $1.$2.'S_W_I_T_C_H: while (1) ';
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125 | unless ($arg) {
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126 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\(/,qr/\)/,qr/[[{(<]/,qr/[]})>]/,undef)
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127 | or do {
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128 | die "Bad $keyword statement (problem in the parentheses?) near $Switch::file line ", line(substr($source,0,pos $source),$line), "\n";
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129 | };
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130 | $arg = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));
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131 | }
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132 | $arg =~ s {^\s*[(]\s*%} { ( \\\%} ||
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133 | $arg =~ s {^\s*[(]\s*m\b} { ( qr} ||
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134 | $arg =~ s {^\s*[(]\s*/} { ( qr/} ||
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135 | $arg =~ s {^\s*[(]\s*qw} { ( \\qw};
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136 | @pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)
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137 | or do {
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138 | 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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139 | };
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140 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));
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141 | $code =~ s/{/{ local \$::_S_W_I_T_C_H; Switch::switch $arg;/;
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142 | $text .= $code . 'continue {last}';
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143 | next component;
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144 | }
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145 | elsif ($Perl5 && $source =~ m/\G(\s*)(case\b)(?!\s*=>)/gc
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146 | || $Perl6 && $source =~ m/\G(\s*)(when\b)(?!\s*=>)/gc
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147 | || $Perl6 && $source =~ m/\G(\s*)(default\b)(?=\s*\{)/gc)
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148 | {
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149 | my $keyword = $2;
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150 | $text .= $1 . ($keyword eq "default"
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151 | ? "if (1)"
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152 | : "if (Switch::case");
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153 |
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154 | if ($keyword eq "default") {
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155 | # Nothing to do
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156 | }
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157 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)) {
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158 | my $code = substr($source,$pos[0],$pos[4]-$pos[0]);
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159 | $text .= " " if $pos[0] < $pos[2];
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160 | $text .= "sub " if is_block $code;
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161 | $text .= filter_blocks($code,line(substr($source,0,$pos[0]),$line)) . ")";
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162 | }
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163 | elsif (@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/[[(]/,qr/[])]/,qr/[[({]/,qr/[])}]/,undef)) {
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164 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));
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165 | $code =~ s {^\s*[(]\s*%} { ( \\\%} ||
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166 | $code =~ s {^\s*[(]\s*m\b} { ( qr} ||
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167 | $code =~ s {^\s*[(]\s*/} { ( qr/} ||
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168 | $code =~ s {^\s*[(]\s*qw} { ( \\qw};
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169 | $text .= " " if $pos[0] < $pos[2];
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170 | $text .= "$code)";
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171 | }
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172 | elsif ($Perl6 && do{@pos = Text::Balanced::_match_variable(\$source,qr/\s*/)}) {
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173 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));
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174 | $code =~ s {^\s*%} { \%} ||
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175 | $code =~ s {^\s*@} { \@};
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176 | $text .= " " if $pos[0] < $pos[2];
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177 | $text .= "$code)";
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178 | }
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179 | elsif ( @pos = Text::Balanced::_match_quotelike(\$source,qr/\s*/,1,0)) {
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180 | my $code = substr($source,$pos[2],$pos[18]-$pos[2]);
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181 | $code = filter_blocks($code,line(substr($source,0,$pos[2]),$line));
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182 | $code =~ s {^\s*m} { qr} ||
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183 | $code =~ s {^\s*/} { qr/} ||
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184 | $code =~ s {^\s*qw} { \\qw};
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185 | $text .= " " if $pos[0] < $pos[2];
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186 | $text .= "$code)";
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187 | }
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188 | elsif ($Perl5 && $source =~ m/\G\s*(([^\$\@{])[^\$\@{]*)(?=\s*{)/gc
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189 | || $Perl6 && $source =~ m/\G\s*([^;{]*)()/gc) {
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190 | my $code = filter_blocks($1,line(substr($source,0,pos $source),$line));
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191 | $text .= ' \\' if $2 eq '%';
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192 | $text .= " $code)";
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193 | }
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194 | else {
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195 | die "Bad $keyword statement (invalid $keyword value?) near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n";
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196 | }
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197 |
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198 | die "Missing opening brace or semi-colon after 'when' value near $Switch::file line ", line(substr($source,0,pos $source), $line), "\n"
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199 | unless !$Perl6 || $source =~ m/\G(\s*)(?=;|\{)/gc;
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200 |
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201 | do{@pos = Text::Balanced::_match_codeblock(\$source,qr/\s*/,qr/\{/,qr/\}/,qr/\{/,qr/\}/,undef)}
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202 | or do {
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203 | if ($source =~ m/\G\s*(?=([};]|\Z))/gc) {
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204 | $casecounter++;
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205 | next component;
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206 | }
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207 | 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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208 | };
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209 | my $code = filter_blocks(substr($source,$pos[0],$pos[4]-$pos[0]),line(substr($source,0,$pos[0]),$line));
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210 | $code =~ s/}(?=\s*\Z)/;last S_W_I_T_C_H }/
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211 | unless $fallthrough;
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212 | $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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213 | $casecounter++;
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214 | next component;
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215 | }
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216 |
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217 | $source =~ m/\G(\s*(-[sm]\s+|\w+|#.*\n|\W))/gc;
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218 | $text .= $1;
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219 | }
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220 | $text;
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221 | }
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222 |
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223 |
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224 |
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225 | sub in
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226 | {
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227 | my ($x,$y) = @_;
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228 | my @numy;
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229 | for my $nextx ( @$x )
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230 | {
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231 | my $numx = ref($nextx) || defined $nextx && (~$nextx&$nextx) eq 0;
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232 | for my $j ( 0..$#$y )
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233 | {
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234 | my $nexty = $y->[$j];
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235 | push @numy, ref($nexty) || defined $nexty && (~$nexty&$nexty) eq 0
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236 | if @numy <= $j;
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237 | return 1 if $numx && $numy[$j] && $nextx==$nexty
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238 | || $nextx eq $nexty;
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239 |
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240 | }
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241 | }
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242 | return "";
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243 | }
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244 |
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245 | sub on_exists
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246 | {
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247 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ };
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248 | [ keys %$ref ]
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249 | }
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250 |
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251 | sub on_defined
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252 | {
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253 | my $ref = @_==1 && ref($_[0]) eq 'HASH' ? $_[0] : { @_ };
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254 | [ grep { defined $ref->{$_} } keys %$ref ]
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255 | }
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256 |
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257 | sub switch(;$)
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258 | {
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259 | my ($s_val) = @_ ? $_[0] : $_;
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260 | my $s_ref = ref $s_val;
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261 |
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262 | if ($s_ref eq 'CODE')
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263 | {
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264 | $::_S_W_I_T_C_H =
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265 | sub { my $c_val = $_[0];
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266 | return $s_val == $c_val if ref $c_val eq 'CODE';
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267 | return $s_val->(@$c_val) if ref $c_val eq 'ARRAY';
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268 | return $s_val->($c_val);
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269 | };
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270 | }
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271 | elsif ($s_ref eq "" && defined $s_val && (~$s_val&$s_val) eq 0) # NUMERIC SCALAR
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272 | {
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273 | $::_S_W_I_T_C_H =
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274 | sub { my $c_val = $_[0];
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275 | my $c_ref = ref $c_val;
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276 | return $s_val == $c_val if $c_ref eq ""
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277 | && defined $c_val
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278 | && (~$c_val&$c_val) eq 0;
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279 | return $s_val eq $c_val if $c_ref eq "";
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280 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY';
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281 | return $c_val->($s_val) if $c_ref eq 'CODE';
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282 | return $c_val->call($s_val) if $c_ref eq 'Switch';
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283 | return scalar $s_val=~/$c_val/
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284 | if $c_ref eq 'Regexp';
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285 | return scalar $c_val->{$s_val}
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286 | if $c_ref eq 'HASH';
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287 | return;
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288 | };
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289 | }
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290 | elsif ($s_ref eq "") # STRING SCALAR
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291 | {
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292 | $::_S_W_I_T_C_H =
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293 | sub { my $c_val = $_[0];
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294 | my $c_ref = ref $c_val;
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295 | return $s_val eq $c_val if $c_ref eq "";
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296 | return in([$s_val],$c_val) if $c_ref eq 'ARRAY';
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297 | return $c_val->($s_val) if $c_ref eq 'CODE';
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298 | return $c_val->call($s_val) if $c_ref eq 'Switch';
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299 | return scalar $s_val=~/$c_val/
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300 | if $c_ref eq 'Regexp';
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301 | return scalar $c_val->{$s_val}
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302 | if $c_ref eq 'HASH';
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303 | return;
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304 | };
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305 | }
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306 | elsif ($s_ref eq 'ARRAY')
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307 | {
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308 | $::_S_W_I_T_C_H =
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309 | sub { my $c_val = $_[0];
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310 | my $c_ref = ref $c_val;
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311 | return in($s_val,[$c_val]) if $c_ref eq "";
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312 | return in($s_val,$c_val) if $c_ref eq 'ARRAY';
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313 | return $c_val->(@$s_val) if $c_ref eq 'CODE';
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314 | return $c_val->call(@$s_val)
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315 | if $c_ref eq 'Switch';
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316 | return scalar grep {$_=~/$c_val/} @$s_val
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317 | if $c_ref eq 'Regexp';
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318 | return scalar grep {$c_val->{$_}} @$s_val
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319 | if $c_ref eq 'HASH';
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320 | return;
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321 | };
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322 | }
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323 | elsif ($s_ref eq 'Regexp')
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324 | {
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325 | $::_S_W_I_T_C_H =
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326 | sub { my $c_val = $_[0];
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327 | my $c_ref = ref $c_val;
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328 | return $c_val=~/s_val/ if $c_ref eq "";
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329 | return scalar grep {$_=~/s_val/} @$c_val
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330 | if $c_ref eq 'ARRAY';
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331 | return $c_val->($s_val) if $c_ref eq 'CODE';
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332 | return $c_val->call($s_val) if $c_ref eq 'Switch';
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333 | return $s_val eq $c_val if $c_ref eq 'Regexp';
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334 | return grep {$_=~/$s_val/ && $c_val->{$_}} keys %$c_val
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335 | if $c_ref eq 'HASH';
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336 | return;
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337 | };
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338 | }
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339 | elsif ($s_ref eq 'HASH')
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340 | {
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341 | $::_S_W_I_T_C_H =
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342 | sub { my $c_val = $_[0];
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343 | my $c_ref = ref $c_val;
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344 | return $s_val->{$c_val} if $c_ref eq "";
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345 | return scalar grep {$s_val->{$_}} @$c_val
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346 | if $c_ref eq 'ARRAY';
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347 | return $c_val->($s_val) if $c_ref eq 'CODE';
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348 | return $c_val->call($s_val) if $c_ref eq 'Switch';
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349 | return grep {$_=~/$c_val/ && $s_val->{"$_"}} keys %$s_val
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350 | if $c_ref eq 'Regexp';
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351 | return $s_val==$c_val if $c_ref eq 'HASH';
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352 | return;
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353 | };
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354 | }
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355 | elsif ($s_ref eq 'Switch')
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356 | {
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357 | $::_S_W_I_T_C_H =
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358 | sub { my $c_val = $_[0];
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359 | return $s_val == $c_val if ref $c_val eq 'Switch';
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360 | return $s_val->call(@$c_val)
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361 | if ref $c_val eq 'ARRAY';
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362 | return $s_val->call($c_val);
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363 | };
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364 | }
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365 | else
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366 | {
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367 | croak "Cannot switch on $s_ref";
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368 | }
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369 | return 1;
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370 | }
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371 |
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372 | sub case($) { local $SIG{__WARN__} = \&carp;
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373 | $::_S_W_I_T_C_H->(@_); }
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374 |
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375 | # IMPLEMENT __
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376 |
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377 | my $placeholder = bless { arity=>1, impl=>sub{$_[1+$_[0]]} };
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378 |
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379 | sub __() { $placeholder }
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380 |
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381 | sub __arg($)
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382 | {
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383 | my $index = $_[0]+1;
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384 | bless { arity=>0, impl=>sub{$_[$index]} };
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385 | }
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386 |
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387 | sub hosub(&@)
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388 | {
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389 | # WRITE THIS
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390 | }
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391 |
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392 | sub call
|
---|
393 | {
|
---|
394 | my ($self,@args) = @_;
|
---|
395 | return $self->{impl}->(0,@args);
|
---|
396 | }
|
---|
397 |
|
---|
398 | sub meta_bop(&)
|
---|
399 | {
|
---|
400 | my ($op) = @_;
|
---|
401 | sub
|
---|
402 | {
|
---|
403 | my ($left, $right, $reversed) = @_;
|
---|
404 | ($right,$left) = @_ if $reversed;
|
---|
405 |
|
---|
406 | my $rop = ref $right eq 'Switch'
|
---|
407 | ? $right
|
---|
408 | : bless { arity=>0, impl=>sub{$right} };
|
---|
409 |
|
---|
410 | my $lop = ref $left eq 'Switch'
|
---|
411 | ? $left
|
---|
412 | : bless { arity=>0, impl=>sub{$left} };
|
---|
413 |
|
---|
414 | my $arity = $lop->{arity} + $rop->{arity};
|
---|
415 |
|
---|
416 | return bless {
|
---|
417 | arity => $arity,
|
---|
418 | impl => sub { my $start = shift;
|
---|
419 | return $op->($lop->{impl}->($start,@_),
|
---|
420 | $rop->{impl}->($start+$lop->{arity},@_));
|
---|
421 | }
|
---|
422 | };
|
---|
423 | };
|
---|
424 | }
|
---|
425 |
|
---|
426 | sub meta_uop(&)
|
---|
427 | {
|
---|
428 | my ($op) = @_;
|
---|
429 | sub
|
---|
430 | {
|
---|
431 | my ($left) = @_;
|
---|
432 |
|
---|
433 | my $lop = ref $left eq 'Switch'
|
---|
434 | ? $left
|
---|
435 | : bless { arity=>0, impl=>sub{$left} };
|
---|
436 |
|
---|
437 | my $arity = $lop->{arity};
|
---|
438 |
|
---|
439 | return bless {
|
---|
440 | arity => $arity,
|
---|
441 | impl => sub { $op->($lop->{impl}->(@_)) }
|
---|
442 | };
|
---|
443 | };
|
---|
444 | }
|
---|
445 |
|
---|
446 |
|
---|
447 | use overload
|
---|
448 | "+" => meta_bop {$_[0] + $_[1]},
|
---|
449 | "-" => meta_bop {$_[0] - $_[1]},
|
---|
450 | "*" => meta_bop {$_[0] * $_[1]},
|
---|
451 | "/" => meta_bop {$_[0] / $_[1]},
|
---|
452 | "%" => meta_bop {$_[0] % $_[1]},
|
---|
453 | "**" => meta_bop {$_[0] ** $_[1]},
|
---|
454 | "<<" => meta_bop {$_[0] << $_[1]},
|
---|
455 | ">>" => meta_bop {$_[0] >> $_[1]},
|
---|
456 | "x" => meta_bop {$_[0] x $_[1]},
|
---|
457 | "." => meta_bop {$_[0] . $_[1]},
|
---|
458 | "<" => meta_bop {$_[0] < $_[1]},
|
---|
459 | "<=" => meta_bop {$_[0] <= $_[1]},
|
---|
460 | ">" => meta_bop {$_[0] > $_[1]},
|
---|
461 | ">=" => meta_bop {$_[0] >= $_[1]},
|
---|
462 | "==" => meta_bop {$_[0] == $_[1]},
|
---|
463 | "!=" => meta_bop {$_[0] != $_[1]},
|
---|
464 | "<=>" => meta_bop {$_[0] <=> $_[1]},
|
---|
465 | "lt" => meta_bop {$_[0] lt $_[1]},
|
---|
466 | "le" => meta_bop {$_[0] le $_[1]},
|
---|
467 | "gt" => meta_bop {$_[0] gt $_[1]},
|
---|
468 | "ge" => meta_bop {$_[0] ge $_[1]},
|
---|
469 | "eq" => meta_bop {$_[0] eq $_[1]},
|
---|
470 | "ne" => meta_bop {$_[0] ne $_[1]},
|
---|
471 | "cmp" => meta_bop {$_[0] cmp $_[1]},
|
---|
472 | "\&" => meta_bop {$_[0] & $_[1]},
|
---|
473 | "^" => meta_bop {$_[0] ^ $_[1]},
|
---|
474 | "|" => meta_bop {$_[0] | $_[1]},
|
---|
475 | "atan2" => meta_bop {atan2 $_[0], $_[1]},
|
---|
476 |
|
---|
477 | "neg" => meta_uop {-$_[0]},
|
---|
478 | "!" => meta_uop {!$_[0]},
|
---|
479 | "~" => meta_uop {~$_[0]},
|
---|
480 | "cos" => meta_uop {cos $_[0]},
|
---|
481 | "sin" => meta_uop {sin $_[0]},
|
---|
482 | "exp" => meta_uop {exp $_[0]},
|
---|
483 | "abs" => meta_uop {abs $_[0]},
|
---|
484 | "log" => meta_uop {log $_[0]},
|
---|
485 | "sqrt" => meta_uop {sqrt $_[0]},
|
---|
486 | "bool" => sub { croak "Can't use && or || in expression containing __" },
|
---|
487 |
|
---|
488 | # "&()" => sub { $_[0]->{impl} },
|
---|
489 |
|
---|
490 | # "||" => meta_bop {$_[0] || $_[1]},
|
---|
491 | # "&&" => meta_bop {$_[0] && $_[1]},
|
---|
492 | # fallback => 1,
|
---|
493 | ;
|
---|
494 | 1;
|
---|
495 |
|
---|
496 | __END__
|
---|
497 |
|
---|
498 |
|
---|
499 | =head1 NAME
|
---|
500 |
|
---|
501 | Switch - A switch statement for Perl
|
---|
502 |
|
---|
503 | =head1 VERSION
|
---|
504 |
|
---|
505 | This document describes version 2.10 of Switch,
|
---|
506 | released Dec 29, 2003.
|
---|
507 |
|
---|
508 | =head1 SYNOPSIS
|
---|
509 |
|
---|
510 | use Switch;
|
---|
511 |
|
---|
512 | switch ($val) {
|
---|
513 |
|
---|
514 | case 1 { print "number 1" }
|
---|
515 | case "a" { print "string a" }
|
---|
516 | case [1..10,42] { print "number in list" }
|
---|
517 | case (@array) { print "number in list" }
|
---|
518 | case /\w+/ { print "pattern" }
|
---|
519 | case qr/\w+/ { print "pattern" }
|
---|
520 | case (%hash) { print "entry in hash" }
|
---|
521 | case (\%hash) { print "entry in hash" }
|
---|
522 | case (\&sub) { print "arg to subroutine" }
|
---|
523 | else { print "previous case not true" }
|
---|
524 | }
|
---|
525 |
|
---|
526 | =head1 BACKGROUND
|
---|
527 |
|
---|
528 | [Skip ahead to L<"DESCRIPTION"> if you don't care about the whys
|
---|
529 | and wherefores of this control structure]
|
---|
530 |
|
---|
531 | In seeking to devise a "Swiss Army" case mechanism suitable for Perl,
|
---|
532 | it is useful to generalize this notion of distributed conditional
|
---|
533 | testing as far as possible. Specifically, the concept of "matching"
|
---|
534 | between the switch value and the various case values need not be
|
---|
535 | restricted to numeric (or string or referential) equality, as it is in other
|
---|
536 | languages. Indeed, as Table 1 illustrates, Perl
|
---|
537 | offers at least eighteen different ways in which two values could
|
---|
538 | generate a match.
|
---|
539 |
|
---|
540 | Table 1: Matching a switch value ($s) with a case value ($c)
|
---|
541 |
|
---|
542 | Switch Case Type of Match Implied Matching Code
|
---|
543 | Value Value
|
---|
544 | ====== ===== ===================== =============
|
---|
545 |
|
---|
546 | number same numeric or referential match if $s == $c;
|
---|
547 | or ref equality
|
---|
548 |
|
---|
549 | object method result of method call match if $s->$c();
|
---|
550 | ref name match if defined $s->$c();
|
---|
551 | or ref
|
---|
552 |
|
---|
553 | other other string equality match if $s eq $c;
|
---|
554 | non-ref non-ref
|
---|
555 | scalar scalar
|
---|
556 |
|
---|
557 | string regexp pattern match match if $s =~ /$c/;
|
---|
558 |
|
---|
559 | array scalar array entry existence match if 0<=$c && $c<@$s;
|
---|
560 | ref array entry definition match if defined $s->[$c];
|
---|
561 | array entry truth match if $s->[$c];
|
---|
562 |
|
---|
563 | array array array intersection match if intersects(@$s, @$c);
|
---|
564 | ref ref (apply this table to
|
---|
565 | all pairs of elements
|
---|
566 | $s->[$i] and
|
---|
567 | $c->[$j])
|
---|
568 |
|
---|
569 | array regexp array grep match if grep /$c/, @$s;
|
---|
570 | ref
|
---|
571 |
|
---|
572 | hash scalar hash entry existence match if exists $s->{$c};
|
---|
573 | ref hash entry definition match if defined $s->{$c};
|
---|
574 | hash entry truth match if $s->{$c};
|
---|
575 |
|
---|
576 | hash regexp hash grep match if grep /$c/, keys %$s;
|
---|
577 | ref
|
---|
578 |
|
---|
579 | sub scalar return value defn match if defined $s->($c);
|
---|
580 | ref return value truth match if $s->($c);
|
---|
581 |
|
---|
582 | sub array return value defn match if defined $s->(@$c);
|
---|
583 | ref ref return value truth match if $s->(@$c);
|
---|
584 |
|
---|
585 |
|
---|
586 | In reality, Table 1 covers 31 alternatives, because only the equality and
|
---|
587 | intersection tests are commutative; in all other cases, the roles of
|
---|
588 | the C<$s> and C<$c> variables could be reversed to produce a
|
---|
589 | different test. For example, instead of testing a single hash for
|
---|
590 | the existence of a series of keys (C<match if exists $s-E<gt>{$c}>),
|
---|
591 | one could test for the existence of a single key in a series of hashes
|
---|
592 | (C<match if exists $c-E<gt>{$s}>).
|
---|
593 |
|
---|
594 | As L<perltodo> observes, a Perl case mechanism must support all these
|
---|
595 | "ways to do it".
|
---|
596 |
|
---|
597 |
|
---|
598 | =head1 DESCRIPTION
|
---|
599 |
|
---|
600 | The Switch.pm module implements a generalized case mechanism that covers
|
---|
601 | the numerous possible combinations of switch and case values described above.
|
---|
602 |
|
---|
603 | The module augments the standard Perl syntax with two new control
|
---|
604 | statements: C<switch> and C<case>. The C<switch> statement takes a
|
---|
605 | single scalar argument of any type, specified in parentheses.
|
---|
606 | C<switch> stores this value as the
|
---|
607 | current switch value in a (localized) control variable.
|
---|
608 | The value is followed by a block which may contain one or more
|
---|
609 | Perl statements (including the C<case> statement described below).
|
---|
610 | The block is unconditionally executed once the switch value has
|
---|
611 | been cached.
|
---|
612 |
|
---|
613 | A C<case> statement takes a single scalar argument (in mandatory
|
---|
614 | parentheses if it's a variable; otherwise the parens are optional) and
|
---|
615 | selects the appropriate type of matching between that argument and the
|
---|
616 | current switch value. The type of matching used is determined by the
|
---|
617 | respective types of the switch value and the C<case> argument, as
|
---|
618 | specified in Table 1. If the match is successful, the mandatory
|
---|
619 | block associated with the C<case> statement is executed.
|
---|
620 |
|
---|
621 | In most other respects, the C<case> statement is semantically identical
|
---|
622 | to an C<if> statement. For example, it can be followed by an C<else>
|
---|
623 | clause, and can be used as a postfix statement qualifier.
|
---|
624 |
|
---|
625 | However, when a C<case> block has been executed control is automatically
|
---|
626 | transferred to the statement after the immediately enclosing C<switch>
|
---|
627 | block, rather than to the next statement within the block. In other
|
---|
628 | words, the success of any C<case> statement prevents other cases in the
|
---|
629 | same scope from executing. But see L<"Allowing fall-through"> below.
|
---|
630 |
|
---|
631 | Together these two new statements provide a fully generalized case
|
---|
632 | mechanism:
|
---|
633 |
|
---|
634 | use Switch;
|
---|
635 |
|
---|
636 | # AND LATER...
|
---|
637 |
|
---|
638 | %special = ( woohoo => 1, d'oh => 1 );
|
---|
639 |
|
---|
640 | while (<>) {
|
---|
641 | switch ($_) {
|
---|
642 |
|
---|
643 | case (%special) { print "homer\n"; } # if $special{$_}
|
---|
644 | case /a-z/i { print "alpha\n"; } # if $_ =~ /a-z/i
|
---|
645 | case [1..9] { print "small num\n"; } # if $_ in [1..9]
|
---|
646 |
|
---|
647 | case { $_[0] >= 10 } { # if $_ >= 10
|
---|
648 | my $age = <>;
|
---|
649 | switch (sub{ $_[0] < $age } ) {
|
---|
650 |
|
---|
651 | case 20 { print "teens\n"; } # if 20 < $age
|
---|
652 | case 30 { print "twenties\n"; } # if 30 < $age
|
---|
653 | else { print "history\n"; }
|
---|
654 | }
|
---|
655 | }
|
---|
656 |
|
---|
657 | print "must be punctuation\n" case /\W/; # if $_ ~= /\W/
|
---|
658 | }
|
---|
659 |
|
---|
660 | Note that C<switch>es can be nested within C<case> (or any other) blocks,
|
---|
661 | and a series of C<case> statements can try different types of matches
|
---|
662 | -- hash membership, pattern match, array intersection, simple equality,
|
---|
663 | etc. -- against the same switch value.
|
---|
664 |
|
---|
665 | The use of intersection tests against an array reference is particularly
|
---|
666 | useful for aggregating integral cases:
|
---|
667 |
|
---|
668 | sub classify_digit
|
---|
669 | {
|
---|
670 | switch ($_[0]) { case 0 { return 'zero' }
|
---|
671 | case [2,4,6,8] { return 'even' }
|
---|
672 | case [1,3,4,7,9] { return 'odd' }
|
---|
673 | case /[A-F]/i { return 'hex' }
|
---|
674 | }
|
---|
675 | }
|
---|
676 |
|
---|
677 |
|
---|
678 | =head2 Allowing fall-through
|
---|
679 |
|
---|
680 | Fall-though (trying another case after one has already succeeded)
|
---|
681 | is usually a Bad Idea in a switch statement. However, this
|
---|
682 | is Perl, not a police state, so there I<is> a way to do it, if you must.
|
---|
683 |
|
---|
684 | If a C<case> block executes an untargeted C<next>, control is
|
---|
685 | immediately transferred to the statement I<after> the C<case> statement
|
---|
686 | (i.e. usually another case), rather than out of the surrounding
|
---|
687 | C<switch> block.
|
---|
688 |
|
---|
689 | For example:
|
---|
690 |
|
---|
691 | switch ($val) {
|
---|
692 | case 1 { handle_num_1(); next } # and try next case...
|
---|
693 | case "1" { handle_str_1(); next } # and try next case...
|
---|
694 | case [0..9] { handle_num_any(); } # and we're done
|
---|
695 | case /\d/ { handle_dig_any(); next } # and try next case...
|
---|
696 | case /.*/ { handle_str_any(); next } # and try next case...
|
---|
697 | }
|
---|
698 |
|
---|
699 | If $val held the number C<1>, the above C<switch> block would call the
|
---|
700 | first three C<handle_...> subroutines, jumping to the next case test
|
---|
701 | each time it encountered a C<next>. After the thrid C<case> block
|
---|
702 | was executed, control would jump to the end of the enclosing
|
---|
703 | C<switch> block.
|
---|
704 |
|
---|
705 | On the other hand, if $val held C<10>, then only the last two C<handle_...>
|
---|
706 | subroutines would be called.
|
---|
707 |
|
---|
708 | Note that this mechanism allows the notion of I<conditional fall-through>.
|
---|
709 | For example:
|
---|
710 |
|
---|
711 | switch ($val) {
|
---|
712 | case [0..9] { handle_num_any(); next if $val < 7; }
|
---|
713 | case /\d/ { handle_dig_any(); }
|
---|
714 | }
|
---|
715 |
|
---|
716 | If an untargeted C<last> statement is executed in a case block, this
|
---|
717 | immediately transfers control out of the enclosing C<switch> block
|
---|
718 | (in other words, there is an implicit C<last> at the end of each
|
---|
719 | normal C<case> block). Thus the previous example could also have been
|
---|
720 | written:
|
---|
721 |
|
---|
722 | switch ($val) {
|
---|
723 | case [0..9] { handle_num_any(); last if $val >= 7; next; }
|
---|
724 | case /\d/ { handle_dig_any(); }
|
---|
725 | }
|
---|
726 |
|
---|
727 |
|
---|
728 | =head2 Automating fall-through
|
---|
729 |
|
---|
730 | In situations where case fall-through should be the norm, rather than an
|
---|
731 | exception, an endless succession of terminal C<next>s is tedious and ugly.
|
---|
732 | Hence, it is possible to reverse the default behaviour by specifying
|
---|
733 | the string "fallthrough" when importing the module. For example, the
|
---|
734 | following code is equivalent to the first example in L<"Allowing fall-through">:
|
---|
735 |
|
---|
736 | use Switch 'fallthrough';
|
---|
737 |
|
---|
738 | switch ($val) {
|
---|
739 | case 1 { handle_num_1(); }
|
---|
740 | case "1" { handle_str_1(); }
|
---|
741 | case [0..9] { handle_num_any(); last }
|
---|
742 | case /\d/ { handle_dig_any(); }
|
---|
743 | case /.*/ { handle_str_any(); }
|
---|
744 | }
|
---|
745 |
|
---|
746 | Note the explicit use of a C<last> to preserve the non-fall-through
|
---|
747 | behaviour of the third case.
|
---|
748 |
|
---|
749 |
|
---|
750 |
|
---|
751 | =head2 Alternative syntax
|
---|
752 |
|
---|
753 | Perl 6 will provide a built-in switch statement with essentially the
|
---|
754 | same semantics as those offered by Switch.pm, but with a different
|
---|
755 | pair of keywords. In Perl 6 C<switch> will be spelled C<given>, and
|
---|
756 | C<case> will be pronounced C<when>. In addition, the C<when> statement
|
---|
757 | will not require switch or case values to be parenthesized.
|
---|
758 |
|
---|
759 | This future syntax is also (largely) available via the Switch.pm module, by
|
---|
760 | importing it with the argument C<"Perl6">. For example:
|
---|
761 |
|
---|
762 | use Switch 'Perl6';
|
---|
763 |
|
---|
764 | given ($val) {
|
---|
765 | when 1 { handle_num_1(); }
|
---|
766 | when ($str1) { handle_str_1(); }
|
---|
767 | when [0..9] { handle_num_any(); last }
|
---|
768 | when /\d/ { handle_dig_any(); }
|
---|
769 | when /.*/ { handle_str_any(); }
|
---|
770 | default { handle anything else; }
|
---|
771 | }
|
---|
772 |
|
---|
773 | Note that scalars still need to be parenthesized, since they would be
|
---|
774 | ambiguous in Perl 5.
|
---|
775 |
|
---|
776 | Note too that you can mix and match both syntaxes by importing the module
|
---|
777 | with:
|
---|
778 |
|
---|
779 | use Switch 'Perl5', 'Perl6';
|
---|
780 |
|
---|
781 |
|
---|
782 | =head2 Higher-order Operations
|
---|
783 |
|
---|
784 | One situation in which C<switch> and C<case> do not provide a good
|
---|
785 | substitute for a cascaded C<if>, is where a switch value needs to
|
---|
786 | be tested against a series of conditions. For example:
|
---|
787 |
|
---|
788 | sub beverage {
|
---|
789 | switch (shift) {
|
---|
790 |
|
---|
791 | case sub { $_[0] < 10 } { return 'milk' }
|
---|
792 | case sub { $_[0] < 20 } { return 'coke' }
|
---|
793 | case sub { $_[0] < 30 } { return 'beer' }
|
---|
794 | case sub { $_[0] < 40 } { return 'wine' }
|
---|
795 | case sub { $_[0] < 50 } { return 'malt' }
|
---|
796 | case sub { $_[0] < 60 } { return 'Moet' }
|
---|
797 | else { return 'milk' }
|
---|
798 | }
|
---|
799 | }
|
---|
800 |
|
---|
801 | The need to specify each condition as a subroutine block is tiresome. To
|
---|
802 | overcome this, when importing Switch.pm, a special "placeholder"
|
---|
803 | subroutine named C<__> [sic] may also be imported. This subroutine
|
---|
804 | converts (almost) any expression in which it appears to a reference to a
|
---|
805 | higher-order function. That is, the expression:
|
---|
806 |
|
---|
807 | use Switch '__';
|
---|
808 |
|
---|
809 | __ < 2 + __
|
---|
810 |
|
---|
811 | is equivalent to:
|
---|
812 |
|
---|
813 | sub { $_[0] < 2 + $_[1] }
|
---|
814 |
|
---|
815 | With C<__>, the previous ugly case statements can be rewritten:
|
---|
816 |
|
---|
817 | case __ < 10 { return 'milk' }
|
---|
818 | case __ < 20 { return 'coke' }
|
---|
819 | case __ < 30 { return 'beer' }
|
---|
820 | case __ < 40 { return 'wine' }
|
---|
821 | case __ < 50 { return 'malt' }
|
---|
822 | case __ < 60 { return 'Moet' }
|
---|
823 | else { return 'milk' }
|
---|
824 |
|
---|
825 | The C<__> subroutine makes extensive use of operator overloading to
|
---|
826 | perform its magic. All operations involving __ are overloaded to
|
---|
827 | produce an anonymous subroutine that implements a lazy version
|
---|
828 | of the original operation.
|
---|
829 |
|
---|
830 | The only problem is that operator overloading does not allow the
|
---|
831 | boolean operators C<&&> and C<||> to be overloaded. So a case statement
|
---|
832 | like this:
|
---|
833 |
|
---|
834 | case 0 <= __ && __ < 10 { return 'digit' }
|
---|
835 |
|
---|
836 | doesn't act as expected, because when it is
|
---|
837 | executed, it constructs two higher order subroutines
|
---|
838 | and then treats the two resulting references as arguments to C<&&>:
|
---|
839 |
|
---|
840 | sub { 0 <= $_[0] } && sub { $_[0] < 10 }
|
---|
841 |
|
---|
842 | This boolean expression is inevitably true, since both references are
|
---|
843 | non-false. Fortunately, the overloaded C<'bool'> operator catches this
|
---|
844 | situation and flags it as a error.
|
---|
845 |
|
---|
846 | =head1 DEPENDENCIES
|
---|
847 |
|
---|
848 | The module is implemented using Filter::Util::Call and Text::Balanced
|
---|
849 | and requires both these modules to be installed.
|
---|
850 |
|
---|
851 | =head1 AUTHOR
|
---|
852 |
|
---|
853 | Damian Conway ([email protected]). The maintainer of this module is now Rafael
|
---|
854 | Garcia-Suarez ([email protected]).
|
---|
855 |
|
---|
856 | =head1 BUGS
|
---|
857 |
|
---|
858 | There are undoubtedly serious bugs lurking somewhere in code this funky :-)
|
---|
859 | Bug reports and other feedback are most welcome.
|
---|
860 |
|
---|
861 | =head1 LIMITATIONS
|
---|
862 |
|
---|
863 | Due to the heuristic nature of Switch.pm's source parsing, the presence
|
---|
864 | of regexes specified with raw C<?...?> delimiters may cause mysterious
|
---|
865 | errors. The workaround is to use C<m?...?> instead.
|
---|
866 |
|
---|
867 | Due to the way source filters work in Perl, you can't use Switch inside
|
---|
868 | an string C<eval>.
|
---|
869 |
|
---|
870 | If your source file is longer then 1 million characters and you have a
|
---|
871 | switch statement that crosses the 1 million (or 2 million, etc.)
|
---|
872 | character boundary you will get mysterious errors. The workaround is to
|
---|
873 | use smaller source files.
|
---|
874 |
|
---|
875 | =head1 COPYRIGHT
|
---|
876 |
|
---|
877 | Copyright (c) 1997-2003, Damian Conway. All Rights Reserved.
|
---|
878 | This module is free software. It may be used, redistributed
|
---|
879 | and/or modified under the same terms as Perl itself.
|
---|