1package feature; 2 3our $VERSION = '1.13'; 4 5# (feature name) => (internal name, used in %^H) 6my %feature = ( 7 switch => 'feature_switch', 8 say => "feature_say", 9 state => "feature_state", 10); 11 12# NB. the latest bundle must be loaded by the -E switch (see toke.c) 13 14my %feature_bundle = ( 15 "5.10" => [qw(switch say state)], 16### "5.11" => [qw(switch say state)], 17); 18 19# special case 20$feature_bundle{"5.9.5"} = $feature_bundle{"5.10"}; 21 22# TODO: 23# - think about versioned features (use feature switch => 2) 24 25=head1 NAME 26 27feature - Perl pragma to enable new syntactic features 28 29=head1 SYNOPSIS 30 31 use feature qw(switch say); 32 given ($foo) { 33 when (1) { say "\$foo == 1" } 34 when ([2,3]) { say "\$foo == 2 || \$foo == 3" } 35 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" } 36 when ($_ > 100) { say "\$foo > 100" } 37 default { say "None of the above" } 38 } 39 40 use feature ':5.10'; # loads all features available in perl 5.10 41 42=head1 DESCRIPTION 43 44It is usually impossible to add new syntax to Perl without breaking 45some existing programs. This pragma provides a way to minimize that 46risk. New syntactic constructs can be enabled by C<use feature 'foo'>, 47and will be parsed only when the appropriate feature pragma is in 48scope. 49 50=head2 Lexical effect 51 52Like other pragmas (C<use strict>, for example), features have a lexical 53effect. C<use feature qw(foo)> will only make the feature "foo" available 54from that point to the end of the enclosing block. 55 56 { 57 use feature 'say'; 58 say "say is available here"; 59 } 60 print "But not here.\n"; 61 62=head2 C<no feature> 63 64Features can also be turned off by using C<no feature "foo">. This too 65has lexical effect. 66 67 use feature 'say'; 68 say "say is available here"; 69 { 70 no feature 'say'; 71 print "But not here.\n"; 72 } 73 say "Yet it is here."; 74 75C<no feature> with no features specified will turn off all features. 76 77=head2 The 'switch' feature 78 79C<use feature 'switch'> tells the compiler to enable the Perl 6 80given/when construct. 81 82See L<perlsyn/"Switch statements"> for details. 83 84=head2 The 'say' feature 85 86C<use feature 'say'> tells the compiler to enable the Perl 6 87C<say> function. 88 89See L<perlfunc/say> for details. 90 91=head2 the 'state' feature 92 93C<use feature 'state'> tells the compiler to enable C<state> 94variables. 95 96See L<perlsub/"Persistent Private Variables"> for details. 97 98=head1 FEATURE BUNDLES 99 100It's possible to load a whole slew of features in one go, using 101a I<feature bundle>. The name of a feature bundle is prefixed with 102a colon, to distinguish it from an actual feature. At present, the 103only feature bundle is C<use feature ":5.10"> which is equivalent 104to C<use feature qw(switch say state)>. 105 106Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has 107no effect: feature bundles are guaranteed to be the same for all sub-versions. 108 109=head1 IMPLICIT LOADING 110 111There are two ways to load the C<feature> pragma implicitly : 112 113=over 4 114 115=item * 116 117By using the C<-E> switch on the command-line instead of C<-e>. It enables 118all available features in the main compilation unit (that is, the one-liner.) 119 120=item * 121 122By requiring explicitly a minimal Perl version number for your program, with 123the C<use VERSION> construct, and when the version is higher than or equal to 1245.10.0. That is, 125 126 use 5.10.0; 127 128will do an implicit 129 130 use feature ':5.10'; 131 132and so on. Note how the trailing sub-version is automatically stripped from the 133version. 134 135But to avoid portability warnings (see L<perlfunc/use>), you may prefer: 136 137 use 5.010; 138 139with the same effect. 140 141=back 142 143=cut 144 145sub import { 146 my $class = shift; 147 if (@_ == 0) { 148 croak("No features specified"); 149 } 150 while (@_) { 151 my $name = shift(@_); 152 if (substr($name, 0, 1) eq ":") { 153 my $v = substr($name, 1); 154 if (!exists $feature_bundle{$v}) { 155 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/; 156 if (!exists $feature_bundle{$v}) { 157 unknown_feature_bundle(substr($name, 1)); 158 } 159 } 160 unshift @_, @{$feature_bundle{$v}}; 161 next; 162 } 163 if (!exists $feature{$name}) { 164 unknown_feature($name); 165 } 166 $^H{$feature{$name}} = 1; 167 } 168} 169 170sub unimport { 171 my $class = shift; 172 173 # A bare C<no feature> should disable *all* features 174 if (!@_) { 175 delete @^H{ values(%feature) }; 176 return; 177 } 178 179 while (@_) { 180 my $name = shift; 181 if (substr($name, 0, 1) eq ":") { 182 my $v = substr($name, 1); 183 if (!exists $feature_bundle{$v}) { 184 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/; 185 if (!exists $feature_bundle{$v}) { 186 unknown_feature_bundle(substr($name, 1)); 187 } 188 } 189 unshift @_, @{$feature_bundle{$v}}; 190 next; 191 } 192 if (!exists($feature{$name})) { 193 unknown_feature($name); 194 } 195 else { 196 delete $^H{$feature{$name}}; 197 } 198 } 199} 200 201sub unknown_feature { 202 my $feature = shift; 203 croak(sprintf('Feature "%s" is not supported by Perl %vd', 204 $feature, $^V)); 205} 206 207sub unknown_feature_bundle { 208 my $feature = shift; 209 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd', 210 $feature, $^V)); 211} 212 213sub croak { 214 require Carp; 215 Carp::croak(@_); 216} 217 2181; 219