1package feature; 2 3our $VERSION = '1.11'; 4 5# (feature name) => (internal name, used in %^H) 6my %feature = ( 7 switch => 'feature_switch', 8 say => "feature_say", 9 state => "feature_state", 10); 11 12my %feature_bundle = ( 13 "5.10.0" => [qw(switch say state)], 14); 15 16# latest version here 17$feature_bundle{"5.10"} = $feature_bundle{sprintf("%vd",$^V)}; 18 19$feature_bundle{"5.9.5"} = $feature_bundle{"5.10.0"}; 20 21# TODO: 22# - think about versioned features (use feature switch => 2) 23 24=head1 NAME 25 26feature - Perl pragma to enable new syntactic features 27 28=head1 SYNOPSIS 29 30 use feature qw(switch say); 31 given ($foo) { 32 when (1) { say "\$foo == 1" } 33 when ([2,3]) { say "\$foo == 2 || \$foo == 3" } 34 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" } 35 when ($_ > 100) { say "\$foo > 100" } 36 default { say "None of the above" } 37 } 38 39 use feature ':5.10'; # loads all features available in perl 5.10 40 41=head1 DESCRIPTION 42 43It is usually impossible to add new syntax to Perl without breaking 44some existing programs. This pragma provides a way to minimize that 45risk. New syntactic constructs can be enabled by C<use feature 'foo'>, 46and will be parsed only when the appropriate feature pragma is in 47scope. 48 49=head2 Lexical effect 50 51Like other pragmas (C<use strict>, for example), features have a lexical 52effect. C<use feature qw(foo)> will only make the feature "foo" available 53from that point to the end of the enclosing block. 54 55 { 56 use feature 'say'; 57 say "say is available here"; 58 } 59 print "But not here.\n"; 60 61=head2 C<no feature> 62 63Features can also be turned off by using C<no feature "foo">. This too 64has lexical effect. 65 66 use feature 'say'; 67 say "say is available here"; 68 { 69 no feature 'say'; 70 print "But not here.\n"; 71 } 72 say "Yet it is here."; 73 74C<no feature> with no features specified will turn off all features. 75 76=head2 The 'switch' feature 77 78C<use feature 'switch'> tells the compiler to enable the Perl 6 79given/when construct. 80 81See L<perlsyn/"Switch statements"> for details. 82 83=head2 The 'say' feature 84 85C<use feature 'say'> tells the compiler to enable the Perl 6 86C<say> function. 87 88See L<perlfunc/say> for details. 89 90=head2 the 'state' feature 91 92C<use feature 'state'> tells the compiler to enable C<state> 93variables. 94 95See L<perlsub/"Persistent Private Variables"> for details. 96 97=head1 FEATURE BUNDLES 98 99It's possible to load a whole slew of features in one go, using 100a I<feature bundle>. The name of a feature bundle is prefixed with 101a colon, to distinguish it from an actual feature. At present, the 102only feature bundles are C<use feature ":5.10"> and C<use feature ":5.10.0">, 103which both are equivalent to C<use feature qw(switch say state)>. 104 105In the forthcoming 5.10.X perl releases, C<use feature ":5.10"> will be 106equivalent to the latest C<use feature ":5.10.X">. 107 108=head1 IMPLICIT LOADING 109 110There are two ways to load the C<feature> pragma implicitly : 111 112=over 4 113 114=item * 115 116By using the C<-E> switch on the command-line instead of C<-e>. It enables 117all available features in the main compilation unit (that is, the one-liner.) 118 119=item * 120 121By requiring explicitly a minimal Perl version number for your program, with 122the C<use VERSION> construct, and when the version is higher than or equal to 1235.10.0. That is, 124 125 use 5.10.0; 126 127will do an implicit 128 129 use feature ':5.10.0'; 130 131and so on. 132 133But to avoid portability warnings (see L<perlfunc/use>), you may prefer: 134 135 use 5.010; 136 137with the same effect. 138 139=back 140 141=cut 142 143sub import { 144 my $class = shift; 145 if (@_ == 0) { 146 croak("No features specified"); 147 } 148 while (@_) { 149 my $name = shift(@_); 150 if (substr($name, 0, 1) eq ":") { 151 my $v = substr($name, 1); 152 if (!exists $feature_bundle{$v}) { 153 unknown_feature_bundle($v); 154 } 155 unshift @_, @{$feature_bundle{$v}}; 156 next; 157 } 158 if (!exists $feature{$name}) { 159 unknown_feature($name); 160 } 161 $^H{$feature{$name}} = 1; 162 } 163} 164 165sub unimport { 166 my $class = shift; 167 168 # A bare C<no feature> should disable *all* features 169 if (!@_) { 170 delete @^H{ values(%feature) }; 171 return; 172 } 173 174 while (@_) { 175 my $name = shift; 176 if (substr($name, 0, 1) eq ":") { 177 my $v = substr($name, 1); 178 if (!exists $feature_bundle{$v}) { 179 unknown_feature_bundle($v); 180 } 181 unshift @_, @{$feature_bundle{$v}}; 182 next; 183 } 184 if (!exists($feature{$name})) { 185 unknown_feature($name); 186 } 187 else { 188 delete $^H{$feature{$name}}; 189 } 190 } 191} 192 193sub unknown_feature { 194 my $feature = shift; 195 croak(sprintf('Feature "%s" is not supported by Perl %vd', 196 $feature, $^V)); 197} 198 199sub unknown_feature_bundle { 200 my $feature = shift; 201 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd', 202 $feature, $^V)); 203} 204 205sub croak { 206 require Carp; 207 Carp::croak(@_); 208} 209 2101; 211