5 # (feature name) => (internal name, used in %^H)
7 switch => 'feature_switch',
9 state => "feature_state",
10 unicode_strings => "feature_unicode",
13 # This gets set (for now) in $^H as well as in %^H,
14 # for runtime speed of the uc/lc/ucfirst/lcfirst functions.
15 # See HINT_UNI_8_BIT in perl.h.
16 our $hint_uni8bit = 0x00000800;
18 # NB. the latest bundle must be loaded by the -E switch (see toke.c)
20 my %feature_bundle = (
21 "5.10" => [qw(switch say state)],
22 "5.11" => [qw(switch say state unicode_strings)],
23 "5.12" => [qw(switch say state unicode_strings)],
24 "5.13" => [qw(switch say state unicode_strings)],
28 $feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};
31 # - think about versioned features (use feature switch => 2)
35 feature - Perl pragma to enable new features
39 use feature qw(switch say);
41 when (1) { say "\$foo == 1" }
42 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
43 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
44 when ($_ > 100) { say "\$foo > 100" }
45 default { say "None of the above" }
48 use feature ':5.10'; # loads all features available in perl 5.10
52 It is usually impossible to add new syntax to Perl without breaking
53 some existing programs. This pragma provides a way to minimize that
54 risk. New syntactic constructs, or new semantic meanings to older
55 constructs, can be enabled by C<use feature 'foo'>, and will be parsed
56 only when the appropriate feature pragma is in scope.
60 Like other pragmas (C<use strict>, for example), features have a lexical
61 effect. C<use feature qw(foo)> will only make the feature "foo" available
62 from that point to the end of the enclosing block.
66 say "say is available here";
68 print "But not here.\n";
72 Features can also be turned off by using C<no feature "foo">. This too
76 say "say is available here";
79 print "But not here.\n";
81 say "Yet it is here.";
83 C<no feature> with no features specified will turn off all features.
85 =head2 The 'switch' feature
87 C<use feature 'switch'> tells the compiler to enable the Perl 6
90 See L<perlsyn/"Switch statements"> for details.
92 =head2 The 'say' feature
94 C<use feature 'say'> tells the compiler to enable the Perl 6
97 See L<perlfunc/say> for details.
99 =head2 the 'state' feature
101 C<use feature 'state'> tells the compiler to enable C<state>
104 See L<perlsub/"Persistent Private Variables"> for details.
106 =head2 the 'unicode_strings' feature
108 C<use feature 'unicode_strings'> tells the compiler to use Unicode semantics
109 in all string operations executed within its scope (unless they are also
110 within the scope of either C<use locale> or C<use bytes>). The same applies
111 to all regular expressions compiled within the scope, even if executed outside
114 C<no feature 'unicode_strings'> tells the compiler to use the traditional
115 Perl semantics wherein the native character set semantics is used unless it is
116 clear to Perl that Unicode is desired. This can lead to some surprises
117 when the behavior suddenly changes. (See
118 L<perlunicode/The "Unicode Bug"> for details.) For this reason, if you are
119 potentially using Unicode in your program, the
120 C<use feature 'unicode_strings'> subpragma is B<strongly> recommended.
122 This subpragma is available starting with Perl 5.11.3, but was not fully
123 implemented until 5.13.8.
125 =head1 FEATURE BUNDLES
127 It's possible to load a whole slew of features in one go, using
128 a I<feature bundle>. The name of a feature bundle is prefixed with
129 a colon, to distinguish it from an actual feature. At present, the
130 only feature bundle is C<use feature ":5.10"> which is equivalent
131 to C<use feature qw(switch say state)>.
133 Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has
134 no effect: feature bundles are guaranteed to be the same for all sub-versions.
136 =head1 IMPLICIT LOADING
138 There are two ways to load the C<feature> pragma implicitly :
144 By using the C<-E> switch on the command-line instead of C<-e>. It enables
145 all available features in the main compilation unit (that is, the one-liner.)
149 By requiring explicitly a minimal Perl version number for your program, with
150 the C<use VERSION> construct, and when the version is higher than or equal to
159 and so on. Note how the trailing sub-version is automatically stripped from the
162 But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
166 with the same effect.
175 croak("No features specified");
178 my $name = shift(@_);
179 if (substr($name, 0, 1) eq ":") {
180 my $v = substr($name, 1);
181 if (!exists $feature_bundle{$v}) {
182 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
183 if (!exists $feature_bundle{$v}) {
184 unknown_feature_bundle(substr($name, 1));
187 unshift @_, @{$feature_bundle{$v}};
190 if (!exists $feature{$name}) {
191 unknown_feature($name);
193 $^H{$feature{$name}} = 1;
194 $^H |= $hint_uni8bit if $name eq 'unicode_strings';
201 # A bare C<no feature> should disable *all* features
203 delete @^H{ values(%feature) };
204 $^H &= ~ $hint_uni8bit;
210 if (substr($name, 0, 1) eq ":") {
211 my $v = substr($name, 1);
212 if (!exists $feature_bundle{$v}) {
213 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
214 if (!exists $feature_bundle{$v}) {
215 unknown_feature_bundle(substr($name, 1));
218 unshift @_, @{$feature_bundle{$v}};
221 if (!exists($feature{$name})) {
222 unknown_feature($name);
225 delete $^H{$feature{$name}};
226 $^H &= ~ $hint_uni8bit if $name eq 'unicode_strings';
231 sub unknown_feature {
233 croak(sprintf('Feature "%s" is not supported by Perl %vd',
237 sub unknown_feature_bundle {
239 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',