5 # (feature name) => (internal name, used in %^H)
8 state => 'feature_state',
9 switch => 'feature_switch',
10 evalbytes => 'feature_evalbytes',
11 current_sub => 'feature___SUB__',
12 unicode_eval => 'feature_unieval',
13 unicode_strings => 'feature_unicode',
16 # This gets set (for now) in $^H as well as in %^H,
17 # for runtime speed of the uc/lc/ucfirst/lcfirst functions.
18 # See HINT_UNI_8_BIT in perl.h.
19 our $hint_uni8bit = 0x00000800;
21 # NB. the latest bundle must be loaded by the -E switch (see toke.c)
23 our %feature_bundle = (
25 "5.10" => [qw(say state switch)],
26 "5.11" => [qw(say state switch unicode_strings)],
27 "5.15" => [qw(say state switch unicode_strings unicode_eval
28 evalbytes current_sub)],
31 # Each of these is the same as the previous bundle
33 $feature_bundle{"5.$_"} = $feature_bundle{"5.".($_-1)}
37 $feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};
40 # - think about versioned features (use feature switch => 2)
44 feature - Perl pragma to enable new features
48 use feature qw(say switch);
50 when (1) { say "\$foo == 1" }
51 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
52 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
53 when ($_ > 100) { say "\$foo > 100" }
54 default { say "None of the above" }
57 use feature ':5.10'; # loads all features available in perl 5.10
61 It is usually impossible to add new syntax to Perl without breaking
62 some existing programs. This pragma provides a way to minimize that
63 risk. New syntactic constructs, or new semantic meanings to older
64 constructs, can be enabled by C<use feature 'foo'>, and will be parsed
65 only when the appropriate feature pragma is in scope. (Nevertheless, the
66 C<CORE::> prefix provides access to all Perl keywords, regardless of this
71 Like other pragmas (C<use strict>, for example), features have a lexical
72 effect. C<use feature qw(foo)> will only make the feature "foo" available
73 from that point to the end of the enclosing block.
77 say "say is available here";
79 print "But not here.\n";
83 Features can also be turned off by using C<no feature "foo">. This too
87 say "say is available here";
90 print "But not here.\n";
92 say "Yet it is here.";
94 C<no feature> with no features specified will turn off all features.
96 =head2 The 'say' feature
98 C<use feature 'say'> tells the compiler to enable the Perl 6
101 See L<perlfunc/say> for details.
103 =head2 the 'state' feature
105 C<use feature 'state'> tells the compiler to enable C<state>
108 See L<perlsub/"Persistent Private Variables"> for details.
110 =head2 The 'switch' feature
112 C<use feature 'switch'> tells the compiler to enable the Perl 6
113 given/when construct.
115 See L<perlsyn/"Switch statements"> for details.
117 =head2 the 'unicode_strings' feature
119 C<use feature 'unicode_strings'> tells the compiler to use Unicode semantics
120 in all string operations executed within its scope (unless they are also
121 within the scope of either C<use locale> or C<use bytes>). The same applies
122 to all regular expressions compiled within the scope, even if executed outside
125 C<no feature 'unicode_strings'> tells the compiler to use the traditional
126 Perl semantics wherein the native character set semantics is used unless it is
127 clear to Perl that Unicode is desired. This can lead to some surprises
128 when the behavior suddenly changes. (See
129 L<perlunicode/The "Unicode Bug"> for details.) For this reason, if you are
130 potentially using Unicode in your program, the
131 C<use feature 'unicode_strings'> subpragma is B<strongly> recommended.
133 This subpragma is available starting with Perl 5.11.3, but was not fully
134 implemented until 5.13.8.
136 =head2 the 'unicode_eval' and 'evalbytes' features
138 Under the C<unicode_eval> feature, Perl's C<eval> function, when passed a
139 string, will evaluate it as a string of characters, ignoring any
140 C<use utf8> declarations. C<use utf8> exists to declare the encoding of
141 the script, which only makes sense for a stream of bytes, not a string of
142 characters. Source filters are forbidden, as they also really only make
143 sense on strings of bytes. Any attempt to activate a source filter will
146 The C<evalbytes> feature enables the C<evalbytes> keyword, which evaluates
147 the argument passed to it as a string of bytes. It dies if the string
148 contains any characters outside the 8-bit range. Source filters work
149 within C<evalbytes>: they apply to the contents of the string being
152 Together, these two features are intended to replace the historical C<eval>
153 function, which has (at least) two bugs in it, that cannot easily be fixed
154 without breaking existing programs:
160 C<eval> behaves differently depending on the internal encoding of the
161 string, sometimes treating its argument as a string of bytes, and sometimes
162 as a string of characters.
166 Source filters activated within C<eval> leak out into whichever I<file>
167 scope is currently being compiled. To give an example with the CPAN module
170 BEGIN { eval "use Semi::Semicolons; # not filtered here " }
173 C<evalbytes> fixes that to work the way one would expect:
175 use feature "evalbytes";
176 BEGIN { evalbytes "use Semi::Semicolons; # filtered " }
181 These two features are available starting with Perl 5.16.
183 =head2 The 'current_sub' feature
185 This provides the C<__SUB__> token that returns a reference to the current
186 subroutine or C<undef> outside of a subroutine.
188 This feature is available starting with Perl 5.16.
190 =head1 FEATURE BUNDLES
192 It's possible to load a whole slew of features in one go, using
193 a I<feature bundle>. The name of a feature bundle is prefixed with
194 a colon, to distinguish it from an actual feature. At present, most
195 feature bundles correspond to Perl releases, e.g. C<use feature
196 ":5.10"> which is equivalent to C<use feature qw(switch say state)>. The
197 only bundle that does not follow this convention is ":default", which is
200 By convention, the feature bundle for any given Perl release includes
201 the features of previous releases, down to and including 5.10, the
202 first official release to provide this facility. Since Perl 5.12
203 only provides one new feature, C<unicode_strings>, and Perl 5.14
204 provides none, C<use feature ":5.14"> is equivalent to C<use feature
205 qw(switch say state unicode_strings)>.
207 Specifying sub-versions such as the C<0> in C<5.14.0> in feature bundles has
208 no effect: feature bundles are guaranteed to be the same for all sub-versions.
210 Note that instead of using release-based feature bundles it is usually
211 better, and shorter, to use implicit loading as described below.
213 =head1 IMPLICIT LOADING
215 There are two ways to load the C<feature> pragma implicitly :
221 By using the C<-E> switch on the command-line instead of C<-e>. It enables
222 all available features in the main compilation unit (that is, the one-liner.)
226 By requiring explicitly a minimal Perl version number for your program, with
227 the C<use VERSION> construct, and when the version is higher than or equal to
237 and so on. Note how the trailing sub-version is automatically stripped from the
240 But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
244 with the same effect.
246 For versions below 5.010, the ":default" feature bundle is automatically
247 loaded, but it is currently empty and has no effect.
256 croak("No features specified");
259 my $name = shift(@_);
260 if (substr($name, 0, 1) eq ":") {
261 my $v = substr($name, 1);
262 if (!exists $feature_bundle{$v}) {
263 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
264 if (!exists $feature_bundle{$v}) {
265 unknown_feature_bundle(substr($name, 1));
268 unshift @_, @{$feature_bundle{$v}};
271 if (!exists $feature{$name}) {
272 unknown_feature($name);
274 $^H{$feature{$name}} = 1;
275 $^H |= $hint_uni8bit if $name eq 'unicode_strings';
282 # A bare C<no feature> should disable *all* features
284 delete @^H{ values(%feature) };
285 $^H &= ~ $hint_uni8bit;
291 if (substr($name, 0, 1) eq ":") {
292 my $v = substr($name, 1);
293 if (!exists $feature_bundle{$v}) {
294 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
295 if (!exists $feature_bundle{$v}) {
296 unknown_feature_bundle(substr($name, 1));
299 unshift @_, @{$feature_bundle{$v}};
302 if (!exists($feature{$name})) {
303 unknown_feature($name);
306 delete $^H{$feature{$name}};
307 $^H &= ~ $hint_uni8bit if $name eq 'unicode_strings';
312 sub unknown_feature {
314 croak(sprintf('Feature "%s" is not supported by Perl %vd',
318 sub unknown_feature_bundle {
320 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',