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0d863452 RH |
1 | package feature; |
2 | ||
84ed0108 | 3 | our $VERSION = '1.24'; |
0d863452 RH |
4 | |
5 | # (feature name) => (internal name, used in %^H) | |
6 | my %feature = ( | |
47e9d84a EB |
7 | say => 'feature_say', |
8 | state => 'feature_state', | |
1863b879 | 9 | switch => 'feature_switch', |
7d789282 | 10 | evalbytes => 'feature_evalbytes', |
84ed0108 | 11 | current_sub => 'feature___SUB__', |
cda6b701 | 12 | unicode_eval => 'feature_unieval', |
47e9d84a | 13 | unicode_strings => 'feature_unicode', |
bc9b29db RH |
14 | ); |
15 | ||
1863b879 RGS |
16 | # This gets set (for now) in $^H as well as in %^H, |
17 | # for runtime speed of the uc/lc/ucfirst/lcfirst functions. | |
b0f41c9d | 18 | # See HINT_UNI_8_BIT in perl.h. |
1863b879 RGS |
19 | our $hint_uni8bit = 0x00000800; |
20 | ||
13a7998c RGS |
21 | # NB. the latest bundle must be loaded by the -E switch (see toke.c) |
22 | ||
0d2bd2aa | 23 | our %feature_bundle = ( |
47e9d84a EB |
24 | "5.10" => [qw(say state switch)], |
25 | "5.11" => [qw(say state switch unicode_strings)], | |
7d789282 | 26 | "5.15" => [qw(say state switch unicode_strings unicode_eval |
84ed0108 | 27 | evalbytes current_sub)], |
0d863452 | 28 | ); |
d052521a | 29 | |
0d2bd2aa FC |
30 | # Each of these is the same as the previous bundle |
31 | for(12...14, 16) { | |
32 | $feature_bundle{"5.$_"} = $feature_bundle{"5.".($_-1)} | |
33 | } | |
34 | ||
82cfb3a2 S |
35 | # special case |
36 | $feature_bundle{"5.9.5"} = $feature_bundle{"5.10"}; | |
7dfde25d | 37 | |
0d863452 | 38 | # TODO: |
1c321dc6 | 39 | # - think about versioned features (use feature switch => 2) |
0d863452 RH |
40 | |
41 | =head1 NAME | |
42 | ||
e1b711da | 43 | feature - Perl pragma to enable new features |
0d863452 RH |
44 | |
45 | =head1 SYNOPSIS | |
46 | ||
47e9d84a | 47 | use feature qw(say switch); |
0d863452 | 48 | given ($foo) { |
bc9b29db RH |
49 | when (1) { say "\$foo == 1" } |
50 | when ([2,3]) { say "\$foo == 2 || \$foo == 3" } | |
51 | when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" } | |
52 | when ($_ > 100) { say "\$foo > 100" } | |
53 | default { say "None of the above" } | |
0d863452 RH |
54 | } |
55 | ||
ec488c7f RGS |
56 | use feature ':5.10'; # loads all features available in perl 5.10 |
57 | ||
0d863452 RH |
58 | =head1 DESCRIPTION |
59 | ||
60 | It is usually impossible to add new syntax to Perl without breaking | |
61 | some existing programs. This pragma provides a way to minimize that | |
1863b879 RGS |
62 | risk. New syntactic constructs, or new semantic meanings to older |
63 | constructs, can be enabled by C<use feature 'foo'>, and will be parsed | |
4a904372 FC |
64 | only when the appropriate feature pragma is in scope. (Nevertheless, the |
65 | C<CORE::> prefix provides access to all Perl keywords, regardless of this | |
66 | pragma.) | |
0d863452 | 67 | |
9eb27be9 RGS |
68 | =head2 Lexical effect |
69 | ||
70 | Like other pragmas (C<use strict>, for example), features have a lexical | |
5e36ed56 | 71 | effect. C<use feature qw(foo)> will only make the feature "foo" available |
9eb27be9 RGS |
72 | from that point to the end of the enclosing block. |
73 | ||
74 | { | |
75 | use feature 'say'; | |
76 | say "say is available here"; | |
77 | } | |
78 | print "But not here.\n"; | |
79 | ||
5e36ed56 RGS |
80 | =head2 C<no feature> |
81 | ||
82 | Features can also be turned off by using C<no feature "foo">. This too | |
83 | has lexical effect. | |
84 | ||
85 | use feature 'say'; | |
86 | say "say is available here"; | |
87 | { | |
88 | no feature 'say'; | |
89 | print "But not here.\n"; | |
90 | } | |
91 | say "Yet it is here."; | |
92 | ||
93 | C<no feature> with no features specified will turn off all features. | |
94 | ||
0d863452 RH |
95 | =head2 The 'say' feature |
96 | ||
97 | C<use feature 'say'> tells the compiler to enable the Perl 6 | |
9eb27be9 | 98 | C<say> function. |
0d863452 RH |
99 | |
100 | See L<perlfunc/say> for details. | |
101 | ||
712d05cf RGS |
102 | =head2 the 'state' feature |
103 | ||
104 | C<use feature 'state'> tells the compiler to enable C<state> | |
9eb27be9 | 105 | variables. |
712d05cf | 106 | |
e60bcc8b RGS |
107 | See L<perlsub/"Persistent Private Variables"> for details. |
108 | ||
47e9d84a EB |
109 | =head2 The 'switch' feature |
110 | ||
111 | C<use feature 'switch'> tells the compiler to enable the Perl 6 | |
112 | given/when construct. | |
113 | ||
114 | See L<perlsyn/"Switch statements"> for details. | |
115 | ||
1863b879 RGS |
116 | =head2 the 'unicode_strings' feature |
117 | ||
20db7501 KW |
118 | C<use feature 'unicode_strings'> tells the compiler to use Unicode semantics |
119 | in all string operations executed within its scope (unless they are also | |
120 | within the scope of either C<use locale> or C<use bytes>). The same applies | |
121 | to all regular expressions compiled within the scope, even if executed outside | |
122 | it. | |
123 | ||
124 | C<no feature 'unicode_strings'> tells the compiler to use the traditional | |
125 | Perl semantics wherein the native character set semantics is used unless it is | |
126 | clear to Perl that Unicode is desired. This can lead to some surprises | |
127 | when the behavior suddenly changes. (See | |
128 | L<perlunicode/The "Unicode Bug"> for details.) For this reason, if you are | |
129 | potentially using Unicode in your program, the | |
130 | C<use feature 'unicode_strings'> subpragma is B<strongly> recommended. | |
131 | ||
132 | This subpragma is available starting with Perl 5.11.3, but was not fully | |
133 | implemented until 5.13.8. | |
1863b879 | 134 | |
7289c5e6 FC |
135 | =head2 the 'unicode_eval' and 'evalbytes' features |
136 | ||
137 | Under the C<unicode_eval> feature, Perl's C<eval> function, when passed a | |
138 | string, will evaluate it as a string of characters, ignoring any | |
139 | C<use utf8> declarations. C<use utf8> exists to declare the encoding of | |
140 | the script, which only makes sense for a stream of bytes, not a string of | |
141 | characters. Source filters are forbidden, as they also really only make | |
142 | sense on strings of bytes. Any attempt to activate a source filter will | |
143 | result in an error. | |
144 | ||
145 | The C<evalbytes> feature enables the C<evalbytes> keyword, which evaluates | |
146 | the argument passed to it as a string of bytes. It dies if the string | |
147 | contains any characters outside the 8-bit range. Source filters work | |
148 | within C<evalbytes>: they apply to the contents of the string being | |
149 | evaluated. | |
150 | ||
151 | Together, these two features are intended to replace the historical C<eval> | |
152 | function, which has (at least) two bugs in it, that cannot easily be fixed | |
153 | without breaking existing programs: | |
154 | ||
155 | =over | |
156 | ||
157 | =item * | |
158 | ||
159 | C<eval> behaves differently depending on the internal encoding of the | |
160 | string, sometimes treating its argument as a string of bytes, and sometimes | |
161 | as a string of characters. | |
162 | ||
163 | =item * | |
164 | ||
165 | Source filters activated within C<eval> leak out into whichever I<file> | |
166 | scope is currently being compiled. To give an example with the CPAN module | |
167 | L<Semi::Semicolons>: | |
168 | ||
169 | BEGIN { eval "use Semi::Semicolons; # not filtered here " } | |
170 | # filtered here! | |
171 | ||
172 | C<evalbytes> fixes that to work the way one would expect: | |
173 | ||
174 | use feature "evalbytes"; | |
175 | BEGIN { evalbytes "use Semi::Semicolons; # filtered " } | |
176 | # not filtered | |
177 | ||
178 | =back | |
179 | ||
180 | These two features are available starting with Perl 5.16. | |
181 | ||
84ed0108 FC |
182 | =head2 The 'current_sub' feature |
183 | ||
184 | This provides the C<__SUB__> token that returns a reference to the current | |
185 | subroutine or C<undef> outside of a subroutine. | |
186 | ||
187 | This feature is available starting with Perl 5.16. | |
188 | ||
bc9b29db RH |
189 | =head1 FEATURE BUNDLES |
190 | ||
191 | It's possible to load a whole slew of features in one go, using | |
192 | a I<feature bundle>. The name of a feature bundle is prefixed with | |
193 | a colon, to distinguish it from an actual feature. At present, the | |
a3a91442 JV |
194 | only feature bundles correspond to Perl releases, e.g. C<use feature |
195 | ":5.10"> which is equivalent to C<use feature qw(switch say state)>. | |
8fd870d9 | 196 | |
a3a91442 JV |
197 | By convention, the feature bundle for any given Perl release includes |
198 | the features of previous releases, down to and including 5.10, the | |
199 | first official release to provide this facility. Since Perl 5.12 | |
200 | only provides one new feature, C<unicode_strings>, and Perl 5.14 | |
201 | provides none, C<use feature ":5.14"> is equivalent to C<use feature | |
202 | qw(switch say state unicode_strings)>. | |
203 | ||
204 | Specifying sub-versions such as the C<0> in C<5.14.0> in feature bundles has | |
82cfb3a2 | 205 | no effect: feature bundles are guaranteed to be the same for all sub-versions. |
bc9b29db | 206 | |
a3a91442 JV |
207 | Note that instead of using release-based feature bundles it is usually |
208 | better, and shorter, to use implicit loading as described below. | |
209 | ||
7dfde25d RGS |
210 | =head1 IMPLICIT LOADING |
211 | ||
212 | There are two ways to load the C<feature> pragma implicitly : | |
213 | ||
214 | =over 4 | |
215 | ||
216 | =item * | |
217 | ||
218 | By using the C<-E> switch on the command-line instead of C<-e>. It enables | |
219 | all available features in the main compilation unit (that is, the one-liner.) | |
220 | ||
221 | =item * | |
222 | ||
223 | By requiring explicitly a minimal Perl version number for your program, with | |
224 | the C<use VERSION> construct, and when the version is higher than or equal to | |
8d115822 | 225 | 5.10.0. That is, |
7dfde25d | 226 | |
8d115822 | 227 | use 5.10.0; |
7dfde25d RGS |
228 | |
229 | will do an implicit | |
230 | ||
82cfb3a2 | 231 | use feature ':5.10'; |
7dfde25d | 232 | |
82cfb3a2 S |
233 | and so on. Note how the trailing sub-version is automatically stripped from the |
234 | version. | |
7dfde25d | 235 | |
8d115822 RB |
236 | But to avoid portability warnings (see L<perlfunc/use>), you may prefer: |
237 | ||
238 | use 5.010; | |
239 | ||
240 | with the same effect. | |
241 | ||
7dfde25d RGS |
242 | =back |
243 | ||
0d863452 RH |
244 | =cut |
245 | ||
246 | sub import { | |
0d863452 RH |
247 | my $class = shift; |
248 | if (@_ == 0) { | |
0d863452 RH |
249 | croak("No features specified"); |
250 | } | |
251 | while (@_) { | |
252 | my $name = shift(@_); | |
89c3975a RGS |
253 | if (substr($name, 0, 1) eq ":") { |
254 | my $v = substr($name, 1); | |
7be54ea7 | 255 | if (!exists $feature_bundle{$v}) { |
82cfb3a2 S |
256 | $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/; |
257 | if (!exists $feature_bundle{$v}) { | |
258 | unknown_feature_bundle(substr($name, 1)); | |
259 | } | |
bc9b29db | 260 | } |
7be54ea7 | 261 | unshift @_, @{$feature_bundle{$v}}; |
bc9b29db RH |
262 | next; |
263 | } | |
0d863452 | 264 | if (!exists $feature{$name}) { |
b42943c4 | 265 | unknown_feature($name); |
0d863452 RH |
266 | } |
267 | $^H{$feature{$name}} = 1; | |
1863b879 | 268 | $^H |= $hint_uni8bit if $name eq 'unicode_strings'; |
0d863452 RH |
269 | } |
270 | } | |
271 | ||
272 | sub unimport { | |
273 | my $class = shift; | |
274 | ||
275 | # A bare C<no feature> should disable *all* features | |
bc9b29db RH |
276 | if (!@_) { |
277 | delete @^H{ values(%feature) }; | |
1863b879 | 278 | $^H &= ~ $hint_uni8bit; |
bc9b29db RH |
279 | return; |
280 | } | |
281 | ||
282 | while (@_) { | |
283 | my $name = shift; | |
89c3975a RGS |
284 | if (substr($name, 0, 1) eq ":") { |
285 | my $v = substr($name, 1); | |
7be54ea7 | 286 | if (!exists $feature_bundle{$v}) { |
82cfb3a2 S |
287 | $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/; |
288 | if (!exists $feature_bundle{$v}) { | |
289 | unknown_feature_bundle(substr($name, 1)); | |
290 | } | |
bc9b29db | 291 | } |
7be54ea7 | 292 | unshift @_, @{$feature_bundle{$v}}; |
bc9b29db RH |
293 | next; |
294 | } | |
0d863452 | 295 | if (!exists($feature{$name})) { |
b42943c4 | 296 | unknown_feature($name); |
0d863452 RH |
297 | } |
298 | else { | |
299 | delete $^H{$feature{$name}}; | |
1863b879 | 300 | $^H &= ~ $hint_uni8bit if $name eq 'unicode_strings'; |
0d863452 RH |
301 | } |
302 | } | |
0d863452 RH |
303 | } |
304 | ||
b42943c4 RGS |
305 | sub unknown_feature { |
306 | my $feature = shift; | |
307 | croak(sprintf('Feature "%s" is not supported by Perl %vd', | |
308 | $feature, $^V)); | |
309 | } | |
310 | ||
311 | sub unknown_feature_bundle { | |
312 | my $feature = shift; | |
313 | croak(sprintf('Feature bundle "%s" is not supported by Perl %vd', | |
314 | $feature, $^V)); | |
315 | } | |
316 | ||
317 | sub croak { | |
318 | require Carp; | |
319 | Carp::croak(@_); | |
320 | } | |
321 | ||
0d863452 | 322 | 1; |