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feature.pm: Note unicode_strings is not complete
[perl5.git] / lib / feature.pm
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1package feature;
2
e836a73b 3our $VERSION = '1.17';
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4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
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7 switch => 'feature_switch',
8 say => "feature_say",
9 state => "feature_state",
10 unicode_strings => "feature_unicode",
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11);
12
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13# This gets set (for now) in $^H as well as in %^H,
14# for runtime speed of the uc/lc/ucfirst/lcfirst functions.
b0f41c9d 15# See HINT_UNI_8_BIT in perl.h.
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16our $hint_uni8bit = 0x00000800;
17
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18# NB. the latest bundle must be loaded by the -E switch (see toke.c)
19
bc9b29db 20my %feature_bundle = (
82cfb3a2 21 "5.10" => [qw(switch say state)],
1863b879 22 "5.11" => [qw(switch say state unicode_strings)],
b7569deb 23 "5.12" => [qw(switch say state unicode_strings)],
e836a73b 24 "5.13" => [qw(switch say state unicode_strings)],
0d863452 25);
d052521a 26
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27# special case
28$feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};
7dfde25d 29
0d863452 30# TODO:
1c321dc6 31# - think about versioned features (use feature switch => 2)
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32
33=head1 NAME
34
e1b711da 35feature - Perl pragma to enable new features
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36
37=head1 SYNOPSIS
38
bc9b29db 39 use feature qw(switch say);
0d863452 40 given ($foo) {
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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" }
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46 }
47
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48 use feature ':5.10'; # loads all features available in perl 5.10
49
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50=head1 DESCRIPTION
51
52It is usually impossible to add new syntax to Perl without breaking
53some existing programs. This pragma provides a way to minimize that
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54risk. New syntactic constructs, or new semantic meanings to older
55constructs, can be enabled by C<use feature 'foo'>, and will be parsed
56only when the appropriate feature pragma is in scope.
0d863452 57
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58=head2 Lexical effect
59
60Like other pragmas (C<use strict>, for example), features have a lexical
5e36ed56 61effect. C<use feature qw(foo)> will only make the feature "foo" available
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62from that point to the end of the enclosing block.
63
64 {
65 use feature 'say';
66 say "say is available here";
67 }
68 print "But not here.\n";
69
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70=head2 C<no feature>
71
72Features can also be turned off by using C<no feature "foo">. This too
73has lexical effect.
74
75 use feature 'say';
76 say "say is available here";
77 {
78 no feature 'say';
79 print "But not here.\n";
80 }
81 say "Yet it is here.";
82
83C<no feature> with no features specified will turn off all features.
84
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85=head2 The 'switch' feature
86
87C<use feature 'switch'> tells the compiler to enable the Perl 6
9eb27be9 88given/when construct.
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89
90See L<perlsyn/"Switch statements"> for details.
91
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92=head2 The 'say' feature
93
94C<use feature 'say'> tells the compiler to enable the Perl 6
9eb27be9 95C<say> function.
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96
97See L<perlfunc/say> for details.
98
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99=head2 the 'state' feature
100
101C<use feature 'state'> tells the compiler to enable C<state>
9eb27be9 102variables.
712d05cf 103
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104See L<perlsub/"Persistent Private Variables"> for details.
105
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106=head2 the 'unicode_strings' feature
107
108C<use feature 'unicode_strings'> tells the compiler to treat
e1b711da 109all strings outside of C<use locale> and C<use bytes> as Unicode. It is
b12c4a8a 110available starting with Perl 5.11.3, but is not fully implemented.
1863b879 111
e1b711da 112See L<perlunicode/The "Unicode Bug"> for details.
1863b879 113
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114=head1 FEATURE BUNDLES
115
116It's possible to load a whole slew of features in one go, using
117a I<feature bundle>. The name of a feature bundle is prefixed with
118a colon, to distinguish it from an actual feature. At present, the
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119only feature bundle is C<use feature ":5.10"> which is equivalent
120to C<use feature qw(switch say state)>.
8fd870d9 121
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122Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has
123no effect: feature bundles are guaranteed to be the same for all sub-versions.
bc9b29db 124
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125=head1 IMPLICIT LOADING
126
127There are two ways to load the C<feature> pragma implicitly :
128
129=over 4
130
131=item *
132
133By using the C<-E> switch on the command-line instead of C<-e>. It enables
134all available features in the main compilation unit (that is, the one-liner.)
135
136=item *
137
138By requiring explicitly a minimal Perl version number for your program, with
139the C<use VERSION> construct, and when the version is higher than or equal to
8d115822 1405.10.0. That is,
7dfde25d 141
8d115822 142 use 5.10.0;
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143
144will do an implicit
145
82cfb3a2 146 use feature ':5.10';
7dfde25d 147
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148and so on. Note how the trailing sub-version is automatically stripped from the
149version.
7dfde25d 150
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151But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
152
153 use 5.010;
154
155with the same effect.
156
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157=back
158
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159=cut
160
161sub import {
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162 my $class = shift;
163 if (@_ == 0) {
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164 croak("No features specified");
165 }
166 while (@_) {
167 my $name = shift(@_);
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168 if (substr($name, 0, 1) eq ":") {
169 my $v = substr($name, 1);
7be54ea7 170 if (!exists $feature_bundle{$v}) {
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171 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
172 if (!exists $feature_bundle{$v}) {
173 unknown_feature_bundle(substr($name, 1));
174 }
bc9b29db 175 }
7be54ea7 176 unshift @_, @{$feature_bundle{$v}};
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177 next;
178 }
0d863452 179 if (!exists $feature{$name}) {
b42943c4 180 unknown_feature($name);
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181 }
182 $^H{$feature{$name}} = 1;
1863b879 183 $^H |= $hint_uni8bit if $name eq 'unicode_strings';
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184 }
185}
186
187sub unimport {
188 my $class = shift;
189
190 # A bare C<no feature> should disable *all* features
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191 if (!@_) {
192 delete @^H{ values(%feature) };
1863b879 193 $^H &= ~ $hint_uni8bit;
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194 return;
195 }
196
197 while (@_) {
198 my $name = shift;
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199 if (substr($name, 0, 1) eq ":") {
200 my $v = substr($name, 1);
7be54ea7 201 if (!exists $feature_bundle{$v}) {
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202 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
203 if (!exists $feature_bundle{$v}) {
204 unknown_feature_bundle(substr($name, 1));
205 }
bc9b29db 206 }
7be54ea7 207 unshift @_, @{$feature_bundle{$v}};
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208 next;
209 }
0d863452 210 if (!exists($feature{$name})) {
b42943c4 211 unknown_feature($name);
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212 }
213 else {
214 delete $^H{$feature{$name}};
1863b879 215 $^H &= ~ $hint_uni8bit if $name eq 'unicode_strings';
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216 }
217 }
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218}
219
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220sub unknown_feature {
221 my $feature = shift;
222 croak(sprintf('Feature "%s" is not supported by Perl %vd',
223 $feature, $^V));
224}
225
226sub unknown_feature_bundle {
227 my $feature = shift;
228 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
229 $feature, $^V));
230}
231
232sub croak {
233 require Carp;
234 Carp::croak(@_);
235}
236
0d863452 2371;