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[perl5.git] / lib / feature.pm
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1package feature;
2
d052521a 3our $VERSION = '1.11';
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4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
7b9ef140 7 switch => 'feature_switch',
7b9ef140 8 say => "feature_say",
712d05cf 9 state => "feature_state",
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10);
11
12my %feature_bundle = (
f23102e2 13 "5.10.0" => [qw(switch say state)],
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d052521a 15
8fd870d9 16# latest version here
d052521a 17$feature_bundle{"5.10"} = $feature_bundle{sprintf("%vd",$^V)};
0d863452 18
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19$feature_bundle{"5.9.5"} = $feature_bundle{"5.10.0"};
20
0d863452 21# TODO:
1c321dc6 22# - think about versioned features (use feature switch => 2)
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23
24=head1 NAME
25
26feature - Perl pragma to enable new syntactic features
27
28=head1 SYNOPSIS
29
bc9b29db 30 use feature qw(switch say);
0d863452 31 given ($foo) {
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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" }
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37 }
38
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39 use feature ':5.10'; # loads all features available in perl 5.10
40
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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
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49=head2 Lexical effect
50
51Like other pragmas (C<use strict>, for example), features have a lexical
5e36ed56 52effect. C<use feature qw(foo)> will only make the feature "foo" available
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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
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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
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76=head2 The 'switch' feature
77
78C<use feature 'switch'> tells the compiler to enable the Perl 6
9eb27be9 79given/when construct.
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80
81See L<perlsyn/"Switch statements"> for details.
82
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83=head2 The 'say' feature
84
85C<use feature 'say'> tells the compiler to enable the Perl 6
9eb27be9 86C<say> function.
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87
88See L<perlfunc/say> for details.
89
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90=head2 the 'state' feature
91
92C<use feature 'state'> tells the compiler to enable C<state>
9eb27be9 93variables.
712d05cf 94
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95See L<perlsub/"Persistent Private Variables"> for details.
96
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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
8fd870d9 102only feature bundles are C<use feature ":5.10"> and C<use feature ":5.10.0">,
f23102e2 103which both are equivalent to C<use feature qw(switch say state)>.
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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">.
bc9b29db 107
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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
1235.9.5. That is,
124
125 use 5.9.5;
126
127will do an implicit
128
129 use feature ':5.9.5';
130
131and so on.
132
133=back
134
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135=cut
136
137sub import {
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138 my $class = shift;
139 if (@_ == 0) {
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140 croak("No features specified");
141 }
142 while (@_) {
143 my $name = shift(@_);
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144 if (substr($name, 0, 1) eq ":") {
145 my $v = substr($name, 1);
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146 if (!exists $feature_bundle{$v}) {
147 unknown_feature_bundle($v);
bc9b29db 148 }
7be54ea7 149 unshift @_, @{$feature_bundle{$v}};
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150 next;
151 }
0d863452 152 if (!exists $feature{$name}) {
b42943c4 153 unknown_feature($name);
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154 }
155 $^H{$feature{$name}} = 1;
156 }
157}
158
159sub unimport {
160 my $class = shift;
161
162 # A bare C<no feature> should disable *all* features
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163 if (!@_) {
164 delete @^H{ values(%feature) };
165 return;
166 }
167
168 while (@_) {
169 my $name = shift;
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170 if (substr($name, 0, 1) eq ":") {
171 my $v = substr($name, 1);
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172 if (!exists $feature_bundle{$v}) {
173 unknown_feature_bundle($v);
bc9b29db 174 }
7be54ea7 175 unshift @_, @{$feature_bundle{$v}};
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176 next;
177 }
0d863452 178 if (!exists($feature{$name})) {
b42943c4 179 unknown_feature($name);
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180 }
181 else {
182 delete $^H{$feature{$name}};
183 }
184 }
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185}
186
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187sub unknown_feature {
188 my $feature = shift;
189 croak(sprintf('Feature "%s" is not supported by Perl %vd',
190 $feature, $^V));
191}
192
193sub unknown_feature_bundle {
194 my $feature = shift;
195 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
196 $feature, $^V));
197}
198
199sub croak {
200 require Carp;
201 Carp::croak(@_);
202}
203
0d863452 2041;