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Re: [PATCH] Re: Perl @ 32984 (also relevant to [perl #33849])
[perl5.git] / lib / feature.pm
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1package feature;
2
270d01de 3our $VERSION = '1.12';
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4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
7b9ef140 7 switch => 'feature_switch',
7b9ef140 8 say => "feature_say",
712d05cf 9 state => "feature_state",
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10);
11
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12# NB. the latest bundle must be loaded by the -E switch (see toke.c)
13
bc9b29db 14my %feature_bundle = (
f23102e2 15 "5.10.0" => [qw(switch say state)],
270d01de 16 "5.11.0" => [qw(switch say state)],
0d863452 17);
d052521a 18
8fd870d9 19# latest version here
270d01de 20$feature_bundle{"5.11"} = $feature_bundle{sprintf("%vd",$^V)};
0d863452 21
270d01de 22$feature_bundle{"5.10"} = $feature_bundle{"5.10.0"};
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23$feature_bundle{"5.9.5"} = $feature_bundle{"5.10.0"};
24
0d863452 25# TODO:
1c321dc6 26# - think about versioned features (use feature switch => 2)
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27
28=head1 NAME
29
30feature - Perl pragma to enable new syntactic features
31
32=head1 SYNOPSIS
33
bc9b29db 34 use feature qw(switch say);
0d863452 35 given ($foo) {
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36 when (1) { say "\$foo == 1" }
37 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
38 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
39 when ($_ > 100) { say "\$foo > 100" }
40 default { say "None of the above" }
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41 }
42
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43 use feature ':5.10'; # loads all features available in perl 5.10
44
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45=head1 DESCRIPTION
46
47It is usually impossible to add new syntax to Perl without breaking
48some existing programs. This pragma provides a way to minimize that
49risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
50and will be parsed only when the appropriate feature pragma is in
51scope.
52
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53=head2 Lexical effect
54
55Like other pragmas (C<use strict>, for example), features have a lexical
5e36ed56 56effect. C<use feature qw(foo)> will only make the feature "foo" available
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57from that point to the end of the enclosing block.
58
59 {
60 use feature 'say';
61 say "say is available here";
62 }
63 print "But not here.\n";
64
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65=head2 C<no feature>
66
67Features can also be turned off by using C<no feature "foo">. This too
68has lexical effect.
69
70 use feature 'say';
71 say "say is available here";
72 {
73 no feature 'say';
74 print "But not here.\n";
75 }
76 say "Yet it is here.";
77
78C<no feature> with no features specified will turn off all features.
79
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80=head2 The 'switch' feature
81
82C<use feature 'switch'> tells the compiler to enable the Perl 6
9eb27be9 83given/when construct.
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84
85See L<perlsyn/"Switch statements"> for details.
86
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87=head2 The 'say' feature
88
89C<use feature 'say'> tells the compiler to enable the Perl 6
9eb27be9 90C<say> function.
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91
92See L<perlfunc/say> for details.
93
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94=head2 the 'state' feature
95
96C<use feature 'state'> tells the compiler to enable C<state>
9eb27be9 97variables.
712d05cf 98
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99See L<perlsub/"Persistent Private Variables"> for details.
100
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101=head1 FEATURE BUNDLES
102
103It's possible to load a whole slew of features in one go, using
104a I<feature bundle>. The name of a feature bundle is prefixed with
105a colon, to distinguish it from an actual feature. At present, the
8fd870d9 106only feature bundles are C<use feature ":5.10"> and C<use feature ":5.10.0">,
f23102e2 107which both are equivalent to C<use feature qw(switch say state)>.
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108
109In the forthcoming 5.10.X perl releases, C<use feature ":5.10"> will be
110equivalent to the latest C<use feature ":5.10.X">.
bc9b29db 111
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112=head1 IMPLICIT LOADING
113
114There are two ways to load the C<feature> pragma implicitly :
115
116=over 4
117
118=item *
119
120By using the C<-E> switch on the command-line instead of C<-e>. It enables
121all available features in the main compilation unit (that is, the one-liner.)
122
123=item *
124
125By requiring explicitly a minimal Perl version number for your program, with
126the C<use VERSION> construct, and when the version is higher than or equal to
8d115822 1275.10.0. That is,
7dfde25d 128
8d115822 129 use 5.10.0;
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130
131will do an implicit
132
8d115822 133 use feature ':5.10.0';
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134
135and so on.
136
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137But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
138
139 use 5.010;
140
141with the same effect.
142
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143=back
144
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145=cut
146
147sub import {
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148 my $class = shift;
149 if (@_ == 0) {
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150 croak("No features specified");
151 }
152 while (@_) {
153 my $name = shift(@_);
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154 if (substr($name, 0, 1) eq ":") {
155 my $v = substr($name, 1);
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156 if (!exists $feature_bundle{$v}) {
157 unknown_feature_bundle($v);
bc9b29db 158 }
7be54ea7 159 unshift @_, @{$feature_bundle{$v}};
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160 next;
161 }
0d863452 162 if (!exists $feature{$name}) {
b42943c4 163 unknown_feature($name);
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164 }
165 $^H{$feature{$name}} = 1;
166 }
167}
168
169sub unimport {
170 my $class = shift;
171
172 # A bare C<no feature> should disable *all* features
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173 if (!@_) {
174 delete @^H{ values(%feature) };
175 return;
176 }
177
178 while (@_) {
179 my $name = shift;
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180 if (substr($name, 0, 1) eq ":") {
181 my $v = substr($name, 1);
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182 if (!exists $feature_bundle{$v}) {
183 unknown_feature_bundle($v);
bc9b29db 184 }
7be54ea7 185 unshift @_, @{$feature_bundle{$v}};
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186 next;
187 }
0d863452 188 if (!exists($feature{$name})) {
b42943c4 189 unknown_feature($name);
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190 }
191 else {
192 delete $^H{$feature{$name}};
193 }
194 }
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195}
196
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197sub unknown_feature {
198 my $feature = shift;
199 croak(sprintf('Feature "%s" is not supported by Perl %vd',
200 $feature, $^V));
201}
202
203sub unknown_feature_bundle {
204 my $feature = shift;
205 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
206 $feature, $^V));
207}
208
209sub croak {
210 require Carp;
211 Carp::croak(@_);
212}
213
0d863452 2141;