This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
Speed-up notice for bisects (skip Encode)
[perl5.git] / lib / feature.pm
CommitLineData
0d863452
RH
1package feature;
2
82cfb3a2 3our $VERSION = '1.13';
0d863452
RH
4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
7b9ef140 7 switch => 'feature_switch',
7b9ef140 8 say => "feature_say",
712d05cf 9 state => "feature_state",
bc9b29db
RH
10);
11
13a7998c
RGS
12# NB. the latest bundle must be loaded by the -E switch (see toke.c)
13
bc9b29db 14my %feature_bundle = (
82cfb3a2
S
15 "5.10" => [qw(switch say state)],
16 "5.11" => [qw(switch say state)],
0d863452 17);
d052521a 18
82cfb3a2
S
19# special case
20$feature_bundle{"5.9.5"} = $feature_bundle{"5.10"};
7dfde25d 21
0d863452 22# TODO:
1c321dc6 23# - think about versioned features (use feature switch => 2)
0d863452
RH
24
25=head1 NAME
26
27feature - Perl pragma to enable new syntactic features
28
29=head1 SYNOPSIS
30
bc9b29db 31 use feature qw(switch say);
0d863452 32 given ($foo) {
bc9b29db
RH
33 when (1) { say "\$foo == 1" }
34 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
35 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
36 when ($_ > 100) { say "\$foo > 100" }
37 default { say "None of the above" }
0d863452
RH
38 }
39
ec488c7f
RGS
40 use feature ':5.10'; # loads all features available in perl 5.10
41
0d863452
RH
42=head1 DESCRIPTION
43
44It is usually impossible to add new syntax to Perl without breaking
45some existing programs. This pragma provides a way to minimize that
46risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
47and will be parsed only when the appropriate feature pragma is in
48scope.
49
9eb27be9
RGS
50=head2 Lexical effect
51
52Like other pragmas (C<use strict>, for example), features have a lexical
5e36ed56 53effect. C<use feature qw(foo)> will only make the feature "foo" available
9eb27be9
RGS
54from that point to the end of the enclosing block.
55
56 {
57 use feature 'say';
58 say "say is available here";
59 }
60 print "But not here.\n";
61
5e36ed56
RGS
62=head2 C<no feature>
63
64Features can also be turned off by using C<no feature "foo">. This too
65has lexical effect.
66
67 use feature 'say';
68 say "say is available here";
69 {
70 no feature 'say';
71 print "But not here.\n";
72 }
73 say "Yet it is here.";
74
75C<no feature> with no features specified will turn off all features.
76
0d863452
RH
77=head2 The 'switch' feature
78
79C<use feature 'switch'> tells the compiler to enable the Perl 6
9eb27be9 80given/when construct.
0d863452
RH
81
82See L<perlsyn/"Switch statements"> for details.
83
0d863452
RH
84=head2 The 'say' feature
85
86C<use feature 'say'> tells the compiler to enable the Perl 6
9eb27be9 87C<say> function.
0d863452
RH
88
89See L<perlfunc/say> for details.
90
712d05cf
RGS
91=head2 the 'state' feature
92
93C<use feature 'state'> tells the compiler to enable C<state>
9eb27be9 94variables.
712d05cf 95
e60bcc8b
RGS
96See L<perlsub/"Persistent Private Variables"> for details.
97
bc9b29db
RH
98=head1 FEATURE BUNDLES
99
100It's possible to load a whole slew of features in one go, using
101a I<feature bundle>. The name of a feature bundle is prefixed with
102a colon, to distinguish it from an actual feature. At present, the
82cfb3a2
S
103only feature bundle is C<use feature ":5.10"> which is equivalent
104to C<use feature qw(switch say state)>.
8fd870d9 105
82cfb3a2
S
106Specifying sub-versions such as the C<0> in C<5.10.0> in feature bundles has
107no effect: feature bundles are guaranteed to be the same for all sub-versions.
bc9b29db 108
7dfde25d
RGS
109=head1 IMPLICIT LOADING
110
111There are two ways to load the C<feature> pragma implicitly :
112
113=over 4
114
115=item *
116
117By using the C<-E> switch on the command-line instead of C<-e>. It enables
118all available features in the main compilation unit (that is, the one-liner.)
119
120=item *
121
122By requiring explicitly a minimal Perl version number for your program, with
123the C<use VERSION> construct, and when the version is higher than or equal to
8d115822 1245.10.0. That is,
7dfde25d 125
8d115822 126 use 5.10.0;
7dfde25d
RGS
127
128will do an implicit
129
82cfb3a2 130 use feature ':5.10';
7dfde25d 131
82cfb3a2
S
132and so on. Note how the trailing sub-version is automatically stripped from the
133version.
7dfde25d 134
8d115822
RB
135But to avoid portability warnings (see L<perlfunc/use>), you may prefer:
136
137 use 5.010;
138
139with the same effect.
140
7dfde25d
RGS
141=back
142
0d863452
RH
143=cut
144
145sub import {
0d863452
RH
146 my $class = shift;
147 if (@_ == 0) {
0d863452
RH
148 croak("No features specified");
149 }
150 while (@_) {
151 my $name = shift(@_);
89c3975a
RGS
152 if (substr($name, 0, 1) eq ":") {
153 my $v = substr($name, 1);
7be54ea7 154 if (!exists $feature_bundle{$v}) {
82cfb3a2
S
155 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
156 if (!exists $feature_bundle{$v}) {
157 unknown_feature_bundle(substr($name, 1));
158 }
bc9b29db 159 }
7be54ea7 160 unshift @_, @{$feature_bundle{$v}};
bc9b29db
RH
161 next;
162 }
0d863452 163 if (!exists $feature{$name}) {
b42943c4 164 unknown_feature($name);
0d863452
RH
165 }
166 $^H{$feature{$name}} = 1;
167 }
168}
169
170sub unimport {
171 my $class = shift;
172
173 # A bare C<no feature> should disable *all* features
bc9b29db
RH
174 if (!@_) {
175 delete @^H{ values(%feature) };
176 return;
177 }
178
179 while (@_) {
180 my $name = shift;
89c3975a
RGS
181 if (substr($name, 0, 1) eq ":") {
182 my $v = substr($name, 1);
7be54ea7 183 if (!exists $feature_bundle{$v}) {
82cfb3a2
S
184 $v =~ s/^([0-9]+)\.([0-9]+).[0-9]+$/$1.$2/;
185 if (!exists $feature_bundle{$v}) {
186 unknown_feature_bundle(substr($name, 1));
187 }
bc9b29db 188 }
7be54ea7 189 unshift @_, @{$feature_bundle{$v}};
bc9b29db
RH
190 next;
191 }
0d863452 192 if (!exists($feature{$name})) {
b42943c4 193 unknown_feature($name);
0d863452
RH
194 }
195 else {
196 delete $^H{$feature{$name}};
197 }
198 }
0d863452
RH
199}
200
b42943c4
RGS
201sub unknown_feature {
202 my $feature = shift;
203 croak(sprintf('Feature "%s" is not supported by Perl %vd',
204 $feature, $^V));
205}
206
207sub unknown_feature_bundle {
208 my $feature = shift;
209 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
210 $feature, $^V));
211}
212
213sub croak {
214 require Carp;
215 Carp::croak(@_);
216}
217
0d863452 2181;