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Move the documentation of the lexical effect of feature.pm to its own
[perl5.git] / lib / feature.pm
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1package feature;
2
712d05cf 3our $VERSION = '1.01';
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4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
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7 switch => 'feature_switch',
8 "~~" => "feature_~~",
9 say => "feature_say",
bc9b29db 10 err => "feature_err",
c0cd9745 11 dor => "feature_err",
712d05cf 12 state => "feature_state",
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13);
14
15my %feature_bundle = (
712d05cf 16 "5.10" => [qw(switch ~~ say err state)],
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17);
18
19
0d863452 20# TODO:
1c321dc6 21# - think about versioned features (use feature switch => 2)
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22
23=head1 NAME
24
25feature - Perl pragma to enable new syntactic features
26
27=head1 SYNOPSIS
28
bc9b29db 29 use feature qw(switch say);
0d863452 30 given ($foo) {
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31 when (1) { say "\$foo == 1" }
32 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
33 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
34 when ($_ > 100) { say "\$foo > 100" }
35 default { say "None of the above" }
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36 }
37
38=head1 DESCRIPTION
39
40It is usually impossible to add new syntax to Perl without breaking
41some existing programs. This pragma provides a way to minimize that
42risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
43and will be parsed only when the appropriate feature pragma is in
44scope.
45
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46=head2 Lexical effect
47
48Like other pragmas (C<use strict>, for example), features have a lexical
49effect. C<use feature qw(foo)> will only make the feature "foo" available
50from that point to the end of the enclosing block.
51
52 {
53 use feature 'say';
54 say "say is available here";
55 }
56 print "But not here.\n";
57
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58=head2 The 'switch' feature
59
60C<use feature 'switch'> tells the compiler to enable the Perl 6
9eb27be9 61given/when construct.
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62
63See L<perlsyn/"Switch statements"> for details.
64
65=head2 The '~~' feature
66
67C<use feature '~~'> tells the compiler to enable the Perl 6
9eb27be9 68smart match C<~~> operator.
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69
70See L<perlsyn/"Smart Matching in Detail"> for details.
71
72=head2 The 'say' feature
73
74C<use feature 'say'> tells the compiler to enable the Perl 6
9eb27be9 75C<say> function.
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76
77See L<perlfunc/say> for details.
78
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79=head2 the 'err' feature
80
81C<use feature 'err'> tells the compiler to enable the C<err>
9eb27be9 82operator.
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83
84C<err> is a low-precedence variant of the C<//> operator:
85see C<perlop> for details.
86
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87=head2 the 'dor' feature
88
89The 'dor' feature is an alias for the 'err' feature.
90
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91=head2 the 'state' feature
92
93C<use feature 'state'> tells the compiler to enable C<state>
9eb27be9 94variables.
712d05cf 95
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96See L<perlsub/"Persistent Private Variables"> for details.
97
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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
103only feature bundle is C<use feature ":5.10">, which is equivalent
712d05cf 104to C<use feature qw(switch ~~ say err state)>.
bc9b29db 105
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106=cut
107
108sub import {
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109 my $class = shift;
110 if (@_ == 0) {
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111 croak("No features specified");
112 }
113 while (@_) {
114 my $name = shift(@_);
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115 if ($name =~ /^:(.*)/) {
116 if (!exists $feature_bundle{$1}) {
b42943c4 117 unknown_feature_bundle($1);
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118 }
119 unshift @_, @{$feature_bundle{$1}};
120 next;
121 }
0d863452 122 if (!exists $feature{$name}) {
b42943c4 123 unknown_feature($name);
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124 }
125 $^H{$feature{$name}} = 1;
126 }
127}
128
129sub unimport {
130 my $class = shift;
131
132 # A bare C<no feature> should disable *all* features
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133 if (!@_) {
134 delete @^H{ values(%feature) };
135 return;
136 }
137
138 while (@_) {
139 my $name = shift;
140 if ($name =~ /^:(.*)/) {
141 if (!exists $feature_bundle{$1}) {
b42943c4 142 unknown_feature_bundle($1);
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143 }
144 unshift @_, @{$feature_bundle{$1}};
145 next;
146 }
0d863452 147 if (!exists($feature{$name})) {
b42943c4 148 unknown_feature($name);
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149 }
150 else {
151 delete $^H{$feature{$name}};
152 }
153 }
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154}
155
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156sub unknown_feature {
157 my $feature = shift;
158 croak(sprintf('Feature "%s" is not supported by Perl %vd',
159 $feature, $^V));
160}
161
162sub unknown_feature_bundle {
163 my $feature = shift;
164 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
165 $feature, $^V));
166}
167
168sub croak {
169 require Carp;
170 Carp::croak(@_);
171}
172
0d863452 1731;