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[perl5.git] / lib / feature.pm
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1package feature;
2
3our $VERSION = '1.00';
7168684c 4$feature::hint_bits = 0x00020000; # HINT_LOCALIZE_HH
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5
6# (feature name) => (internal name, used in %^H)
7my %feature = (
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8 switch => 'feature_switch',
9 "~~" => "feature_~~",
10 say => "feature_say",
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11 err => "feature_err",
12);
13
14my %feature_bundle = (
15 "5.10" => [qw(switch ~~ say err)],
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16);
17
18
19# Here are some notes that probably shouldn't be in the public
20# documentation, but which it's useful to have somewhere.
21#
22# One side-effect of the change is that C<prototype("CORE::continue")>
23# no longer throws the error C<Can't find an opnumber for "continue">.
24# One of the tests in t/op/cproto.t had to be changed to accommodate
25# this, but it really shouldn't affect real-world code.
26#
27# TODO:
28# - sort out the smartmatch semantics
29# - think about versioned features (use switch => 2)
30#
31# -- Robin 2005-12
32
33=head1 NAME
34
35feature - Perl pragma to enable new syntactic features
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
48=head1 DESCRIPTION
49
50It is usually impossible to add new syntax to Perl without breaking
51some existing programs. This pragma provides a way to minimize that
52risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
53and will be parsed only when the appropriate feature pragma is in
54scope.
55
56=head2 The 'switch' feature
57
58C<use feature 'switch'> tells the compiler to enable the Perl 6
59given/when construct from here to the end of the enclosing BLOCK.
60
61See L<perlsyn/"Switch statements"> for details.
62
63=head2 The '~~' feature
64
65C<use feature '~~'> tells the compiler to enable the Perl 6
66smart match C<~~> operator from here to the end of the enclosing BLOCK.
67
68See L<perlsyn/"Smart Matching in Detail"> for details.
69
70=head2 The 'say' feature
71
72C<use feature 'say'> tells the compiler to enable the Perl 6
73C<say> function from here to the end of the enclosing BLOCK.
74
75See L<perlfunc/say> for details.
76
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77=head2 the 'err' feature
78
79C<use feature 'err'> tells the compiler to enable the C<err>
80operator from here to the end of the enclosing BLOCK.
81
82C<err> is a low-precedence variant of the C<//> operator:
83see C<perlop> for details.
84
85=head1 FEATURE BUNDLES
86
87It's possible to load a whole slew of features in one go, using
88a I<feature bundle>. The name of a feature bundle is prefixed with
89a colon, to distinguish it from an actual feature. At present, the
90only feature bundle is C<use feature ":5.10">, which is equivalent
91to C<use feature qw(switch ~~ say err)>.
92
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93=cut
94
95sub import {
96 $^H |= $feature::hint_bits; # Need this or %^H won't work
97
98 my $class = shift;
99 if (@_ == 0) {
100 require Carp;
101 Carp->import("croak");
102 croak("No features specified");
103 }
104 while (@_) {
105 my $name = shift(@_);
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106 if ($name =~ /^:(.*)/) {
107 if (!exists $feature_bundle{$1}) {
108 require Carp;
109 Carp->import("croak");
110 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
111 $1, $^V));
112 }
113 unshift @_, @{$feature_bundle{$1}};
114 next;
115 }
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116 if (!exists $feature{$name}) {
117 require Carp;
118 Carp->import("croak");
119 croak(sprintf('Feature "%s" is not supported by Perl %vd',
120 $name, $^V));
121 }
122 $^H{$feature{$name}} = 1;
123 }
124}
125
126sub unimport {
127 my $class = shift;
128
129 # A bare C<no feature> should disable *all* features
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130 if (!@_) {
131 delete @^H{ values(%feature) };
132 return;
133 }
134
135 while (@_) {
136 my $name = shift;
137 if ($name =~ /^:(.*)/) {
138 if (!exists $feature_bundle{$1}) {
139 require Carp;
140 Carp->import("croak");
141 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
142 $1, $^V));
143 }
144 unshift @_, @{$feature_bundle{$1}};
145 next;
146 }
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147 if (!exists($feature{$name})) {
148 require Carp;
149 Carp->import("croak");
150 croak(sprintf('Feature "%s" is not supported by Perl %vd',
151 $name, $^V));
152 }
153 else {
154 delete $^H{$feature{$name}};
155 }
156 }
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157}
158
1591;