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[perl5.git] / lib / feature.pm
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1package feature;
2
712d05cf 3our $VERSION = '1.01';
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4
5# (feature name) => (internal name, used in %^H)
6my %feature = (
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7 switch => 'feature_switch',
8 "~~" => "feature_~~",
9 say => "feature_say",
bc9b29db 10 err => "feature_err",
c0cd9745 11 dor => "feature_err",
712d05cf 12 state => "feature_state",
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13);
14
15my %feature_bundle = (
712d05cf 16 "5.10" => [qw(switch ~~ say err state)],
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17);
18
19
20# Here are some notes that probably shouldn't be in the public
21# documentation, but which it's useful to have somewhere.
22#
23# One side-effect of the change is that C<prototype("CORE::continue")>
24# no longer throws the error C<Can't find an opnumber for "continue">.
25# One of the tests in t/op/cproto.t had to be changed to accommodate
26# this, but it really shouldn't affect real-world code.
27#
28# TODO:
29# - sort out the smartmatch semantics
30# - think about versioned features (use switch => 2)
31#
32# -- Robin 2005-12
33
34=head1 NAME
35
36feature - Perl pragma to enable new syntactic features
37
38=head1 SYNOPSIS
39
bc9b29db 40 use feature qw(switch say);
0d863452 41 given ($foo) {
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42 when (1) { say "\$foo == 1" }
43 when ([2,3]) { say "\$foo == 2 || \$foo == 3" }
44 when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
45 when ($_ > 100) { say "\$foo > 100" }
46 default { say "None of the above" }
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47 }
48
49=head1 DESCRIPTION
50
51It is usually impossible to add new syntax to Perl without breaking
52some existing programs. This pragma provides a way to minimize that
53risk. New syntactic constructs can be enabled by C<use feature 'foo'>,
54and will be parsed only when the appropriate feature pragma is in
55scope.
56
57=head2 The 'switch' feature
58
59C<use feature 'switch'> tells the compiler to enable the Perl 6
60given/when construct from here to the end of the enclosing BLOCK.
61
62See L<perlsyn/"Switch statements"> for details.
63
64=head2 The '~~' feature
65
66C<use feature '~~'> tells the compiler to enable the Perl 6
67smart match C<~~> operator from here to the end of the enclosing BLOCK.
68
69See L<perlsyn/"Smart Matching in Detail"> for details.
70
71=head2 The 'say' feature
72
73C<use feature 'say'> tells the compiler to enable the Perl 6
74C<say> function from here to the end of the enclosing BLOCK.
75
76See L<perlfunc/say> for details.
77
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78=head2 the 'err' feature
79
80C<use feature 'err'> tells the compiler to enable the C<err>
81operator from here to the end of the enclosing BLOCK.
82
83C<err> is a low-precedence variant of the C<//> operator:
84see C<perlop> for details.
85
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86=head2 the 'dor' feature
87
88The 'dor' feature is an alias for the 'err' feature.
89
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90=head2 the 'state' feature
91
92C<use feature 'state'> tells the compiler to enable C<state>
93variables from here to the end of the enclosing BLOCK.
94
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95=head1 FEATURE BUNDLES
96
97It's possible to load a whole slew of features in one go, using
98a I<feature bundle>. The name of a feature bundle is prefixed with
99a colon, to distinguish it from an actual feature. At present, the
100only feature bundle is C<use feature ":5.10">, which is equivalent
712d05cf 101to C<use feature qw(switch ~~ say err state)>.
bc9b29db 102
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103=cut
104
105sub import {
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106 my $class = shift;
107 if (@_ == 0) {
108 require Carp;
109 Carp->import("croak");
110 croak("No features specified");
111 }
112 while (@_) {
113 my $name = shift(@_);
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114 if ($name =~ /^:(.*)/) {
115 if (!exists $feature_bundle{$1}) {
116 require Carp;
117 Carp->import("croak");
118 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
119 $1, $^V));
120 }
121 unshift @_, @{$feature_bundle{$1}};
122 next;
123 }
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124 if (!exists $feature{$name}) {
125 require Carp;
126 Carp->import("croak");
127 croak(sprintf('Feature "%s" is not supported by Perl %vd',
128 $name, $^V));
129 }
130 $^H{$feature{$name}} = 1;
131 }
132}
133
134sub unimport {
135 my $class = shift;
136
137 # A bare C<no feature> should disable *all* features
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138 if (!@_) {
139 delete @^H{ values(%feature) };
140 return;
141 }
142
143 while (@_) {
144 my $name = shift;
145 if ($name =~ /^:(.*)/) {
146 if (!exists $feature_bundle{$1}) {
147 require Carp;
148 Carp->import("croak");
149 croak(sprintf('Feature bundle "%s" is not supported by Perl %vd',
150 $1, $^V));
151 }
152 unshift @_, @{$feature_bundle{$1}};
153 next;
154 }
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155 if (!exists($feature{$name})) {
156 require Carp;
157 Carp->import("croak");
158 croak(sprintf('Feature "%s" is not supported by Perl %vd',
159 $name, $^V));
160 }
161 else {
162 delete $^H{$feature{$name}};
163 }
164 }
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165}
166
1671;