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Make Unicode constants under use utf8 work again
[perl5.git] / dist / constant / lib / constant.pm
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54310121 1package constant;
6515510f 2use 5.005;
83763826 3use strict;
d3a7d8c7 4use warnings::register;
17f410f9 5
6515510f 6use vars qw($VERSION %declared);
d12b49d6 7$VERSION = '1.21';
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8
9#=======================================================================
10
83763826 11# Some names are evil choices.
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12my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
13$keywords{UNITCHECK}++ if $] > 5.009;
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14
15my %forced_into_main = map +($_, 1),
16 qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
17
18my %forbidden = (%keywords, %forced_into_main);
19
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20my $str_end = $] >= 5.006 ? "\\z" : "\\Z";
21my $normal_constant_name = qr/^_?[^\W_0-9]\w*$str_end/;
22my $tolerable = qr/^[A-Za-z_]\w*$str_end/;
23my $boolean = qr/^[01]?$str_end/;
24
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25BEGIN {
26 # We'd like to do use constant _CAN_PCS => $] > 5.009002
27 # but that's a bit tricky before we load the constant module :-)
28 # By doing this, we save 1 run time check for *every* call to import.
29 no strict 'refs';
30 my $const = $] > 5.009002;
31 *_CAN_PCS = sub () {$const};
32}
33
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34#=======================================================================
35# import() - import symbols into user's namespace
36#
37# What we actually do is define a function in the caller's namespace
38# which returns the value. The function we create will normally
39# be inlined as a constant, thereby avoiding further sub calling
40# overhead.
41#=======================================================================
42sub import {
43 my $class = shift;
44 return unless @_; # Ignore 'use constant;'
b35226bb 45 my $constants;
3cb88d13 46 my $multiple = ref $_[0];
39a108ce 47 my $pkg = caller;
f7fd2659 48 my $flush_mro;
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49 my $symtab;
50
c5764f70 51 if (_CAN_PCS) {
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52 no strict 'refs';
53 $symtab = \%{$pkg . '::'};
54 };
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55
56 if ( $multiple ) {
57 if (ref $_[0] ne 'HASH') {
58 require Carp;
59 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
60 }
b35226bb 61 $constants = shift;
3cb88d13 62 } else {
15dc519f 63 unless (defined $_[0]) {
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64 require Carp;
65 Carp::croak("Can't use undef as constant name");
66 }
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67 $constants->{+shift} = undef;
68 }
3cb88d13 69
15dc519f 70 foreach my $name ( keys %$constants ) {
3cb88d13 71 # Normal constant name
672c0ce9 72 if ($name =~ $normal_constant_name and !$forbidden{$name}) {
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73 # Everything is okay
74
75 # Name forced into main, but we're not in main. Fatal.
76 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
77 require Carp;
78 Carp::croak("Constant name '$name' is forced into main::");
79
80 # Starts with double underscore. Fatal.
81 } elsif ($name =~ /^__/) {
82 require Carp;
83 Carp::croak("Constant name '$name' begins with '__'");
84
85 # Maybe the name is tolerable
672c0ce9 86 } elsif ($name =~ $tolerable) {
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87 # Then we'll warn only if you've asked for warnings
88 if (warnings::enabled()) {
89 if ($keywords{$name}) {
90 warnings::warn("Constant name '$name' is a Perl keyword");
91 } elsif ($forced_into_main{$name}) {
92 warnings::warn("Constant name '$name' is " .
93 "forced into package main::");
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94 }
95 }
96
97 # Looks like a boolean
98 # use constant FRED == fred;
672c0ce9 99 } elsif ($name =~ $boolean) {
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100 require Carp;
101 if (@_) {
102 Carp::croak("Constant name '$name' is invalid");
83763826 103 } else {
3cb88d13 104 Carp::croak("Constant name looks like boolean value");
83763826 105 }
83763826 106
83763826 107 } else {
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108 # Must have bad characters
109 require Carp;
110 Carp::croak("Constant name '$name' has invalid characters");
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111 }
112
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113 {
114 no strict 'refs';
115 my $full_name = "${pkg}::$name";
116 $declared{$full_name}++;
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117 if ($multiple || @_ == 1) {
118 my $scalar = $multiple ? $constants->{$name} : $_[0];
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119
120 # Work around perl bug #xxxxx: Sub names (actually glob
121 # names in general) ignore the UTF8 flag. So we have to
122 # turn it off to get the "right" symbol table entry.
123 utf8::is_utf8 $name and utf8::encode $name;
124
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125 # The constant serves to optimise this entire block out on
126 # 5.8 and earlier.
127 if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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128 # No typeglob yet, so we can use a reference as space-
129 # efficient proxy for a constant subroutine
130 # The check in Perl_ck_rvconst knows that inlinable
131 # constants from cv_const_sv are read only. So we have to:
132 Internals::SvREADONLY($scalar, 1);
133 $symtab->{$name} = \$scalar;
f7fd2659 134 ++$flush_mro;
3cb88d13 135 } else {
e040ff70 136 *$full_name = sub () { $scalar };
3cb88d13 137 }
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138 } elsif (@_) {
139 my @list = @_;
140 *$full_name = sub () { @list };
141 } else {
142 *$full_name = sub () { };
3cb88d13 143 }
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144 }
145 }
f7fd2659 146 # Flush the cache exactly once if we make any direct symbol table changes.
a8ae8fee 147 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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148}
149
1501;
151
152__END__
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153
154=head1 NAME
155
156constant - Perl pragma to declare constants
157
158=head1 SYNOPSIS
159
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160 use constant PI => 4 * atan2(1, 1);
161 use constant DEBUG => 0;
162
163 print "Pi equals ", PI, "...\n" if DEBUG;
164
3cb88d13 165 use constant {
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166 SEC => 0,
167 MIN => 1,
168 HOUR => 2,
169 MDAY => 3,
170 MON => 4,
171 YEAR => 5,
172 WDAY => 6,
173 YDAY => 7,
174 ISDST => 8,
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175 };
176
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177 use constant WEEKDAYS => qw(
178 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
179 );
180
181 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
182
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183=head1 DESCRIPTION
184
6515510f 185This pragma allows you to declare constants at compile-time.
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186
187When you declare a constant such as C<PI> using the method shown
188above, each machine your script runs upon can have as many digits
189of accuracy as it can use. Also, your program will be easier to
190read, more likely to be maintained (and maintained correctly), and
191far less likely to send a space probe to the wrong planet because
192nobody noticed the one equation in which you wrote C<3.14195>.
193
d3383c75 194When a constant is used in an expression, Perl replaces it with its
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195value at compile time, and may then optimize the expression further.
196In particular, any code in an C<if (CONSTANT)> block will be optimized
197away if the constant is false.
198
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199=head1 NOTES
200
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201As with all C<use> directives, defining a constant happens at
202compile time. Thus, it's probably not correct to put a constant
203declaration inside of a conditional statement (like C<if ($foo)
204{ use constant ... }>).
54310121 205
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206Constants defined using this module cannot be interpolated into
207strings like variables. However, concatenation works just fine:
54310121 208
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209 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
210 print "Pi equals ".PI."...\n"; # right
54310121 211
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212Even though a reference may be declared as a constant, the reference may
213point to data which may be changed, as this code shows.
214
215 use constant ARRAY => [ 1,2,3,4 ];
216 print ARRAY->[1];
217 ARRAY->[1] = " be changed";
218 print ARRAY->[1];
219
220Dereferencing constant references incorrectly (such as using an array
221subscript on a constant hash reference, or vice versa) will be trapped at
222compile time.
54310121 223
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224Constants belong to the package they are defined in. To refer to a
225constant defined in another package, specify the full package name, as
226in C<Some::Package::CONSTANT>. Constants may be exported by modules,
227and may also be called as either class or instance methods, that is,
228as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
229C<$obj> is an instance of C<Some::Package>. Subclasses may define
230their own constants to override those in their base class.
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231
232The use of all caps for constant names is merely a convention,
233although it is recommended in order to make constants stand out
234and to help avoid collisions with other barewords, keywords, and
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235subroutine names. Constant names must begin with a letter or
236underscore. Names beginning with a double underscore are reserved. Some
237poor choices for names will generate warnings, if warnings are enabled at
238compile time.
54310121 239
a747501d 240=head2 List constants
54310121 241
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242Constants may be lists of more (or less) than one value. A constant
243with no values evaluates to C<undef> in scalar context. Note that
244constants with more than one value do I<not> return their last value in
245scalar context as one might expect. They currently return the number
246of values, but B<this may change in the future>. Do not use constants
247with multiple values in scalar context.
3cb88d13 248
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249B<NOTE:> This implies that the expression defining the value of a
250constant is evaluated in list context. This may produce surprises:
54310121 251
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252 use constant TIMESTAMP => localtime; # WRONG!
253 use constant TIMESTAMP => scalar localtime; # right
54310121 254
a747501d 255The first line above defines C<TIMESTAMP> as a 9-element list, as
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256returned by C<localtime()> in list context. To set it to the string
257returned by C<localtime()> in scalar context, an explicit C<scalar>
a747501d 258keyword is required.
54310121 259
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260List constants are lists, not arrays. To index or slice them, they
261must be placed in parentheses.
54310121 262
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263 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
264 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 265
a747501d 266=head2 Defining multiple constants at once
b0d6893f 267
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268Instead of writing multiple C<use constant> statements, you may define
269multiple constants in a single statement by giving, instead of the
270constant name, a reference to a hash where the keys are the names of
271the constants to be defined. Obviously, all constants defined using
272this method must have a single value.
273
274 use constant {
275 FOO => "A single value",
276 BAR => "This", "won't", "work!", # Error!
277 };
278
279This is a fundamental limitation of the way hashes are constructed in
280Perl. The error messages produced when this happens will often be
281quite cryptic -- in the worst case there may be none at all, and
282you'll only later find that something is broken.
283
284When defining multiple constants, you cannot use the values of other
285constants defined in the same declaration. This is because the
286calling package doesn't know about any constant within that group
287until I<after> the C<use> statement is finished.
288
289 use constant {
290 BITMASK => 0xAFBAEBA8,
291 NEGMASK => ~BITMASK, # Error!
292 };
293
294=head2 Magic constants
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295
296Magical values and references can be made into constants at compile
297time, allowing for way cool stuff like this. (These error numbers
298aren't totally portable, alas.)
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299
300 use constant E2BIG => ($! = 7);
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301 print E2BIG, "\n"; # something like "Arg list too long"
302 print 0+E2BIG, "\n"; # "7"
54310121 303
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304You can't produce a tied constant by giving a tied scalar as the
305value. References to tied variables, however, can be used as
306constants without any problems.
307
a747501d 308=head1 TECHNICAL NOTES
b0d6893f 309
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310In the current implementation, scalar constants are actually
311inlinable subroutines. As of version 5.004 of Perl, the appropriate
312scalar constant is inserted directly in place of some subroutine
313calls, thereby saving the overhead of a subroutine call. See
314L<perlsub/"Constant Functions"> for details about how and when this
315happens.
3cb88d13 316
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317In the rare case in which you need to discover at run time whether a
318particular constant has been declared via this module, you may use
319this function to examine the hash C<%constant::declared>. If the given
320constant name does not include a package name, the current package is
321used.
322
323 sub declared ($) {
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324 use constant 1.01; # don't omit this!
325 my $name = shift;
326 $name =~ s/^::/main::/;
327 my $pkg = caller;
328 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
329 $constant::declared{$full_name};
83763826 330 }
779c5bc9 331
6515510f 332=head1 CAVEATS
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333
334In the current version of Perl, list constants are not inlined
335and some symbols may be redefined without generating a warning.
336
a747501d 337It is not possible to have a subroutine or a keyword with the same
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338name as a constant in the same package. This is probably a Good Thing.
339
340A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
341ENV INC SIG> is not allowed anywhere but in package C<main::>, for
342technical reasons.
343
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344Unlike constants in some languages, these cannot be overridden
345on the command line or via environment variables.
346
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347You can get into trouble if you use constants in a context which
348automatically quotes barewords (as is true for any subroutine call).
349For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
350be interpreted as a string. Use C<$hash{CONSTANT()}> or
351C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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352kicking in. Similarly, since the C<< => >> operator quotes a bareword
353immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 354(or simply use a comma in place of the big arrow) instead of
a747501d 355C<< CONSTANT => 'value' >>.
a3cb178b 356
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357=head1 SEE ALSO
358
359L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
360
361L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
362but uses C<SvREADONLY> instead of C<tie>.
363
364L<Attribute::Constant> - Make read-only variables via attribute
365
366L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
367
368L<Hash::Util> - A selection of general-utility hash subroutines (mostly
369to lock/unlock keys and values)
370
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371=head1 BUGS
372
373Please report any bugs or feature requests via the perlbug(1) utility.
374
375=head1 AUTHORS
54310121 376
83763826 377Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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378many other folks.
379
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380Multiple constant declarations at once added by Casey West,
381E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 382
a747501d 383Documentation mostly rewritten by Ilmari Karonen,
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384E<lt>F<perl@itz.pp.sci.fi>E<gt>.
385
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386This program is maintained by the Perl 5 Porters.
387The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
388E<lt>F<sebastien@aperghis.net>E<gt>.
389
d3383c75 390=head1 COPYRIGHT & LICENSE
54310121 391
83763826 392Copyright (C) 1997, 1999 Tom Phoenix
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393
394This module is free software; you can redistribute it or modify it
395under the same terms as Perl itself.
396
397=cut