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Make dual-lived constant.pm work on 5.8 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($VERSION %declared);
2d1c5561 7$VERSION = '1.24';
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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};
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32
33 # Before this makes its way into a dev perl release, we have to do
34 # browser-sniffing, as it were....
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35 *{chr 256} = \3;
36 if (exists ${__PACKAGE__."::"}{"\xc4\x80"}) {
37 delete ${__PACKAGE__."::"}{"\xc4\x80"};
38 *_DOWNGRADE = sub () {1};
39 }
40 else {
41 delete ${__PACKAGE__."::"}{chr 256};
42 *_DOWNGRADE = sub () {0};
43 }
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44}
45
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46#=======================================================================
47# import() - import symbols into user's namespace
48#
49# What we actually do is define a function in the caller's namespace
50# which returns the value. The function we create will normally
51# be inlined as a constant, thereby avoiding further sub calling
52# overhead.
53#=======================================================================
54sub import {
55 my $class = shift;
56 return unless @_; # Ignore 'use constant;'
b35226bb 57 my $constants;
3cb88d13 58 my $multiple = ref $_[0];
39a108ce 59 my $pkg = caller;
f7fd2659 60 my $flush_mro;
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61 my $symtab;
62
c5764f70 63 if (_CAN_PCS) {
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64 no strict 'refs';
65 $symtab = \%{$pkg . '::'};
66 };
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67
68 if ( $multiple ) {
69 if (ref $_[0] ne 'HASH') {
70 require Carp;
71 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
72 }
b35226bb 73 $constants = shift;
3cb88d13 74 } else {
15dc519f 75 unless (defined $_[0]) {
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76 require Carp;
77 Carp::croak("Can't use undef as constant name");
78 }
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79 $constants->{+shift} = undef;
80 }
3cb88d13 81
15dc519f 82 foreach my $name ( keys %$constants ) {
3cb88d13 83 # Normal constant name
672c0ce9 84 if ($name =~ $normal_constant_name and !$forbidden{$name}) {
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85 # Everything is okay
86
87 # Name forced into main, but we're not in main. Fatal.
88 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
89 require Carp;
90 Carp::croak("Constant name '$name' is forced into main::");
91
92 # Starts with double underscore. Fatal.
93 } elsif ($name =~ /^__/) {
94 require Carp;
95 Carp::croak("Constant name '$name' begins with '__'");
96
97 # Maybe the name is tolerable
672c0ce9 98 } elsif ($name =~ $tolerable) {
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99 # Then we'll warn only if you've asked for warnings
100 if (warnings::enabled()) {
101 if ($keywords{$name}) {
102 warnings::warn("Constant name '$name' is a Perl keyword");
103 } elsif ($forced_into_main{$name}) {
104 warnings::warn("Constant name '$name' is " .
105 "forced into package main::");
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106 }
107 }
108
109 # Looks like a boolean
110 # use constant FRED == fred;
672c0ce9 111 } elsif ($name =~ $boolean) {
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112 require Carp;
113 if (@_) {
114 Carp::croak("Constant name '$name' is invalid");
83763826 115 } else {
3cb88d13 116 Carp::croak("Constant name looks like boolean value");
83763826 117 }
83763826 118
83763826 119 } else {
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120 # Must have bad characters
121 require Carp;
122 Carp::croak("Constant name '$name' has invalid characters");
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123 }
124
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125 {
126 no strict 'refs';
127 my $full_name = "${pkg}::$name";
128 $declared{$full_name}++;
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129 if ($multiple || @_ == 1) {
130 my $scalar = $multiple ? $constants->{$name} : $_[0];
d12b49d6 131
2d1c5561 132 if (_DOWNGRADE) { # for 5.8 to 5.14
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133 # Work around perl bug #31991: Sub names (actually glob
134 # names in general) ignore the UTF8 flag. So we have to
135 # turn it off to get the "right" symbol table entry.
136 utf8::is_utf8 $name and utf8::encode $name;
137 }
d12b49d6 138
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139 # The constant serves to optimise this entire block out on
140 # 5.8 and earlier.
141 if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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142 # No typeglob yet, so we can use a reference as space-
143 # efficient proxy for a constant subroutine
144 # The check in Perl_ck_rvconst knows that inlinable
145 # constants from cv_const_sv are read only. So we have to:
146 Internals::SvREADONLY($scalar, 1);
147 $symtab->{$name} = \$scalar;
f7fd2659 148 ++$flush_mro;
3cb88d13 149 } else {
e040ff70 150 *$full_name = sub () { $scalar };
3cb88d13 151 }
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152 } elsif (@_) {
153 my @list = @_;
154 *$full_name = sub () { @list };
155 } else {
156 *$full_name = sub () { };
3cb88d13 157 }
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158 }
159 }
f7fd2659 160 # Flush the cache exactly once if we make any direct symbol table changes.
a8ae8fee 161 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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162}
163
1641;
165
166__END__
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167
168=head1 NAME
169
170constant - Perl pragma to declare constants
171
172=head1 SYNOPSIS
173
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174 use constant PI => 4 * atan2(1, 1);
175 use constant DEBUG => 0;
176
177 print "Pi equals ", PI, "...\n" if DEBUG;
178
3cb88d13 179 use constant {
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180 SEC => 0,
181 MIN => 1,
182 HOUR => 2,
183 MDAY => 3,
184 MON => 4,
185 YEAR => 5,
186 WDAY => 6,
187 YDAY => 7,
188 ISDST => 8,
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189 };
190
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191 use constant WEEKDAYS => qw(
192 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
193 );
194
195 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
196
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197=head1 DESCRIPTION
198
6515510f 199This pragma allows you to declare constants at compile-time.
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200
201When you declare a constant such as C<PI> using the method shown
202above, each machine your script runs upon can have as many digits
203of accuracy as it can use. Also, your program will be easier to
204read, more likely to be maintained (and maintained correctly), and
205far less likely to send a space probe to the wrong planet because
206nobody noticed the one equation in which you wrote C<3.14195>.
207
d3383c75 208When a constant is used in an expression, Perl replaces it with its
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209value at compile time, and may then optimize the expression further.
210In particular, any code in an C<if (CONSTANT)> block will be optimized
211away if the constant is false.
212
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213=head1 NOTES
214
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215As with all C<use> directives, defining a constant happens at
216compile time. Thus, it's probably not correct to put a constant
217declaration inside of a conditional statement (like C<if ($foo)
218{ use constant ... }>).
54310121 219
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220Constants defined using this module cannot be interpolated into
221strings like variables. However, concatenation works just fine:
54310121 222
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223 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
224 print "Pi equals ".PI."...\n"; # right
54310121 225
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226Even though a reference may be declared as a constant, the reference may
227point to data which may be changed, as this code shows.
228
229 use constant ARRAY => [ 1,2,3,4 ];
230 print ARRAY->[1];
231 ARRAY->[1] = " be changed";
232 print ARRAY->[1];
233
234Dereferencing constant references incorrectly (such as using an array
235subscript on a constant hash reference, or vice versa) will be trapped at
236compile time.
54310121 237
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238Constants belong to the package they are defined in. To refer to a
239constant defined in another package, specify the full package name, as
240in C<Some::Package::CONSTANT>. Constants may be exported by modules,
241and may also be called as either class or instance methods, that is,
242as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
243C<$obj> is an instance of C<Some::Package>. Subclasses may define
244their own constants to override those in their base class.
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245
246The use of all caps for constant names is merely a convention,
247although it is recommended in order to make constants stand out
248and to help avoid collisions with other barewords, keywords, and
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249subroutine names. Constant names must begin with a letter or
250underscore. Names beginning with a double underscore are reserved. Some
251poor choices for names will generate warnings, if warnings are enabled at
252compile time.
54310121 253
a747501d 254=head2 List constants
54310121 255
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256Constants may be lists of more (or less) than one value. A constant
257with no values evaluates to C<undef> in scalar context. Note that
258constants with more than one value do I<not> return their last value in
259scalar context as one might expect. They currently return the number
260of values, but B<this may change in the future>. Do not use constants
261with multiple values in scalar context.
3cb88d13 262
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263B<NOTE:> This implies that the expression defining the value of a
264constant is evaluated in list context. This may produce surprises:
54310121 265
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266 use constant TIMESTAMP => localtime; # WRONG!
267 use constant TIMESTAMP => scalar localtime; # right
54310121 268
a747501d 269The first line above defines C<TIMESTAMP> as a 9-element list, as
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270returned by C<localtime()> in list context. To set it to the string
271returned by C<localtime()> in scalar context, an explicit C<scalar>
a747501d 272keyword is required.
54310121 273
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274List constants are lists, not arrays. To index or slice them, they
275must be placed in parentheses.
54310121 276
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277 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
278 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 279
a747501d 280=head2 Defining multiple constants at once
b0d6893f 281
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282Instead of writing multiple C<use constant> statements, you may define
283multiple constants in a single statement by giving, instead of the
284constant name, a reference to a hash where the keys are the names of
285the constants to be defined. Obviously, all constants defined using
286this method must have a single value.
287
288 use constant {
289 FOO => "A single value",
290 BAR => "This", "won't", "work!", # Error!
291 };
292
293This is a fundamental limitation of the way hashes are constructed in
294Perl. The error messages produced when this happens will often be
295quite cryptic -- in the worst case there may be none at all, and
296you'll only later find that something is broken.
297
298When defining multiple constants, you cannot use the values of other
299constants defined in the same declaration. This is because the
300calling package doesn't know about any constant within that group
301until I<after> the C<use> statement is finished.
302
303 use constant {
304 BITMASK => 0xAFBAEBA8,
305 NEGMASK => ~BITMASK, # Error!
306 };
307
308=head2 Magic constants
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309
310Magical values and references can be made into constants at compile
311time, allowing for way cool stuff like this. (These error numbers
312aren't totally portable, alas.)
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313
314 use constant E2BIG => ($! = 7);
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315 print E2BIG, "\n"; # something like "Arg list too long"
316 print 0+E2BIG, "\n"; # "7"
54310121 317
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318You can't produce a tied constant by giving a tied scalar as the
319value. References to tied variables, however, can be used as
320constants without any problems.
321
a747501d 322=head1 TECHNICAL NOTES
b0d6893f 323
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324In the current implementation, scalar constants are actually
325inlinable subroutines. As of version 5.004 of Perl, the appropriate
326scalar constant is inserted directly in place of some subroutine
327calls, thereby saving the overhead of a subroutine call. See
328L<perlsub/"Constant Functions"> for details about how and when this
329happens.
3cb88d13 330
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331In the rare case in which you need to discover at run time whether a
332particular constant has been declared via this module, you may use
333this function to examine the hash C<%constant::declared>. If the given
334constant name does not include a package name, the current package is
335used.
336
337 sub declared ($) {
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338 use constant 1.01; # don't omit this!
339 my $name = shift;
340 $name =~ s/^::/main::/;
341 my $pkg = caller;
342 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
343 $constant::declared{$full_name};
83763826 344 }
779c5bc9 345
6515510f 346=head1 CAVEATS
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347
348In the current version of Perl, list constants are not inlined
349and some symbols may be redefined without generating a warning.
350
a747501d 351It is not possible to have a subroutine or a keyword with the same
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352name as a constant in the same package. This is probably a Good Thing.
353
354A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
355ENV INC SIG> is not allowed anywhere but in package C<main::>, for
356technical reasons.
357
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358Unlike constants in some languages, these cannot be overridden
359on the command line or via environment variables.
360
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361You can get into trouble if you use constants in a context which
362automatically quotes barewords (as is true for any subroutine call).
363For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
364be interpreted as a string. Use C<$hash{CONSTANT()}> or
365C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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366kicking in. Similarly, since the C<< => >> operator quotes a bareword
367immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 368(or simply use a comma in place of the big arrow) instead of
a747501d 369C<< CONSTANT => 'value' >>.
a3cb178b 370
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371=head1 SEE ALSO
372
373L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
374
375L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
376but uses C<SvREADONLY> instead of C<tie>.
377
378L<Attribute::Constant> - Make read-only variables via attribute
379
380L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
381
382L<Hash::Util> - A selection of general-utility hash subroutines (mostly
383to lock/unlock keys and values)
384
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385=head1 BUGS
386
387Please report any bugs or feature requests via the perlbug(1) utility.
388
389=head1 AUTHORS
54310121 390
83763826 391Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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392many other folks.
393
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394Multiple constant declarations at once added by Casey West,
395E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 396
a747501d 397Documentation mostly rewritten by Ilmari Karonen,
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398E<lt>F<perl@itz.pp.sci.fi>E<gt>.
399
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400This program is maintained by the Perl 5 Porters.
401The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
402E<lt>F<sebastien@aperghis.net>E<gt>.
403
d3383c75 404=head1 COPYRIGHT & LICENSE
54310121 405
83763826 406Copyright (C) 1997, 1999 Tom Phoenix
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407
408This module is free software; you can redistribute it or modify it
409under the same terms as Perl itself.
410
411=cut