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constant.pm: Make list constants read-only
[perl5.git] / dist / constant / lib / constant.pm
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54310121 1package constant;
eb10a876 2use 5.008;
83763826 3use strict;
d3a7d8c7 4use warnings::register;
17f410f9 5
6515510f 6use vars qw($VERSION %declared);
f2952d39 7$VERSION = '1.28';
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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 $normal_constant_name = qr/^_?[^\W_0-9]\w*\z/;
21my $tolerable = qr/^[A-Za-z_]\w*\z/;
22my $boolean = qr/^[01]?\z/;
672c0ce9 23
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24BEGIN {
25 # We'd like to do use constant _CAN_PCS => $] > 5.009002
26 # but that's a bit tricky before we load the constant module :-)
27 # By doing this, we save 1 run time check for *every* call to import.
c5764f70 28 my $const = $] > 5.009002;
2b7e302a 29 my $downgrade = $] < 5.015004; # && $] >= 5.008
15635cbf 30 my $constarray = $] > 5.019001;
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31 if ($const) {
32 Internals::SvREADONLY($const, 1);
33 Internals::SvREADONLY($downgrade, 1);
34 $constant::{_CAN_PCS} = \$const;
35 $constant::{_DOWNGRADE} = \$downgrade;
15635cbf 36 $constant::{_CAN_PCS_FOR_ARRAY} = \$constarray;
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37 }
38 else {
39 no strict 'refs';
40 *{"_CAN_PCS"} = sub () {$const};
41 *{"_DOWNGRADE"} = sub () { $downgrade };
15635cbf 42 *{"_CAN_PCS_FOR_ARRAY"} = sub () { $constarray };
94d5c174 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 $name;
137 }
d12b49d6 138
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139 # The constant serves to optimise this entire block out on
140 # 5.8 and earlier.
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141 if (_CAN_PCS) {
142 # Use a reference as a proxy for a constant subroutine.
143 # If this is not a glob yet, it saves space. If it is
144 # a glob, we must still create it this way to get the
145 # right internal flags set, as constants are distinct
146 # from subroutines created with sub(){...}.
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147 # The check in Perl_ck_rvconst knows that inlinable
148 # constants from cv_const_sv are read only. So we have to:
149 Internals::SvREADONLY($scalar, 1);
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150 if ($symtab && !exists $symtab->{$name}) {
151 $symtab->{$name} = \$scalar;
152 ++$flush_mro;
153 }
154 else {
155 local $constant::{_dummy} = \$scalar;
156 *$full_name = \&{"_dummy"};
157 }
3cb88d13 158 } else {
e040ff70 159 *$full_name = sub () { $scalar };
3cb88d13 160 }
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161 } elsif (@_) {
162 my @list = @_;
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163 if (_CAN_PCS_FOR_ARRAY) {
164 Internals::SvREADONLY(@list, 1);
165 Internals::SvREADONLY($list[$_], 1) for 0..$#list;
166 if ($symtab && !exists $symtab->{$name}) {
167 $symtab->{$name} = \@list;
168 $flush_mro++;
169 }
170 else {
171 local $constant::{_dummy} = \@list;
172 *$full_name = \&{"_dummy"};
173 }
174 }
175 else { *$full_name = sub () { @list }; }
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176 } else {
177 *$full_name = sub () { };
3cb88d13 178 }
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179 }
180 }
f7fd2659 181 # Flush the cache exactly once if we make any direct symbol table changes.
a8ae8fee 182 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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183}
184
1851;
186
187__END__
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188
189=head1 NAME
190
191constant - Perl pragma to declare constants
192
193=head1 SYNOPSIS
194
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195 use constant PI => 4 * atan2(1, 1);
196 use constant DEBUG => 0;
197
198 print "Pi equals ", PI, "...\n" if DEBUG;
199
3cb88d13 200 use constant {
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201 SEC => 0,
202 MIN => 1,
203 HOUR => 2,
204 MDAY => 3,
205 MON => 4,
206 YEAR => 5,
207 WDAY => 6,
208 YDAY => 7,
209 ISDST => 8,
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210 };
211
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212 use constant WEEKDAYS => qw(
213 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
214 );
215
216 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
217
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218=head1 DESCRIPTION
219
6515510f 220This pragma allows you to declare constants at compile-time.
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221
222When you declare a constant such as C<PI> using the method shown
223above, each machine your script runs upon can have as many digits
224of accuracy as it can use. Also, your program will be easier to
225read, more likely to be maintained (and maintained correctly), and
226far less likely to send a space probe to the wrong planet because
227nobody noticed the one equation in which you wrote C<3.14195>.
228
d3383c75 229When a constant is used in an expression, Perl replaces it with its
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230value at compile time, and may then optimize the expression further.
231In particular, any code in an C<if (CONSTANT)> block will be optimized
232away if the constant is false.
233
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234=head1 NOTES
235
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236As with all C<use> directives, defining a constant happens at
237compile time. Thus, it's probably not correct to put a constant
238declaration inside of a conditional statement (like C<if ($foo)
239{ use constant ... }>).
54310121 240
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241Constants defined using this module cannot be interpolated into
242strings like variables. However, concatenation works just fine:
54310121 243
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244 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
245 print "Pi equals ".PI."...\n"; # right
54310121 246
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247Even though a reference may be declared as a constant, the reference may
248point to data which may be changed, as this code shows.
249
250 use constant ARRAY => [ 1,2,3,4 ];
251 print ARRAY->[1];
252 ARRAY->[1] = " be changed";
253 print ARRAY->[1];
254
255Dereferencing constant references incorrectly (such as using an array
256subscript on a constant hash reference, or vice versa) will be trapped at
257compile time.
54310121 258
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259Constants belong to the package they are defined in. To refer to a
260constant defined in another package, specify the full package name, as
261in C<Some::Package::CONSTANT>. Constants may be exported by modules,
262and may also be called as either class or instance methods, that is,
263as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
264C<$obj> is an instance of C<Some::Package>. Subclasses may define
265their own constants to override those in their base class.
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266
267The use of all caps for constant names is merely a convention,
268although it is recommended in order to make constants stand out
269and to help avoid collisions with other barewords, keywords, and
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270subroutine names. Constant names must begin with a letter or
271underscore. Names beginning with a double underscore are reserved. Some
272poor choices for names will generate warnings, if warnings are enabled at
273compile time.
54310121 274
a747501d 275=head2 List constants
54310121 276
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277Constants may be lists of more (or less) than one value. A constant
278with no values evaluates to C<undef> in scalar context. Note that
279constants with more than one value do I<not> return their last value in
280scalar context as one might expect. They currently return the number
281of values, but B<this may change in the future>. Do not use constants
282with multiple values in scalar context.
3cb88d13 283
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284B<NOTE:> This implies that the expression defining the value of a
285constant is evaluated in list context. This may produce surprises:
54310121 286
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287 use constant TIMESTAMP => localtime; # WRONG!
288 use constant TIMESTAMP => scalar localtime; # right
54310121 289
a747501d 290The first line above defines C<TIMESTAMP> as a 9-element list, as
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291returned by C<localtime()> in list context. To set it to the string
292returned by C<localtime()> in scalar context, an explicit C<scalar>
a747501d 293keyword is required.
54310121 294
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295List constants are lists, not arrays. To index or slice them, they
296must be placed in parentheses.
54310121 297
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298 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
299 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 300
a747501d 301=head2 Defining multiple constants at once
b0d6893f 302
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303Instead of writing multiple C<use constant> statements, you may define
304multiple constants in a single statement by giving, instead of the
305constant name, a reference to a hash where the keys are the names of
306the constants to be defined. Obviously, all constants defined using
307this method must have a single value.
308
309 use constant {
310 FOO => "A single value",
311 BAR => "This", "won't", "work!", # Error!
312 };
313
314This is a fundamental limitation of the way hashes are constructed in
315Perl. The error messages produced when this happens will often be
316quite cryptic -- in the worst case there may be none at all, and
317you'll only later find that something is broken.
318
319When defining multiple constants, you cannot use the values of other
320constants defined in the same declaration. This is because the
321calling package doesn't know about any constant within that group
322until I<after> the C<use> statement is finished.
323
324 use constant {
325 BITMASK => 0xAFBAEBA8,
326 NEGMASK => ~BITMASK, # Error!
327 };
328
329=head2 Magic constants
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330
331Magical values and references can be made into constants at compile
332time, allowing for way cool stuff like this. (These error numbers
333aren't totally portable, alas.)
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334
335 use constant E2BIG => ($! = 7);
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336 print E2BIG, "\n"; # something like "Arg list too long"
337 print 0+E2BIG, "\n"; # "7"
54310121 338
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339You can't produce a tied constant by giving a tied scalar as the
340value. References to tied variables, however, can be used as
341constants without any problems.
342
a747501d 343=head1 TECHNICAL NOTES
b0d6893f 344
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345In the current implementation, scalar constants are actually
346inlinable subroutines. As of version 5.004 of Perl, the appropriate
347scalar constant is inserted directly in place of some subroutine
348calls, thereby saving the overhead of a subroutine call. See
349L<perlsub/"Constant Functions"> for details about how and when this
350happens.
3cb88d13 351
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352In the rare case in which you need to discover at run time whether a
353particular constant has been declared via this module, you may use
354this function to examine the hash C<%constant::declared>. If the given
355constant name does not include a package name, the current package is
356used.
357
358 sub declared ($) {
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359 use constant 1.01; # don't omit this!
360 my $name = shift;
361 $name =~ s/^::/main::/;
362 my $pkg = caller;
363 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
364 $constant::declared{$full_name};
83763826 365 }
779c5bc9 366
6515510f 367=head1 CAVEATS
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368
369In the current version of Perl, list constants are not inlined
370and some symbols may be redefined without generating a warning.
371
a747501d 372It is not possible to have a subroutine or a keyword with the same
83763826
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373name as a constant in the same package. This is probably a Good Thing.
374
375A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
376ENV INC SIG> is not allowed anywhere but in package C<main::>, for
377technical reasons.
378
54310121
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379Unlike constants in some languages, these cannot be overridden
380on the command line or via environment variables.
381
a3cb178b
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382You can get into trouble if you use constants in a context which
383automatically quotes barewords (as is true for any subroutine call).
384For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
385be interpreted as a string. Use C<$hash{CONSTANT()}> or
386C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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387kicking in. Similarly, since the C<< => >> operator quotes a bareword
388immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 389(or simply use a comma in place of the big arrow) instead of
a747501d 390C<< CONSTANT => 'value' >>.
a3cb178b 391
d3383c75
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392=head1 SEE ALSO
393
394L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
395
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396L<Attribute::Constant> - Make read-only variables via attribute
397
398L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
399
400L<Hash::Util> - A selection of general-utility hash subroutines (mostly
401to lock/unlock keys and values)
402
6515510f
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403=head1 BUGS
404
405Please report any bugs or feature requests via the perlbug(1) utility.
406
407=head1 AUTHORS
54310121 408
83763826 409Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
54310121
PP
410many other folks.
411
e1e60e72
CW
412Multiple constant declarations at once added by Casey West,
413E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 414
a747501d 415Documentation mostly rewritten by Ilmari Karonen,
b0d6893f
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416E<lt>F<perl@itz.pp.sci.fi>E<gt>.
417
6515510f
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418This program is maintained by the Perl 5 Porters.
419The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
420E<lt>F<sebastien@aperghis.net>E<gt>.
421
d3383c75 422=head1 COPYRIGHT & LICENSE
54310121 423
83763826 424Copyright (C) 1997, 1999 Tom Phoenix
54310121
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425
426This module is free software; you can redistribute it or modify it
427under the same terms as Perl itself.
428
429=cut