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TODO some of the new sigdispatch tests on VMS.
[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($VERSION %declared);
15dc519f 7$VERSION = '1.20';
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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 # The constant serves to optimise this entire block out on
120 # 5.8 and earlier.
121 if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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122 # No typeglob yet, so we can use a reference as space-
123 # efficient proxy for a constant subroutine
124 # The check in Perl_ck_rvconst knows that inlinable
125 # constants from cv_const_sv are read only. So we have to:
126 Internals::SvREADONLY($scalar, 1);
127 $symtab->{$name} = \$scalar;
f7fd2659 128 ++$flush_mro;
3cb88d13 129 } else {
e040ff70 130 *$full_name = sub () { $scalar };
3cb88d13 131 }
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132 } elsif (@_) {
133 my @list = @_;
134 *$full_name = sub () { @list };
135 } else {
136 *$full_name = sub () { };
3cb88d13 137 }
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138 }
139 }
f7fd2659 140 # Flush the cache exactly once if we make any direct symbol table changes.
a8ae8fee 141 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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142}
143
1441;
145
146__END__
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147
148=head1 NAME
149
150constant - Perl pragma to declare constants
151
152=head1 SYNOPSIS
153
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154 use constant PI => 4 * atan2(1, 1);
155 use constant DEBUG => 0;
156
157 print "Pi equals ", PI, "...\n" if DEBUG;
158
3cb88d13 159 use constant {
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160 SEC => 0,
161 MIN => 1,
162 HOUR => 2,
163 MDAY => 3,
164 MON => 4,
165 YEAR => 5,
166 WDAY => 6,
167 YDAY => 7,
168 ISDST => 8,
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169 };
170
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171 use constant WEEKDAYS => qw(
172 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
173 );
174
175 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
176
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177=head1 DESCRIPTION
178
6515510f 179This pragma allows you to declare constants at compile-time.
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180
181When you declare a constant such as C<PI> using the method shown
182above, each machine your script runs upon can have as many digits
183of accuracy as it can use. Also, your program will be easier to
184read, more likely to be maintained (and maintained correctly), and
185far less likely to send a space probe to the wrong planet because
186nobody noticed the one equation in which you wrote C<3.14195>.
187
d3383c75 188When a constant is used in an expression, Perl replaces it with its
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189value at compile time, and may then optimize the expression further.
190In particular, any code in an C<if (CONSTANT)> block will be optimized
191away if the constant is false.
192
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193=head1 NOTES
194
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195As with all C<use> directives, defining a constant happens at
196compile time. Thus, it's probably not correct to put a constant
197declaration inside of a conditional statement (like C<if ($foo)
198{ use constant ... }>).
54310121 199
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200Constants defined using this module cannot be interpolated into
201strings like variables. However, concatenation works just fine:
54310121 202
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203 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
204 print "Pi equals ".PI."...\n"; # right
54310121 205
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206Even though a reference may be declared as a constant, the reference may
207point to data which may be changed, as this code shows.
208
209 use constant ARRAY => [ 1,2,3,4 ];
210 print ARRAY->[1];
211 ARRAY->[1] = " be changed";
212 print ARRAY->[1];
213
214Dereferencing constant references incorrectly (such as using an array
215subscript on a constant hash reference, or vice versa) will be trapped at
216compile time.
54310121 217
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218Constants belong to the package they are defined in. To refer to a
219constant defined in another package, specify the full package name, as
220in C<Some::Package::CONSTANT>. Constants may be exported by modules,
221and may also be called as either class or instance methods, that is,
222as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
223C<$obj> is an instance of C<Some::Package>. Subclasses may define
224their own constants to override those in their base class.
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225
226The use of all caps for constant names is merely a convention,
227although it is recommended in order to make constants stand out
228and to help avoid collisions with other barewords, keywords, and
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229subroutine names. Constant names must begin with a letter or
230underscore. Names beginning with a double underscore are reserved. Some
231poor choices for names will generate warnings, if warnings are enabled at
232compile time.
54310121 233
a747501d 234=head2 List constants
54310121 235
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236Constants may be lists of more (or less) than one value. A constant
237with no values evaluates to C<undef> in scalar context. Note that
238constants with more than one value do I<not> return their last value in
239scalar context as one might expect. They currently return the number
240of values, but B<this may change in the future>. Do not use constants
241with multiple values in scalar context.
3cb88d13 242
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243B<NOTE:> This implies that the expression defining the value of a
244constant is evaluated in list context. This may produce surprises:
54310121 245
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246 use constant TIMESTAMP => localtime; # WRONG!
247 use constant TIMESTAMP => scalar localtime; # right
54310121 248
a747501d 249The first line above defines C<TIMESTAMP> as a 9-element list, as
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250returned by C<localtime()> in list context. To set it to the string
251returned by C<localtime()> in scalar context, an explicit C<scalar>
a747501d 252keyword is required.
54310121 253
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254List constants are lists, not arrays. To index or slice them, they
255must be placed in parentheses.
54310121 256
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257 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
258 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 259
a747501d 260=head2 Defining multiple constants at once
b0d6893f 261
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262Instead of writing multiple C<use constant> statements, you may define
263multiple constants in a single statement by giving, instead of the
264constant name, a reference to a hash where the keys are the names of
265the constants to be defined. Obviously, all constants defined using
266this method must have a single value.
267
268 use constant {
269 FOO => "A single value",
270 BAR => "This", "won't", "work!", # Error!
271 };
272
273This is a fundamental limitation of the way hashes are constructed in
274Perl. The error messages produced when this happens will often be
275quite cryptic -- in the worst case there may be none at all, and
276you'll only later find that something is broken.
277
278When defining multiple constants, you cannot use the values of other
279constants defined in the same declaration. This is because the
280calling package doesn't know about any constant within that group
281until I<after> the C<use> statement is finished.
282
283 use constant {
284 BITMASK => 0xAFBAEBA8,
285 NEGMASK => ~BITMASK, # Error!
286 };
287
288=head2 Magic constants
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289
290Magical values and references can be made into constants at compile
291time, allowing for way cool stuff like this. (These error numbers
292aren't totally portable, alas.)
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293
294 use constant E2BIG => ($! = 7);
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295 print E2BIG, "\n"; # something like "Arg list too long"
296 print 0+E2BIG, "\n"; # "7"
54310121 297
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298You can't produce a tied constant by giving a tied scalar as the
299value. References to tied variables, however, can be used as
300constants without any problems.
301
a747501d 302=head1 TECHNICAL NOTES
b0d6893f 303
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304In the current implementation, scalar constants are actually
305inlinable subroutines. As of version 5.004 of Perl, the appropriate
306scalar constant is inserted directly in place of some subroutine
307calls, thereby saving the overhead of a subroutine call. See
308L<perlsub/"Constant Functions"> for details about how and when this
309happens.
3cb88d13 310
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311In the rare case in which you need to discover at run time whether a
312particular constant has been declared via this module, you may use
313this function to examine the hash C<%constant::declared>. If the given
314constant name does not include a package name, the current package is
315used.
316
317 sub declared ($) {
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318 use constant 1.01; # don't omit this!
319 my $name = shift;
320 $name =~ s/^::/main::/;
321 my $pkg = caller;
322 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
323 $constant::declared{$full_name};
83763826 324 }
779c5bc9 325
6515510f 326=head1 CAVEATS
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327
328In the current version of Perl, list constants are not inlined
329and some symbols may be redefined without generating a warning.
330
a747501d 331It is not possible to have a subroutine or a keyword with the same
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332name as a constant in the same package. This is probably a Good Thing.
333
334A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
335ENV INC SIG> is not allowed anywhere but in package C<main::>, for
336technical reasons.
337
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338Unlike constants in some languages, these cannot be overridden
339on the command line or via environment variables.
340
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341You can get into trouble if you use constants in a context which
342automatically quotes barewords (as is true for any subroutine call).
343For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
344be interpreted as a string. Use C<$hash{CONSTANT()}> or
345C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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346kicking in. Similarly, since the C<< => >> operator quotes a bareword
347immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 348(or simply use a comma in place of the big arrow) instead of
a747501d 349C<< CONSTANT => 'value' >>.
a3cb178b 350
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351=head1 SEE ALSO
352
353L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
354
355L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
356but uses C<SvREADONLY> instead of C<tie>.
357
358L<Attribute::Constant> - Make read-only variables via attribute
359
360L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
361
362L<Hash::Util> - A selection of general-utility hash subroutines (mostly
363to lock/unlock keys and values)
364
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365=head1 BUGS
366
367Please report any bugs or feature requests via the perlbug(1) utility.
368
369=head1 AUTHORS
54310121 370
83763826 371Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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372many other folks.
373
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374Multiple constant declarations at once added by Casey West,
375E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 376
a747501d 377Documentation mostly rewritten by Ilmari Karonen,
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378E<lt>F<perl@itz.pp.sci.fi>E<gt>.
379
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380This program is maintained by the Perl 5 Porters.
381The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
382E<lt>F<sebastien@aperghis.net>E<gt>.
383
d3383c75 384=head1 COPYRIGHT & LICENSE
54310121 385
83763826 386Copyright (C) 1997, 1999 Tom Phoenix
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387
388This module is free software; you can redistribute it or modify it
389under the same terms as Perl itself.
390
391=cut