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Move constant from ext/ to dist/
[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($VERSION %declared);
7064c127 7$VERSION = '1.19';
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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 {
b35226bb 63 $constants->{+shift} = undef;
83763826 64 }
3cb88d13 65
b35226bb 66 foreach my $name ( keys %$constants ) {
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67 unless (defined $name) {
68 require Carp;
69 Carp::croak("Can't use undef as constant name");
70 }
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71
72 # Normal constant name
672c0ce9 73 if ($name =~ $normal_constant_name and !$forbidden{$name}) {
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74 # Everything is okay
75
76 # Name forced into main, but we're not in main. Fatal.
77 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
78 require Carp;
79 Carp::croak("Constant name '$name' is forced into main::");
80
81 # Starts with double underscore. Fatal.
82 } elsif ($name =~ /^__/) {
83 require Carp;
84 Carp::croak("Constant name '$name' begins with '__'");
85
86 # Maybe the name is tolerable
672c0ce9 87 } elsif ($name =~ $tolerable) {
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88 # Then we'll warn only if you've asked for warnings
89 if (warnings::enabled()) {
90 if ($keywords{$name}) {
91 warnings::warn("Constant name '$name' is a Perl keyword");
92 } elsif ($forced_into_main{$name}) {
93 warnings::warn("Constant name '$name' is " .
94 "forced into package main::");
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95 }
96 }
97
98 # Looks like a boolean
99 # use constant FRED == fred;
672c0ce9 100 } elsif ($name =~ $boolean) {
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101 require Carp;
102 if (@_) {
103 Carp::croak("Constant name '$name' is invalid");
83763826 104 } else {
3cb88d13 105 Carp::croak("Constant name looks like boolean value");
83763826 106 }
83763826 107
83763826 108 } else {
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109 # Must have bad characters
110 require Carp;
111 Carp::croak("Constant name '$name' has invalid characters");
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112 }
113
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114 {
115 no strict 'refs';
116 my $full_name = "${pkg}::$name";
117 $declared{$full_name}++;
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118 if ($multiple || @_ == 1) {
119 my $scalar = $multiple ? $constants->{$name} : $_[0];
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120 # The constant serves to optimise this entire block out on
121 # 5.8 and earlier.
122 if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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123 # No typeglob yet, so we can use a reference as space-
124 # efficient proxy for a constant subroutine
125 # The check in Perl_ck_rvconst knows that inlinable
126 # constants from cv_const_sv are read only. So we have to:
127 Internals::SvREADONLY($scalar, 1);
128 $symtab->{$name} = \$scalar;
f7fd2659 129 ++$flush_mro;
3cb88d13 130 } else {
e040ff70 131 *$full_name = sub () { $scalar };
3cb88d13 132 }
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133 } elsif (@_) {
134 my @list = @_;
135 *$full_name = sub () { @list };
136 } else {
137 *$full_name = sub () { };
3cb88d13 138 }
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139 }
140 }
f7fd2659 141 # Flush the cache exactly once if we make any direct symbol table changes.
a8ae8fee 142 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
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143}
144
1451;
146
147__END__
54310121 148
149=head1 NAME
150
151constant - Perl pragma to declare constants
152
153=head1 SYNOPSIS
154
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155 use constant PI => 4 * atan2(1, 1);
156 use constant DEBUG => 0;
157
158 print "Pi equals ", PI, "...\n" if DEBUG;
159
3cb88d13 160 use constant {
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161 SEC => 0,
162 MIN => 1,
163 HOUR => 2,
164 MDAY => 3,
165 MON => 4,
166 YEAR => 5,
167 WDAY => 6,
168 YDAY => 7,
169 ISDST => 8,
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170 };
171
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172 use constant WEEKDAYS => qw(
173 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
174 );
175
176 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
177
54310121 178=head1 DESCRIPTION
179
6515510f 180This pragma allows you to declare constants at compile-time.
54310121 181
182When you declare a constant such as C<PI> using the method shown
183above, each machine your script runs upon can have as many digits
184of accuracy as it can use. Also, your program will be easier to
185read, more likely to be maintained (and maintained correctly), and
186far less likely to send a space probe to the wrong planet because
187nobody noticed the one equation in which you wrote C<3.14195>.
188
d3383c75 189When a constant is used in an expression, Perl replaces it with its
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190value at compile time, and may then optimize the expression further.
191In particular, any code in an C<if (CONSTANT)> block will be optimized
192away if the constant is false.
193
54310121 194=head1 NOTES
195
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196As with all C<use> directives, defining a constant happens at
197compile time. Thus, it's probably not correct to put a constant
198declaration inside of a conditional statement (like C<if ($foo)
199{ use constant ... }>).
54310121 200
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201Constants defined using this module cannot be interpolated into
202strings like variables. However, concatenation works just fine:
54310121 203
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204 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
205 print "Pi equals ".PI."...\n"; # right
54310121 206
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207Even though a reference may be declared as a constant, the reference may
208point to data which may be changed, as this code shows.
209
210 use constant ARRAY => [ 1,2,3,4 ];
211 print ARRAY->[1];
212 ARRAY->[1] = " be changed";
213 print ARRAY->[1];
214
215Dereferencing constant references incorrectly (such as using an array
216subscript on a constant hash reference, or vice versa) will be trapped at
217compile time.
54310121 218
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219Constants belong to the package they are defined in. To refer to a
220constant defined in another package, specify the full package name, as
221in C<Some::Package::CONSTANT>. Constants may be exported by modules,
222and may also be called as either class or instance methods, that is,
223as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
224C<$obj> is an instance of C<Some::Package>. Subclasses may define
225their own constants to override those in their base class.
54310121 226
227The use of all caps for constant names is merely a convention,
228although it is recommended in order to make constants stand out
229and to help avoid collisions with other barewords, keywords, and
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230subroutine names. Constant names must begin with a letter or
231underscore. Names beginning with a double underscore are reserved. Some
232poor choices for names will generate warnings, if warnings are enabled at
233compile time.
54310121 234
a747501d 235=head2 List constants
54310121 236
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237Constants may be lists of more (or less) than one value. A constant
238with no values evaluates to C<undef> in scalar context. Note that
239constants with more than one value do I<not> return their last value in
240scalar context as one might expect. They currently return the number
241of values, but B<this may change in the future>. Do not use constants
242with multiple values in scalar context.
3cb88d13 243
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244B<NOTE:> This implies that the expression defining the value of a
245constant is evaluated in list context. This may produce surprises:
54310121 246
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247 use constant TIMESTAMP => localtime; # WRONG!
248 use constant TIMESTAMP => scalar localtime; # right
54310121 249
a747501d 250The first line above defines C<TIMESTAMP> as a 9-element list, as
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251returned by C<localtime()> in list context. To set it to the string
252returned by C<localtime()> in scalar context, an explicit C<scalar>
a747501d 253keyword is required.
54310121 254
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255List constants are lists, not arrays. To index or slice them, they
256must be placed in parentheses.
54310121 257
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258 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
259 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 260
a747501d 261=head2 Defining multiple constants at once
b0d6893f 262
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263Instead of writing multiple C<use constant> statements, you may define
264multiple constants in a single statement by giving, instead of the
265constant name, a reference to a hash where the keys are the names of
266the constants to be defined. Obviously, all constants defined using
267this method must have a single value.
268
269 use constant {
270 FOO => "A single value",
271 BAR => "This", "won't", "work!", # Error!
272 };
273
274This is a fundamental limitation of the way hashes are constructed in
275Perl. The error messages produced when this happens will often be
276quite cryptic -- in the worst case there may be none at all, and
277you'll only later find that something is broken.
278
279When defining multiple constants, you cannot use the values of other
280constants defined in the same declaration. This is because the
281calling package doesn't know about any constant within that group
282until I<after> the C<use> statement is finished.
283
284 use constant {
285 BITMASK => 0xAFBAEBA8,
286 NEGMASK => ~BITMASK, # Error!
287 };
288
289=head2 Magic constants
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290
291Magical values and references can be made into constants at compile
292time, allowing for way cool stuff like this. (These error numbers
293aren't totally portable, alas.)
54310121 294
295 use constant E2BIG => ($! = 7);
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296 print E2BIG, "\n"; # something like "Arg list too long"
297 print 0+E2BIG, "\n"; # "7"
54310121 298
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299You can't produce a tied constant by giving a tied scalar as the
300value. References to tied variables, however, can be used as
301constants without any problems.
302
a747501d 303=head1 TECHNICAL NOTES
b0d6893f 304
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305In the current implementation, scalar constants are actually
306inlinable subroutines. As of version 5.004 of Perl, the appropriate
307scalar constant is inserted directly in place of some subroutine
308calls, thereby saving the overhead of a subroutine call. See
309L<perlsub/"Constant Functions"> for details about how and when this
310happens.
3cb88d13 311
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312In the rare case in which you need to discover at run time whether a
313particular constant has been declared via this module, you may use
314this function to examine the hash C<%constant::declared>. If the given
315constant name does not include a package name, the current package is
316used.
317
318 sub declared ($) {
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319 use constant 1.01; # don't omit this!
320 my $name = shift;
321 $name =~ s/^::/main::/;
322 my $pkg = caller;
323 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
324 $constant::declared{$full_name};
83763826 325 }
779c5bc9 326
6515510f 327=head1 CAVEATS
54310121 328
329In the current version of Perl, list constants are not inlined
330and some symbols may be redefined without generating a warning.
331
a747501d 332It is not possible to have a subroutine or a keyword with the same
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333name as a constant in the same package. This is probably a Good Thing.
334
335A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
336ENV INC SIG> is not allowed anywhere but in package C<main::>, for
337technical reasons.
338
54310121 339Unlike constants in some languages, these cannot be overridden
340on the command line or via environment variables.
341
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342You can get into trouble if you use constants in a context which
343automatically quotes barewords (as is true for any subroutine call).
344For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
345be interpreted as a string. Use C<$hash{CONSTANT()}> or
346C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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347kicking in. Similarly, since the C<< => >> operator quotes a bareword
348immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 349(or simply use a comma in place of the big arrow) instead of
a747501d 350C<< CONSTANT => 'value' >>.
a3cb178b 351
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352=head1 SEE ALSO
353
354L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
355
356L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
357but uses C<SvREADONLY> instead of C<tie>.
358
359L<Attribute::Constant> - Make read-only variables via attribute
360
361L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
362
363L<Hash::Util> - A selection of general-utility hash subroutines (mostly
364to lock/unlock keys and values)
365
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366=head1 BUGS
367
368Please report any bugs or feature requests via the perlbug(1) utility.
369
370=head1 AUTHORS
54310121 371
83763826 372Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
54310121 373many other folks.
374
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375Multiple constant declarations at once added by Casey West,
376E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 377
a747501d 378Documentation mostly rewritten by Ilmari Karonen,
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379E<lt>F<perl@itz.pp.sci.fi>E<gt>.
380
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381This program is maintained by the Perl 5 Porters.
382The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
383E<lt>F<sebastien@aperghis.net>E<gt>.
384
d3383c75 385=head1 COPYRIGHT & LICENSE
54310121 386
83763826 387Copyright (C) 1997, 1999 Tom Phoenix
54310121 388
389This module is free software; you can redistribute it or modify it
390under the same terms as Perl itself.
391
392=cut