4 use warnings::register;
6 use vars qw($VERSION %declared);
9 #=======================================================================
11 # Some names are evil choices.
12 my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
13 $keywords{UNITCHECK}++ if $] > 5.009;
15 my %forced_into_main = map +($_, 1),
16 qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
18 my %forbidden = (%keywords, %forced_into_main);
20 my $str_end = $] >= 5.006 ? "\\z" : "\\Z";
21 my $normal_constant_name = qr/^_?[^\W_0-9]\w*$str_end/;
22 my $tolerable = qr/^[A-Za-z_]\w*$str_end/;
23 my $boolean = qr/^[01]?$str_end/;
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.
30 my $const = $] > 5.009002;
31 *_CAN_PCS = sub () {$const};
34 #=======================================================================
35 # import() - import symbols into user's namespace
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
41 #=======================================================================
44 return unless @_; # Ignore 'use constant;'
46 my $multiple = ref $_[0];
53 $symtab = \%{$pkg . '::'};
57 if (ref $_[0] ne 'HASH') {
59 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
63 $constants->{+shift} = undef;
66 foreach my $name ( keys %$constants ) {
67 unless (defined $name) {
69 Carp::croak("Can't use undef as constant name");
72 # Normal constant name
73 if ($name =~ $normal_constant_name and !$forbidden{$name}) {
76 # Name forced into main, but we're not in main. Fatal.
77 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
79 Carp::croak("Constant name '$name' is forced into main::");
81 # Starts with double underscore. Fatal.
82 } elsif ($name =~ /^__/) {
84 Carp::croak("Constant name '$name' begins with '__'");
86 # Maybe the name is tolerable
87 } elsif ($name =~ $tolerable) {
88 # Then we'll warn only if you've asked for warnings
89 if (warnings::enabled()) {
90 if ($keywords{$name}) {
91 warnings::warn("Constant name '$name' is a Perl keyword");
92 } elsif ($forced_into_main{$name}) {
93 warnings::warn("Constant name '$name' is " .
94 "forced into package main::");
98 # Looks like a boolean
99 # use constant FRED == fred;
100 } elsif ($name =~ $boolean) {
103 Carp::croak("Constant name '$name' is invalid");
105 Carp::croak("Constant name looks like boolean value");
109 # Must have bad characters
111 Carp::croak("Constant name '$name' has invalid characters");
116 my $full_name = "${pkg}::$name";
117 $declared{$full_name}++;
118 if ($multiple || @_ == 1) {
119 my $scalar = $multiple ? $constants->{$name} : $_[0];
120 # The constant serves to optimise this entire block out on
122 if (_CAN_PCS && $symtab && !exists $symtab->{$name}) {
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;
131 *$full_name = sub () { $scalar };
135 *$full_name = sub () { @list };
137 *$full_name = sub () { };
141 # Flush the cache exactly once if we make any direct symbol table changes.
142 mro::method_changed_in($pkg) if _CAN_PCS && $flush_mro;
151 constant - Perl pragma to declare constants
155 use constant PI => 4 * atan2(1, 1);
156 use constant DEBUG => 0;
158 print "Pi equals ", PI, "...\n" if DEBUG;
172 use constant WEEKDAYS => qw(
173 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
176 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
180 This pragma allows you to declare constants at compile-time.
182 When you declare a constant such as C<PI> using the method shown
183 above, each machine your script runs upon can have as many digits
184 of accuracy as it can use. Also, your program will be easier to
185 read, more likely to be maintained (and maintained correctly), and
186 far less likely to send a space probe to the wrong planet because
187 nobody noticed the one equation in which you wrote C<3.14195>.
189 When a constant is used in an expression, Perl replaces it with its
190 value at compile time, and may then optimize the expression further.
191 In particular, any code in an C<if (CONSTANT)> block will be optimized
192 away if the constant is false.
196 As with all C<use> directives, defining a constant happens at
197 compile time. Thus, it's probably not correct to put a constant
198 declaration inside of a conditional statement (like C<if ($foo)
199 { use constant ... }>).
201 Constants defined using this module cannot be interpolated into
202 strings like variables. However, concatenation works just fine:
204 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
205 print "Pi equals ".PI."...\n"; # right
207 Even though a reference may be declared as a constant, the reference may
208 point to data which may be changed, as this code shows.
210 use constant ARRAY => [ 1,2,3,4 ];
212 ARRAY->[1] = " be changed";
215 Dereferencing constant references incorrectly (such as using an array
216 subscript on a constant hash reference, or vice versa) will be trapped at
219 Constants belong to the package they are defined in. To refer to a
220 constant defined in another package, specify the full package name, as
221 in C<Some::Package::CONSTANT>. Constants may be exported by modules,
222 and may also be called as either class or instance methods, that is,
223 as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
224 C<$obj> is an instance of C<Some::Package>. Subclasses may define
225 their own constants to override those in their base class.
227 The use of all caps for constant names is merely a convention,
228 although it is recommended in order to make constants stand out
229 and to help avoid collisions with other barewords, keywords, and
230 subroutine names. Constant names must begin with a letter or
231 underscore. Names beginning with a double underscore are reserved. Some
232 poor choices for names will generate warnings, if warnings are enabled at
235 =head2 List constants
237 Constants may be lists of more (or less) than one value. A constant
238 with no values evaluates to C<undef> in scalar context. Note that
239 constants with more than one value do I<not> return their last value in
240 scalar context as one might expect. They currently return the number
241 of values, but B<this may change in the future>. Do not use constants
242 with multiple values in scalar context.
244 B<NOTE:> This implies that the expression defining the value of a
245 constant is evaluated in list context. This may produce surprises:
247 use constant TIMESTAMP => localtime; # WRONG!
248 use constant TIMESTAMP => scalar localtime; # right
250 The first line above defines C<TIMESTAMP> as a 9-element list, as
251 returned by C<localtime()> in list context. To set it to the string
252 returned by C<localtime()> in scalar context, an explicit C<scalar>
255 List constants are lists, not arrays. To index or slice them, they
256 must be placed in parentheses.
258 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
259 my @workdays = (WEEKDAYS)[1 .. 5]; # right
261 =head2 Defining multiple constants at once
263 Instead of writing multiple C<use constant> statements, you may define
264 multiple constants in a single statement by giving, instead of the
265 constant name, a reference to a hash where the keys are the names of
266 the constants to be defined. Obviously, all constants defined using
267 this method must have a single value.
270 FOO => "A single value",
271 BAR => "This", "won't", "work!", # Error!
274 This is a fundamental limitation of the way hashes are constructed in
275 Perl. The error messages produced when this happens will often be
276 quite cryptic -- in the worst case there may be none at all, and
277 you'll only later find that something is broken.
279 When defining multiple constants, you cannot use the values of other
280 constants defined in the same declaration. This is because the
281 calling package doesn't know about any constant within that group
282 until I<after> the C<use> statement is finished.
285 BITMASK => 0xAFBAEBA8,
286 NEGMASK => ~BITMASK, # Error!
289 =head2 Magic constants
291 Magical values and references can be made into constants at compile
292 time, allowing for way cool stuff like this. (These error numbers
293 aren't totally portable, alas.)
295 use constant E2BIG => ($! = 7);
296 print E2BIG, "\n"; # something like "Arg list too long"
297 print 0+E2BIG, "\n"; # "7"
299 You can't produce a tied constant by giving a tied scalar as the
300 value. References to tied variables, however, can be used as
301 constants without any problems.
303 =head1 TECHNICAL NOTES
305 In the current implementation, scalar constants are actually
306 inlinable subroutines. As of version 5.004 of Perl, the appropriate
307 scalar constant is inserted directly in place of some subroutine
308 calls, thereby saving the overhead of a subroutine call. See
309 L<perlsub/"Constant Functions"> for details about how and when this
312 In the rare case in which you need to discover at run time whether a
313 particular constant has been declared via this module, you may use
314 this function to examine the hash C<%constant::declared>. If the given
315 constant name does not include a package name, the current package is
319 use constant 1.01; # don't omit this!
321 $name =~ s/^::/main::/;
323 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
324 $constant::declared{$full_name};
329 In the current version of Perl, list constants are not inlined
330 and some symbols may be redefined without generating a warning.
332 It is not possible to have a subroutine or a keyword with the same
333 name as a constant in the same package. This is probably a Good Thing.
335 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
336 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
339 Unlike constants in some languages, these cannot be overridden
340 on the command line or via environment variables.
342 You can get into trouble if you use constants in a context which
343 automatically quotes barewords (as is true for any subroutine call).
344 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
345 be interpreted as a string. Use C<$hash{CONSTANT()}> or
346 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
347 kicking in. Similarly, since the C<< => >> operator quotes a bareword
348 immediately to its left, you have to say C<< CONSTANT() => 'value' >>
349 (or simply use a comma in place of the big arrow) instead of
350 C<< CONSTANT => 'value' >>.
354 L<Readonly> - Facility for creating read-only scalars, arrays, hashes.
356 L<Const> - Facility for creating read-only variables. Similar to C<Readonly>,
357 but uses C<SvREADONLY> instead of C<tie>.
359 L<Attribute::Constant> - Make read-only variables via attribute
361 L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag
363 L<Hash::Util> - A selection of general-utility hash subroutines (mostly
364 to lock/unlock keys and values)
368 Please report any bugs or feature requests via the perlbug(1) utility.
372 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
375 Multiple constant declarations at once added by Casey West,
376 E<lt>F<casey@geeknest.com>E<gt>.
378 Documentation mostly rewritten by Ilmari Karonen,
379 E<lt>F<perl@itz.pp.sci.fi>E<gt>.
381 This program is maintained by the Perl 5 Porters.
382 The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
383 E<lt>F<sebastien@aperghis.net>E<gt>.
385 =head1 COPYRIGHT & LICENSE
387 Copyright (C) 1997, 1999 Tom Phoenix
389 This module is free software; you can redistribute it or modify it
390 under the same terms as Perl itself.