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