5 use warnings::register;
7 our($VERSION, %declared);
10 #=======================================================================
12 # Some names are evil choices.
13 my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
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 #=======================================================================
21 # import() - import symbols into user's namespace
23 # What we actually do is define a function in the caller's namespace
24 # which returns the value. The function we create will normally
25 # be inlined as a constant, thereby avoiding further sub calling
27 #=======================================================================
30 return unless @_; # Ignore 'use constant;'
32 my $multiple = ref $_[0];
38 $symtab = \%{$pkg . '::'};
42 if (ref $_[0] ne 'HASH') {
44 Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
48 $constants->{+shift} = undef;
51 foreach my $name ( keys %$constants ) {
52 unless (defined $name) {
54 Carp::croak("Can't use undef as constant name");
57 # Normal constant name
58 if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) {
61 # Name forced into main, but we're not in main. Fatal.
62 } elsif ($forced_into_main{$name} and $pkg ne 'main') {
64 Carp::croak("Constant name '$name' is forced into main::");
66 # Starts with double underscore. Fatal.
67 } elsif ($name =~ /^__/) {
69 Carp::croak("Constant name '$name' begins with '__'");
71 # Maybe the name is tolerable
72 } elsif ($name =~ /^[A-Za-z_]\w*\z/) {
73 # Then we'll warn only if you've asked for warnings
74 if (warnings::enabled()) {
75 if ($keywords{$name}) {
76 warnings::warn("Constant name '$name' is a Perl keyword");
77 } elsif ($forced_into_main{$name}) {
78 warnings::warn("Constant name '$name' is " .
79 "forced into package main::");
83 # Looks like a boolean
84 # use constant FRED == fred;
85 } elsif ($name =~ /^[01]?\z/) {
88 Carp::croak("Constant name '$name' is invalid");
90 Carp::croak("Constant name looks like boolean value");
94 # Must have bad characters
96 Carp::croak("Constant name '$name' has invalid characters");
101 my $full_name = "${pkg}::$name";
102 $declared{$full_name}++;
103 if ($multiple || @_ == 1) {
104 my $scalar = $multiple ? $constants->{$name} : $_[0];
105 if ($symtab && !exists $symtab->{$name}) {
106 # No typeglob yet, so we can use a reference as space-
107 # efficient proxy for a constant subroutine
108 # The check in Perl_ck_rvconst knows that inlinable
109 # constants from cv_const_sv are read only. So we have to:
110 Internals::SvREADONLY($scalar, 1);
111 $symtab->{$name} = \$scalar;
112 Internals::inc_sub_generation;
114 if(!exists $symtab->{$name}) {
115 print STDERR "$name $scalar\n";
117 *$full_name = sub () { $scalar };
121 *$full_name = sub () { @list };
123 *$full_name = sub () { };
135 constant - Perl pragma to declare constants
139 use constant PI => 4 * atan2(1, 1);
140 use constant DEBUG => 0;
142 print "Pi equals ", PI, "...\n" if DEBUG;
156 use constant WEEKDAYS => qw(
157 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
160 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
164 This will declare a symbol to be a constant with the given value.
166 When you declare a constant such as C<PI> using the method shown
167 above, each machine your script runs upon can have as many digits
168 of accuracy as it can use. Also, your program will be easier to
169 read, more likely to be maintained (and maintained correctly), and
170 far less likely to send a space probe to the wrong planet because
171 nobody noticed the one equation in which you wrote C<3.14195>.
173 When a constant is used in an expression, perl replaces it with its
174 value at compile time, and may then optimize the expression further.
175 In particular, any code in an C<if (CONSTANT)> block will be optimized
176 away if the constant is false.
180 As with all C<use> directives, defining a constant happens at
181 compile time. Thus, it's probably not correct to put a constant
182 declaration inside of a conditional statement (like C<if ($foo)
183 { use constant ... }>).
185 Constants defined using this module cannot be interpolated into
186 strings like variables. However, concatenation works just fine:
188 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
189 print "Pi equals ".PI."...\n"; # right
191 Even though a reference may be declared as a constant, the reference may
192 point to data which may be changed, as this code shows.
194 use constant ARRAY => [ 1,2,3,4 ];
196 ARRAY->[1] = " be changed";
199 Dereferencing constant references incorrectly (such as using an array
200 subscript on a constant hash reference, or vice versa) will be trapped at
203 Constants belong to the package they are defined in. To refer to a
204 constant defined in another package, specify the full package name, as
205 in C<Some::Package::CONSTANT>. Constants may be exported by modules,
206 and may also be called as either class or instance methods, that is,
207 as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
208 C<$obj> is an instance of C<Some::Package>. Subclasses may define
209 their own constants to override those in their base class.
211 The use of all caps for constant names is merely a convention,
212 although it is recommended in order to make constants stand out
213 and to help avoid collisions with other barewords, keywords, and
214 subroutine names. Constant names must begin with a letter or
215 underscore. Names beginning with a double underscore are reserved. Some
216 poor choices for names will generate warnings, if warnings are enabled at
219 =head2 List constants
221 Constants may be lists of more (or less) than one value. A constant
222 with no values evaluates to C<undef> in scalar context. Note that
223 constants with more than one value do I<not> return their last value in
224 scalar context as one might expect. They currently return the number
225 of values, but B<this may change in the future>. Do not use constants
226 with multiple values in scalar context.
228 B<NOTE:> This implies that the expression defining the value of a
229 constant is evaluated in list context. This may produce surprises:
231 use constant TIMESTAMP => localtime; # WRONG!
232 use constant TIMESTAMP => scalar localtime; # right
234 The first line above defines C<TIMESTAMP> as a 9-element list, as
235 returned by localtime() in list context. To set it to the string
236 returned by localtime() in scalar context, an explicit C<scalar>
239 List constants are lists, not arrays. To index or slice them, they
240 must be placed in parentheses.
242 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
243 my @workdays = (WEEKDAYS)[1 .. 5]; # right
245 =head2 Defining multiple constants at once
247 Instead of writing multiple C<use constant> statements, you may define
248 multiple constants in a single statement by giving, instead of the
249 constant name, a reference to a hash where the keys are the names of
250 the constants to be defined. Obviously, all constants defined using
251 this method must have a single value.
254 FOO => "A single value",
255 BAR => "This", "won't", "work!", # Error!
258 This is a fundamental limitation of the way hashes are constructed in
259 Perl. The error messages produced when this happens will often be
260 quite cryptic -- in the worst case there may be none at all, and
261 you'll only later find that something is broken.
263 When defining multiple constants, you cannot use the values of other
264 constants defined in the same declaration. This is because the
265 calling package doesn't know about any constant within that group
266 until I<after> the C<use> statement is finished.
269 BITMASK => 0xAFBAEBA8,
270 NEGMASK => ~BITMASK, # Error!
273 =head2 Magic constants
275 Magical values and references can be made into constants at compile
276 time, allowing for way cool stuff like this. (These error numbers
277 aren't totally portable, alas.)
279 use constant E2BIG => ($! = 7);
280 print E2BIG, "\n"; # something like "Arg list too long"
281 print 0+E2BIG, "\n"; # "7"
283 You can't produce a tied constant by giving a tied scalar as the
284 value. References to tied variables, however, can be used as
285 constants without any problems.
287 =head1 TECHNICAL NOTES
289 In the current implementation, scalar constants are actually
290 inlinable subroutines. As of version 5.004 of Perl, the appropriate
291 scalar constant is inserted directly in place of some subroutine
292 calls, thereby saving the overhead of a subroutine call. See
293 L<perlsub/"Constant Functions"> for details about how and when this
296 In the rare case in which you need to discover at run time whether a
297 particular constant has been declared via this module, you may use
298 this function to examine the hash C<%constant::declared>. If the given
299 constant name does not include a package name, the current package is
303 use constant 1.01; # don't omit this!
305 $name =~ s/^::/main::/;
307 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
308 $constant::declared{$full_name};
313 In the current version of Perl, list constants are not inlined
314 and some symbols may be redefined without generating a warning.
316 It is not possible to have a subroutine or a keyword with the same
317 name as a constant in the same package. This is probably a Good Thing.
319 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
320 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
323 Unlike constants in some languages, these cannot be overridden
324 on the command line or via environment variables.
326 You can get into trouble if you use constants in a context which
327 automatically quotes barewords (as is true for any subroutine call).
328 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
329 be interpreted as a string. Use C<$hash{CONSTANT()}> or
330 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
331 kicking in. Similarly, since the C<< => >> operator quotes a bareword
332 immediately to its left, you have to say C<< CONSTANT() => 'value' >>
333 (or simply use a comma in place of the big arrow) instead of
334 C<< CONSTANT => 'value' >>.
338 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
341 Multiple constant declarations at once added by Casey West,
342 E<lt>F<casey@geeknest.com>E<gt>.
344 Documentation mostly rewritten by Ilmari Karonen,
345 E<lt>F<perl@itz.pp.sci.fi>E<gt>.
349 Copyright (C) 1997, 1999 Tom Phoenix
351 This module is free software; you can redistribute it or modify it
352 under the same terms as Perl itself.