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;
113 if(!exists $symtab->{$name}) {
114 print STDERR "$name $scalar\n";
116 *$full_name = sub () { $scalar };
120 *$full_name = sub () { @list };
122 *$full_name = sub () { };
134 constant - Perl pragma to declare constants
138 use constant PI => 4 * atan2(1, 1);
139 use constant DEBUG => 0;
141 print "Pi equals ", PI, "...\n" if DEBUG;
155 use constant WEEKDAYS => qw(
156 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
159 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
163 This will declare a symbol to be a constant with the given value.
165 When you declare a constant such as C<PI> using the method shown
166 above, each machine your script runs upon can have as many digits
167 of accuracy as it can use. Also, your program will be easier to
168 read, more likely to be maintained (and maintained correctly), and
169 far less likely to send a space probe to the wrong planet because
170 nobody noticed the one equation in which you wrote C<3.14195>.
172 When a constant is used in an expression, perl replaces it with its
173 value at compile time, and may then optimize the expression further.
174 In particular, any code in an C<if (CONSTANT)> block will be optimized
175 away if the constant is false.
179 As with all C<use> directives, defining a constant happens at
180 compile time. Thus, it's probably not correct to put a constant
181 declaration inside of a conditional statement (like C<if ($foo)
182 { use constant ... }>).
184 Constants defined using this module cannot be interpolated into
185 strings like variables. However, concatenation works just fine:
187 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
188 print "Pi equals ".PI."...\n"; # right
190 Even though a reference may be declared as a constant, the reference may
191 point to data which may be changed, as this code shows.
193 use constant ARRAY => [ 1,2,3,4 ];
195 ARRAY->[1] = " be changed";
198 Dereferencing constant references incorrectly (such as using an array
199 subscript on a constant hash reference, or vice versa) will be trapped at
202 Constants belong to the package they are defined in. To refer to a
203 constant defined in another package, specify the full package name, as
204 in C<Some::Package::CONSTANT>. Constants may be exported by modules,
205 and may also be called as either class or instance methods, that is,
206 as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
207 C<$obj> is an instance of C<Some::Package>. Subclasses may define
208 their own constants to override those in their base class.
210 The use of all caps for constant names is merely a convention,
211 although it is recommended in order to make constants stand out
212 and to help avoid collisions with other barewords, keywords, and
213 subroutine names. Constant names must begin with a letter or
214 underscore. Names beginning with a double underscore are reserved. Some
215 poor choices for names will generate warnings, if warnings are enabled at
218 =head2 List constants
220 Constants may be lists of more (or less) than one value. A constant
221 with no values evaluates to C<undef> in scalar context. Note that
222 constants with more than one value do I<not> return their last value in
223 scalar context as one might expect. They currently return the number
224 of values, but B<this may change in the future>. Do not use constants
225 with multiple values in scalar context.
227 B<NOTE:> This implies that the expression defining the value of a
228 constant is evaluated in list context. This may produce surprises:
230 use constant TIMESTAMP => localtime; # WRONG!
231 use constant TIMESTAMP => scalar localtime; # right
233 The first line above defines C<TIMESTAMP> as a 9-element list, as
234 returned by localtime() in list context. To set it to the string
235 returned by localtime() in scalar context, an explicit C<scalar>
238 List constants are lists, not arrays. To index or slice them, they
239 must be placed in parentheses.
241 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
242 my @workdays = (WEEKDAYS)[1 .. 5]; # right
244 =head2 Defining multiple constants at once
246 Instead of writing multiple C<use constant> statements, you may define
247 multiple constants in a single statement by giving, instead of the
248 constant name, a reference to a hash where the keys are the names of
249 the constants to be defined. Obviously, all constants defined using
250 this method must have a single value.
253 FOO => "A single value",
254 BAR => "This", "won't", "work!", # Error!
257 This is a fundamental limitation of the way hashes are constructed in
258 Perl. The error messages produced when this happens will often be
259 quite cryptic -- in the worst case there may be none at all, and
260 you'll only later find that something is broken.
262 When defining multiple constants, you cannot use the values of other
263 constants defined in the same declaration. This is because the
264 calling package doesn't know about any constant within that group
265 until I<after> the C<use> statement is finished.
268 BITMASK => 0xAFBAEBA8,
269 NEGMASK => ~BITMASK, # Error!
272 =head2 Magic constants
274 Magical values and references can be made into constants at compile
275 time, allowing for way cool stuff like this. (These error numbers
276 aren't totally portable, alas.)
278 use constant E2BIG => ($! = 7);
279 print E2BIG, "\n"; # something like "Arg list too long"
280 print 0+E2BIG, "\n"; # "7"
282 You can't produce a tied constant by giving a tied scalar as the
283 value. References to tied variables, however, can be used as
284 constants without any problems.
286 =head1 TECHNICAL NOTES
288 In the current implementation, scalar constants are actually
289 inlinable subroutines. As of version 5.004 of Perl, the appropriate
290 scalar constant is inserted directly in place of some subroutine
291 calls, thereby saving the overhead of a subroutine call. See
292 L<perlsub/"Constant Functions"> for details about how and when this
295 In the rare case in which you need to discover at run time whether a
296 particular constant has been declared via this module, you may use
297 this function to examine the hash C<%constant::declared>. If the given
298 constant name does not include a package name, the current package is
302 use constant 1.01; # don't omit this!
304 $name =~ s/^::/main::/;
306 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
307 $constant::declared{$full_name};
312 In the current version of Perl, list constants are not inlined
313 and some symbols may be redefined without generating a warning.
315 It is not possible to have a subroutine or a keyword with the same
316 name as a constant in the same package. This is probably a Good Thing.
318 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
319 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
322 Unlike constants in some languages, these cannot be overridden
323 on the command line or via environment variables.
325 You can get into trouble if you use constants in a context which
326 automatically quotes barewords (as is true for any subroutine call).
327 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
328 be interpreted as a string. Use C<$hash{CONSTANT()}> or
329 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
330 kicking in. Similarly, since the C<< => >> operator quotes a bareword
331 immediately to its left, you have to say C<< CONSTANT() => 'value' >>
332 (or simply use a comma in place of the big arrow) instead of
333 C<< CONSTANT => 'value' >>.
337 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
340 Multiple constant declarations at once added by Casey West,
341 E<lt>F<casey@geeknest.com>E<gt>.
343 Documentation mostly rewritten by Ilmari Karonen,
344 E<lt>F<perl@itz.pp.sci.fi>E<gt>.
348 Copyright (C) 1997, 1999 Tom Phoenix
350 This module is free software; you can redistribute it or modify it
351 under the same terms as Perl itself.