Commit | Line | Data |
---|---|---|
54310121 | 1 | package constant; |
6515510f | 2 | use 5.005; |
83763826 | 3 | use strict; |
d3a7d8c7 | 4 | use warnings::register; |
17f410f9 | 5 | |
6515510f | 6 | use vars qw($VERSION %declared); |
d9696651 | 7 | $VERSION = '1.17'; |
83763826 GS |
8 | |
9 | #======================================================================= | |
10 | ||
83763826 | 11 | # Some names are evil choices. |
83b99c4f NC |
12 | my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD }; |
13 | $keywords{UNITCHECK}++ if $] > 5.009; | |
83763826 GS |
14 | |
15 | my %forced_into_main = map +($_, 1), | |
16 | qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG }; | |
17 | ||
18 | my %forbidden = (%keywords, %forced_into_main); | |
19 | ||
20 | #======================================================================= | |
21 | # import() - import symbols into user's namespace | |
22 | # | |
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 | |
26 | # overhead. | |
27 | #======================================================================= | |
28 | sub import { | |
29 | my $class = shift; | |
30 | return unless @_; # Ignore 'use constant;' | |
b35226bb | 31 | my $constants; |
3cb88d13 | 32 | my $multiple = ref $_[0]; |
39a108ce | 33 | my $pkg = caller; |
e040ff70 | 34 | my $symtab; |
6515510f | 35 | my $str_end = $] >= 5.006 ? "\\z" : "\\Z"; |
e040ff70 NC |
36 | |
37 | if ($] > 5.009002) { | |
38 | no strict 'refs'; | |
39 | $symtab = \%{$pkg . '::'}; | |
40 | }; | |
3cb88d13 CT |
41 | |
42 | if ( $multiple ) { | |
43 | if (ref $_[0] ne 'HASH') { | |
44 | require Carp; | |
45 | Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'"); | |
46 | } | |
b35226bb | 47 | $constants = shift; |
3cb88d13 | 48 | } else { |
b35226bb | 49 | $constants->{+shift} = undef; |
83763826 | 50 | } |
3cb88d13 | 51 | |
b35226bb | 52 | foreach my $name ( keys %$constants ) { |
3cb88d13 CT |
53 | unless (defined $name) { |
54 | require Carp; | |
55 | Carp::croak("Can't use undef as constant name"); | |
56 | } | |
3cb88d13 CT |
57 | |
58 | # Normal constant name | |
6515510f | 59 | if ($name =~ /^_?[^\W_0-9]\w*$str_end/ and !$forbidden{$name}) { |
3cb88d13 CT |
60 | # Everything is okay |
61 | ||
62 | # Name forced into main, but we're not in main. Fatal. | |
63 | } elsif ($forced_into_main{$name} and $pkg ne 'main') { | |
64 | require Carp; | |
65 | Carp::croak("Constant name '$name' is forced into main::"); | |
66 | ||
67 | # Starts with double underscore. Fatal. | |
68 | } elsif ($name =~ /^__/) { | |
69 | require Carp; | |
70 | Carp::croak("Constant name '$name' begins with '__'"); | |
71 | ||
72 | # Maybe the name is tolerable | |
6515510f | 73 | } elsif ($name =~ /^[A-Za-z_]\w*$str_end/) { |
3cb88d13 CT |
74 | # Then we'll warn only if you've asked for warnings |
75 | if (warnings::enabled()) { | |
76 | if ($keywords{$name}) { | |
77 | warnings::warn("Constant name '$name' is a Perl keyword"); | |
78 | } elsif ($forced_into_main{$name}) { | |
79 | warnings::warn("Constant name '$name' is " . | |
80 | "forced into package main::"); | |
3cb88d13 CT |
81 | } |
82 | } | |
83 | ||
84 | # Looks like a boolean | |
85 | # use constant FRED == fred; | |
6515510f | 86 | } elsif ($name =~ /^[01]?$str_end/) { |
3cb88d13 CT |
87 | require Carp; |
88 | if (@_) { | |
89 | Carp::croak("Constant name '$name' is invalid"); | |
83763826 | 90 | } else { |
3cb88d13 | 91 | Carp::croak("Constant name looks like boolean value"); |
83763826 | 92 | } |
83763826 | 93 | |
83763826 | 94 | } else { |
3cb88d13 CT |
95 | # Must have bad characters |
96 | require Carp; | |
97 | Carp::croak("Constant name '$name' has invalid characters"); | |
83763826 GS |
98 | } |
99 | ||
3cb88d13 CT |
100 | { |
101 | no strict 'refs'; | |
102 | my $full_name = "${pkg}::$name"; | |
103 | $declared{$full_name}++; | |
e040ff70 NC |
104 | if ($multiple || @_ == 1) { |
105 | my $scalar = $multiple ? $constants->{$name} : $_[0]; | |
106 | if ($symtab && !exists $symtab->{$name}) { | |
107 | # No typeglob yet, so we can use a reference as space- | |
108 | # efficient proxy for a constant subroutine | |
109 | # The check in Perl_ck_rvconst knows that inlinable | |
110 | # constants from cv_const_sv are read only. So we have to: | |
111 | Internals::SvREADONLY($scalar, 1); | |
112 | $symtab->{$name} = \$scalar; | |
e1a479c5 | 113 | mro::method_changed_in($pkg); |
3cb88d13 | 114 | } else { |
e040ff70 | 115 | *$full_name = sub () { $scalar }; |
3cb88d13 | 116 | } |
e040ff70 NC |
117 | } elsif (@_) { |
118 | my @list = @_; | |
119 | *$full_name = sub () { @list }; | |
120 | } else { | |
121 | *$full_name = sub () { }; | |
3cb88d13 | 122 | } |
83763826 GS |
123 | } |
124 | } | |
83763826 GS |
125 | } |
126 | ||
127 | 1; | |
128 | ||
129 | __END__ | |
54310121 | 130 | |
131 | =head1 NAME | |
132 | ||
133 | constant - Perl pragma to declare constants | |
134 | ||
135 | =head1 SYNOPSIS | |
136 | ||
a747501d AMS |
137 | use constant PI => 4 * atan2(1, 1); |
138 | use constant DEBUG => 0; | |
139 | ||
140 | print "Pi equals ", PI, "...\n" if DEBUG; | |
141 | ||
3cb88d13 | 142 | use constant { |
a747501d AMS |
143 | SEC => 0, |
144 | MIN => 1, | |
145 | HOUR => 2, | |
146 | MDAY => 3, | |
147 | MON => 4, | |
148 | YEAR => 5, | |
149 | WDAY => 6, | |
150 | YDAY => 7, | |
151 | ISDST => 8, | |
3cb88d13 CT |
152 | }; |
153 | ||
a747501d AMS |
154 | use constant WEEKDAYS => qw( |
155 | Sunday Monday Tuesday Wednesday Thursday Friday Saturday | |
156 | ); | |
157 | ||
158 | print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; | |
159 | ||
54310121 | 160 | =head1 DESCRIPTION |
161 | ||
6515510f | 162 | This pragma allows you to declare constants at compile-time. |
54310121 | 163 | |
164 | When you declare a constant such as C<PI> using the method shown | |
165 | above, each machine your script runs upon can have as many digits | |
166 | of accuracy as it can use. Also, your program will be easier to | |
167 | read, more likely to be maintained (and maintained correctly), and | |
168 | far less likely to send a space probe to the wrong planet because | |
169 | nobody noticed the one equation in which you wrote C<3.14195>. | |
170 | ||
d3383c75 | 171 | When a constant is used in an expression, Perl replaces it with its |
a747501d AMS |
172 | value at compile time, and may then optimize the expression further. |
173 | In particular, any code in an C<if (CONSTANT)> block will be optimized | |
174 | away if the constant is false. | |
175 | ||
54310121 | 176 | =head1 NOTES |
177 | ||
a747501d AMS |
178 | As with all C<use> directives, defining a constant happens at |
179 | compile time. Thus, it's probably not correct to put a constant | |
180 | declaration inside of a conditional statement (like C<if ($foo) | |
181 | { use constant ... }>). | |
54310121 | 182 | |
a747501d AMS |
183 | Constants defined using this module cannot be interpolated into |
184 | strings like variables. However, concatenation works just fine: | |
54310121 | 185 | |
a747501d AMS |
186 | print "Pi equals PI...\n"; # WRONG: does not expand "PI" |
187 | print "Pi equals ".PI."...\n"; # right | |
54310121 | 188 | |
a747501d AMS |
189 | Even though a reference may be declared as a constant, the reference may |
190 | point to data which may be changed, as this code shows. | |
191 | ||
192 | use constant ARRAY => [ 1,2,3,4 ]; | |
193 | print ARRAY->[1]; | |
194 | ARRAY->[1] = " be changed"; | |
195 | print ARRAY->[1]; | |
196 | ||
197 | Dereferencing constant references incorrectly (such as using an array | |
198 | subscript on a constant hash reference, or vice versa) will be trapped at | |
199 | compile time. | |
54310121 | 200 | |
a747501d AMS |
201 | Constants belong to the package they are defined in. To refer to a |
202 | constant defined in another package, specify the full package name, as | |
203 | in C<Some::Package::CONSTANT>. Constants may be exported by modules, | |
204 | and may also be called as either class or instance methods, that is, | |
205 | as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where | |
206 | C<$obj> is an instance of C<Some::Package>. Subclasses may define | |
207 | their own constants to override those in their base class. | |
54310121 | 208 | |
209 | The use of all caps for constant names is merely a convention, | |
210 | although it is recommended in order to make constants stand out | |
211 | and to help avoid collisions with other barewords, keywords, and | |
83763826 GS |
212 | subroutine names. Constant names must begin with a letter or |
213 | underscore. Names beginning with a double underscore are reserved. Some | |
214 | poor choices for names will generate warnings, if warnings are enabled at | |
215 | compile time. | |
54310121 | 216 | |
a747501d | 217 | =head2 List constants |
54310121 | 218 | |
a747501d AMS |
219 | Constants may be lists of more (or less) than one value. A constant |
220 | with no values evaluates to C<undef> in scalar context. Note that | |
221 | constants with more than one value do I<not> return their last value in | |
222 | scalar context as one might expect. They currently return the number | |
223 | of values, but B<this may change in the future>. Do not use constants | |
224 | with multiple values in scalar context. | |
3cb88d13 | 225 | |
a747501d AMS |
226 | B<NOTE:> This implies that the expression defining the value of a |
227 | constant is evaluated in list context. This may produce surprises: | |
54310121 | 228 | |
a747501d AMS |
229 | use constant TIMESTAMP => localtime; # WRONG! |
230 | use constant TIMESTAMP => scalar localtime; # right | |
54310121 | 231 | |
a747501d | 232 | The first line above defines C<TIMESTAMP> as a 9-element list, as |
6515510f AT |
233 | returned by C<localtime()> in list context. To set it to the string |
234 | returned by C<localtime()> in scalar context, an explicit C<scalar> | |
a747501d | 235 | keyword is required. |
54310121 | 236 | |
a747501d AMS |
237 | List constants are lists, not arrays. To index or slice them, they |
238 | must be placed in parentheses. | |
54310121 | 239 | |
a747501d AMS |
240 | my @workdays = WEEKDAYS[1 .. 5]; # WRONG! |
241 | my @workdays = (WEEKDAYS)[1 .. 5]; # right | |
b0d6893f | 242 | |
a747501d | 243 | =head2 Defining multiple constants at once |
b0d6893f | 244 | |
a747501d AMS |
245 | Instead of writing multiple C<use constant> statements, you may define |
246 | multiple constants in a single statement by giving, instead of the | |
247 | constant name, a reference to a hash where the keys are the names of | |
248 | the constants to be defined. Obviously, all constants defined using | |
249 | this method must have a single value. | |
250 | ||
251 | use constant { | |
252 | FOO => "A single value", | |
253 | BAR => "This", "won't", "work!", # Error! | |
254 | }; | |
255 | ||
256 | This is a fundamental limitation of the way hashes are constructed in | |
257 | Perl. The error messages produced when this happens will often be | |
258 | quite cryptic -- in the worst case there may be none at all, and | |
259 | you'll only later find that something is broken. | |
260 | ||
261 | When defining multiple constants, you cannot use the values of other | |
262 | constants defined in the same declaration. This is because the | |
263 | calling package doesn't know about any constant within that group | |
264 | until I<after> the C<use> statement is finished. | |
265 | ||
266 | use constant { | |
267 | BITMASK => 0xAFBAEBA8, | |
268 | NEGMASK => ~BITMASK, # Error! | |
269 | }; | |
270 | ||
271 | =head2 Magic constants | |
b0d6893f IK |
272 | |
273 | Magical values and references can be made into constants at compile | |
274 | time, allowing for way cool stuff like this. (These error numbers | |
275 | aren't totally portable, alas.) | |
54310121 | 276 | |
277 | use constant E2BIG => ($! = 7); | |
a747501d AMS |
278 | print E2BIG, "\n"; # something like "Arg list too long" |
279 | print 0+E2BIG, "\n"; # "7" | |
54310121 | 280 | |
b0d6893f IK |
281 | You can't produce a tied constant by giving a tied scalar as the |
282 | value. References to tied variables, however, can be used as | |
283 | constants without any problems. | |
284 | ||
a747501d | 285 | =head1 TECHNICAL NOTES |
b0d6893f | 286 | |
a747501d AMS |
287 | In the current implementation, scalar constants are actually |
288 | inlinable subroutines. As of version 5.004 of Perl, the appropriate | |
289 | scalar constant is inserted directly in place of some subroutine | |
290 | calls, thereby saving the overhead of a subroutine call. See | |
291 | L<perlsub/"Constant Functions"> for details about how and when this | |
292 | happens. | |
3cb88d13 | 293 | |
83763826 GS |
294 | In the rare case in which you need to discover at run time whether a |
295 | particular constant has been declared via this module, you may use | |
296 | this function to examine the hash C<%constant::declared>. If the given | |
297 | constant name does not include a package name, the current package is | |
298 | used. | |
299 | ||
300 | sub declared ($) { | |
a747501d AMS |
301 | use constant 1.01; # don't omit this! |
302 | my $name = shift; | |
303 | $name =~ s/^::/main::/; | |
304 | my $pkg = caller; | |
305 | my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; | |
306 | $constant::declared{$full_name}; | |
83763826 | 307 | } |
779c5bc9 | 308 | |
6515510f | 309 | =head1 CAVEATS |
54310121 | 310 | |
311 | In the current version of Perl, list constants are not inlined | |
312 | and some symbols may be redefined without generating a warning. | |
313 | ||
a747501d | 314 | It is not possible to have a subroutine or a keyword with the same |
83763826 GS |
315 | name as a constant in the same package. This is probably a Good Thing. |
316 | ||
317 | A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT | |
318 | ENV INC SIG> is not allowed anywhere but in package C<main::>, for | |
319 | technical reasons. | |
320 | ||
54310121 | 321 | Unlike constants in some languages, these cannot be overridden |
322 | on the command line or via environment variables. | |
323 | ||
a3cb178b GS |
324 | You can get into trouble if you use constants in a context which |
325 | automatically quotes barewords (as is true for any subroutine call). | |
326 | For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will | |
327 | be interpreted as a string. Use C<$hash{CONSTANT()}> or | |
328 | C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from | |
a747501d AMS |
329 | kicking in. Similarly, since the C<< => >> operator quotes a bareword |
330 | immediately to its left, you have to say C<< CONSTANT() => 'value' >> | |
83763826 | 331 | (or simply use a comma in place of the big arrow) instead of |
a747501d | 332 | C<< CONSTANT => 'value' >>. |
a3cb178b | 333 | |
d3383c75 AT |
334 | =head1 SEE ALSO |
335 | ||
336 | L<Readonly> - Facility for creating read-only scalars, arrays, hashes. | |
337 | ||
338 | L<Const> - Facility for creating read-only variables. Similar to C<Readonly>, | |
339 | but uses C<SvREADONLY> instead of C<tie>. | |
340 | ||
341 | L<Attribute::Constant> - Make read-only variables via attribute | |
342 | ||
343 | L<Scalar::Readonly> - Perl extension to the C<SvREADONLY> scalar flag | |
344 | ||
345 | L<Hash::Util> - A selection of general-utility hash subroutines (mostly | |
346 | to lock/unlock keys and values) | |
347 | ||
6515510f AT |
348 | =head1 BUGS |
349 | ||
350 | Please report any bugs or feature requests via the perlbug(1) utility. | |
351 | ||
352 | =head1 AUTHORS | |
54310121 | 353 | |
83763826 | 354 | Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from |
54310121 | 355 | many other folks. |
356 | ||
e1e60e72 CW |
357 | Multiple constant declarations at once added by Casey West, |
358 | E<lt>F<casey@geeknest.com>E<gt>. | |
3cb88d13 | 359 | |
a747501d | 360 | Documentation mostly rewritten by Ilmari Karonen, |
b0d6893f IK |
361 | E<lt>F<perl@itz.pp.sci.fi>E<gt>. |
362 | ||
6515510f AT |
363 | This program is maintained by the Perl 5 Porters. |
364 | The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni | |
365 | E<lt>F<sebastien@aperghis.net>E<gt>. | |
366 | ||
d3383c75 | 367 | =head1 COPYRIGHT & LICENSE |
54310121 | 368 | |
83763826 | 369 | Copyright (C) 1997, 1999 Tom Phoenix |
54310121 | 370 | |
371 | This module is free software; you can redistribute it or modify it | |
372 | under the same terms as Perl itself. | |
373 | ||
374 | =cut |