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1 | package constant; |
2 | |
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3 | use strict; |
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4 | use 5.006_00; |
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5 | use warnings::register; |
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6 | |
7 | our($VERSION, %declared); |
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8 | $VERSION = '1.07'; |
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9 | |
10 | #======================================================================= |
11 | |
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12 | # Some names are evil choices. |
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13 | my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD }; |
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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;' |
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31 | my $constants; |
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32 | my $multiple = ref $_[0]; |
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33 | my $pkg = caller; |
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34 | my $symtab; |
35 | |
36 | if ($] > 5.009002) { |
37 | no strict 'refs'; |
38 | $symtab = \%{$pkg . '::'}; |
39 | }; |
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40 | |
41 | if ( $multiple ) { |
42 | if (ref $_[0] ne 'HASH') { |
43 | require Carp; |
44 | Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'"); |
45 | } |
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46 | $constants = shift; |
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47 | } else { |
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48 | $constants->{+shift} = undef; |
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49 | } |
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50 | |
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51 | foreach my $name ( keys %$constants ) { |
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52 | unless (defined $name) { |
53 | require Carp; |
54 | Carp::croak("Can't use undef as constant name"); |
55 | } |
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56 | |
57 | # Normal constant name |
58 | if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) { |
59 | # Everything is okay |
60 | |
61 | # Name forced into main, but we're not in main. Fatal. |
62 | } elsif ($forced_into_main{$name} and $pkg ne 'main') { |
63 | require Carp; |
64 | Carp::croak("Constant name '$name' is forced into main::"); |
65 | |
66 | # Starts with double underscore. Fatal. |
67 | } elsif ($name =~ /^__/) { |
68 | require Carp; |
69 | Carp::croak("Constant name '$name' begins with '__'"); |
70 | |
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 '$name' is " . |
79 | "forced into package main::"); |
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80 | } |
81 | } |
82 | |
83 | # Looks like a boolean |
84 | # use constant FRED == fred; |
85 | } elsif ($name =~ /^[01]?\z/) { |
86 | require Carp; |
87 | if (@_) { |
88 | Carp::croak("Constant name '$name' is invalid"); |
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89 | } else { |
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90 | Carp::croak("Constant name looks like boolean value"); |
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91 | } |
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92 | |
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93 | } else { |
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94 | # Must have bad characters |
95 | require Carp; |
96 | Carp::croak("Constant name '$name' has invalid characters"); |
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97 | } |
98 | |
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99 | { |
100 | no strict 'refs'; |
101 | my $full_name = "${pkg}::$name"; |
102 | $declared{$full_name}++; |
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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; |
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112 | Internals::inc_sub_generation; |
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113 | } else { |
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114 | if(!exists $symtab->{$name}) { |
115 | print STDERR "$name $scalar\n"; |
116 | } |
117 | *$full_name = sub () { $scalar }; |
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118 | } |
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119 | } elsif (@_) { |
120 | my @list = @_; |
121 | *$full_name = sub () { @list }; |
122 | } else { |
123 | *$full_name = sub () { }; |
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124 | } |
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125 | } |
126 | } |
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127 | } |
128 | |
129 | 1; |
130 | |
131 | __END__ |
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132 | |
133 | =head1 NAME |
134 | |
135 | constant - Perl pragma to declare constants |
136 | |
137 | =head1 SYNOPSIS |
138 | |
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139 | use constant PI => 4 * atan2(1, 1); |
140 | use constant DEBUG => 0; |
141 | |
142 | print "Pi equals ", PI, "...\n" if DEBUG; |
143 | |
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144 | use constant { |
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145 | SEC => 0, |
146 | MIN => 1, |
147 | HOUR => 2, |
148 | MDAY => 3, |
149 | MON => 4, |
150 | YEAR => 5, |
151 | WDAY => 6, |
152 | YDAY => 7, |
153 | ISDST => 8, |
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154 | }; |
155 | |
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156 | use constant WEEKDAYS => qw( |
157 | Sunday Monday Tuesday Wednesday Thursday Friday Saturday |
158 | ); |
159 | |
160 | print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; |
161 | |
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162 | =head1 DESCRIPTION |
163 | |
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164 | This will declare a symbol to be a constant with the given value. |
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165 | |
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>. |
172 | |
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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. |
177 | |
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178 | =head1 NOTES |
179 | |
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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 ... }>). |
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184 | |
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185 | Constants defined using this module cannot be interpolated into |
186 | strings like variables. However, concatenation works just fine: |
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187 | |
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188 | print "Pi equals PI...\n"; # WRONG: does not expand "PI" |
189 | print "Pi equals ".PI."...\n"; # right |
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190 | |
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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. |
193 | |
194 | use constant ARRAY => [ 1,2,3,4 ]; |
195 | print ARRAY->[1]; |
196 | ARRAY->[1] = " be changed"; |
197 | print ARRAY->[1]; |
198 | |
199 | Dereferencing constant references incorrectly (such as using an array |
200 | subscript on a constant hash reference, or vice versa) will be trapped at |
201 | compile time. |
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202 | |
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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. |
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210 | |
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 |
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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 |
217 | compile time. |
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218 | |
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219 | =head2 List constants |
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220 | |
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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. |
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227 | |
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228 | B<NOTE:> This implies that the expression defining the value of a |
229 | constant is evaluated in list context. This may produce surprises: |
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230 | |
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231 | use constant TIMESTAMP => localtime; # WRONG! |
232 | use constant TIMESTAMP => scalar localtime; # right |
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233 | |
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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> |
237 | keyword is required. |
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238 | |
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239 | List constants are lists, not arrays. To index or slice them, they |
240 | must be placed in parentheses. |
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241 | |
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242 | my @workdays = WEEKDAYS[1 .. 5]; # WRONG! |
243 | my @workdays = (WEEKDAYS)[1 .. 5]; # right |
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244 | |
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245 | =head2 Defining multiple constants at once |
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246 | |
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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. |
252 | |
253 | use constant { |
254 | FOO => "A single value", |
255 | BAR => "This", "won't", "work!", # Error! |
256 | }; |
257 | |
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. |
262 | |
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. |
267 | |
268 | use constant { |
269 | BITMASK => 0xAFBAEBA8, |
270 | NEGMASK => ~BITMASK, # Error! |
271 | }; |
272 | |
273 | =head2 Magic constants |
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274 | |
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.) |
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278 | |
279 | use constant E2BIG => ($! = 7); |
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280 | print E2BIG, "\n"; # something like "Arg list too long" |
281 | print 0+E2BIG, "\n"; # "7" |
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282 | |
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283 | You can't produce a tied constant by giving a tied scalar as the |
284 | value. References to tied variables, however, can be used as |
285 | constants without any problems. |
286 | |
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287 | =head1 TECHNICAL NOTES |
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288 | |
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289 | In the current implementation, scalar constants are actually |
290 | inlinable subroutines. As of version 5.004 of Perl, the appropriate |
291 | scalar constant is inserted directly in place of some subroutine |
292 | calls, thereby saving the overhead of a subroutine call. See |
293 | L<perlsub/"Constant Functions"> for details about how and when this |
294 | happens. |
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295 | |
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296 | In the rare case in which you need to discover at run time whether a |
297 | particular constant has been declared via this module, you may use |
298 | this function to examine the hash C<%constant::declared>. If the given |
299 | constant name does not include a package name, the current package is |
300 | used. |
301 | |
302 | sub declared ($) { |
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303 | use constant 1.01; # don't omit this! |
304 | my $name = shift; |
305 | $name =~ s/^::/main::/; |
306 | my $pkg = caller; |
307 | my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; |
308 | $constant::declared{$full_name}; |
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309 | } |
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310 | |
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311 | =head1 BUGS |
312 | |
313 | In the current version of Perl, list constants are not inlined |
314 | and some symbols may be redefined without generating a warning. |
315 | |
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316 | It is not possible to have a subroutine or a keyword with the same |
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317 | name as a constant in the same package. This is probably a Good Thing. |
318 | |
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 |
321 | technical reasons. |
322 | |
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323 | Unlike constants in some languages, these cannot be overridden |
324 | on the command line or via environment variables. |
325 | |
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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 |
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331 | kicking in. Similarly, since the C<< => >> operator quotes a bareword |
332 | immediately to its left, you have to say C<< CONSTANT() => 'value' >> |
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333 | (or simply use a comma in place of the big arrow) instead of |
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334 | C<< CONSTANT => 'value' >>. |
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335 | |
54310121 |
336 | =head1 AUTHOR |
337 | |
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338 | Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from |
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339 | many other folks. |
340 | |
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341 | Multiple constant declarations at once added by Casey West, |
342 | E<lt>F<casey@geeknest.com>E<gt>. |
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343 | |
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344 | Documentation mostly rewritten by Ilmari Karonen, |
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345 | E<lt>F<perl@itz.pp.sci.fi>E<gt>. |
346 | |
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347 | =head1 COPYRIGHT |
348 | |
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349 | Copyright (C) 1997, 1999 Tom Phoenix |
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350 | |
351 | This module is free software; you can redistribute it or modify it |
352 | under the same terms as Perl itself. |
353 | |
354 | =cut |