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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 | $VERSION = '1.04'; |
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9 | |
10 | #======================================================================= |
11 | |
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12 | # Some names are evil choices. |
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13 | my %keywords = map +($_, 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 = (); |
32 | my $multiple = ref $_[0]; |
33 | |
34 | if ( $multiple ) { |
35 | if (ref $_[0] ne 'HASH') { |
36 | require Carp; |
37 | Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'"); |
38 | } |
39 | %constants = %{+shift}; |
40 | } else { |
41 | $constants{+shift} = undef; |
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42 | } |
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43 | |
44 | foreach my $name ( keys %constants ) { |
45 | unless (defined $name) { |
46 | require Carp; |
47 | Carp::croak("Can't use undef as constant name"); |
48 | } |
49 | my $pkg = caller; |
50 | |
51 | # Normal constant name |
52 | if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) { |
53 | # Everything is okay |
54 | |
55 | # Name forced into main, but we're not in main. Fatal. |
56 | } elsif ($forced_into_main{$name} and $pkg ne 'main') { |
57 | require Carp; |
58 | Carp::croak("Constant name '$name' is forced into main::"); |
59 | |
60 | # Starts with double underscore. Fatal. |
61 | } elsif ($name =~ /^__/) { |
62 | require Carp; |
63 | Carp::croak("Constant name '$name' begins with '__'"); |
64 | |
65 | # Maybe the name is tolerable |
66 | } elsif ($name =~ /^[A-Za-z_]\w*\z/) { |
67 | # Then we'll warn only if you've asked for warnings |
68 | if (warnings::enabled()) { |
69 | if ($keywords{$name}) { |
70 | warnings::warn("Constant name '$name' is a Perl keyword"); |
71 | } elsif ($forced_into_main{$name}) { |
72 | warnings::warn("Constant name '$name' is " . |
73 | "forced into package main::"); |
74 | } else { |
75 | # Catch-all - what did I miss? If you get this error, |
76 | # please let me know what your constant's name was. |
77 | # Write to <rootbeer@redcat.com>. Thanks! |
78 | warnings::warn("Constant name '$name' has unknown problems"); |
79 | } |
80 | } |
81 | |
82 | # Looks like a boolean |
83 | # use constant FRED == fred; |
84 | } elsif ($name =~ /^[01]?\z/) { |
85 | require Carp; |
86 | if (@_) { |
87 | Carp::croak("Constant name '$name' is invalid"); |
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88 | } else { |
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89 | Carp::croak("Constant name looks like boolean value"); |
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90 | } |
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91 | |
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92 | } else { |
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93 | # Must have bad characters |
94 | require Carp; |
95 | Carp::croak("Constant name '$name' has invalid characters"); |
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96 | } |
97 | |
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98 | { |
99 | no strict 'refs'; |
100 | my $full_name = "${pkg}::$name"; |
101 | $declared{$full_name}++; |
102 | if ($multiple) { |
103 | my $scalar = $constants{$name}; |
104 | *$full_name = sub () { $scalar }; |
105 | } else { |
106 | if (@_ == 1) { |
107 | my $scalar = $_[0]; |
108 | *$full_name = sub () { $scalar }; |
109 | } elsif (@_) { |
110 | my @list = @_; |
111 | *$full_name = sub () { @list }; |
112 | } else { |
113 | *$full_name = sub () { }; |
114 | } |
115 | } |
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116 | } |
117 | } |
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118 | } |
119 | |
120 | 1; |
121 | |
122 | __END__ |
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123 | |
124 | =head1 NAME |
125 | |
126 | constant - Perl pragma to declare constants |
127 | |
128 | =head1 SYNOPSIS |
129 | |
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130 | use constant PI => 4 * atan2(1, 1); |
131 | use constant DEBUG => 0; |
132 | |
133 | print "Pi equals ", PI, "...\n" if DEBUG; |
134 | |
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135 | use constant { |
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136 | SEC => 0, |
137 | MIN => 1, |
138 | HOUR => 2, |
139 | MDAY => 3, |
140 | MON => 4, |
141 | YEAR => 5, |
142 | WDAY => 6, |
143 | YDAY => 7, |
144 | ISDST => 8, |
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145 | }; |
146 | |
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147 | use constant WEEKDAYS => qw( |
148 | Sunday Monday Tuesday Wednesday Thursday Friday Saturday |
149 | ); |
150 | |
151 | print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; |
152 | |
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153 | =head1 DESCRIPTION |
154 | |
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155 | This will declare a symbol to be a constant with the given value. |
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156 | |
157 | When you declare a constant such as C<PI> using the method shown |
158 | above, each machine your script runs upon can have as many digits |
159 | of accuracy as it can use. Also, your program will be easier to |
160 | read, more likely to be maintained (and maintained correctly), and |
161 | far less likely to send a space probe to the wrong planet because |
162 | nobody noticed the one equation in which you wrote C<3.14195>. |
163 | |
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164 | When a constant is used in an expression, perl replaces it with its |
165 | value at compile time, and may then optimize the expression further. |
166 | In particular, any code in an C<if (CONSTANT)> block will be optimized |
167 | away if the constant is false. |
168 | |
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169 | =head1 NOTES |
170 | |
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171 | As with all C<use> directives, defining a constant happens at |
172 | compile time. Thus, it's probably not correct to put a constant |
173 | declaration inside of a conditional statement (like C<if ($foo) |
174 | { use constant ... }>). |
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175 | |
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176 | Constants defined using this module cannot be interpolated into |
177 | strings like variables. However, concatenation works just fine: |
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178 | |
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179 | print "Pi equals PI...\n"; # WRONG: does not expand "PI" |
180 | print "Pi equals ".PI."...\n"; # right |
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181 | |
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182 | Even though a reference may be declared as a constant, the reference may |
183 | point to data which may be changed, as this code shows. |
184 | |
185 | use constant ARRAY => [ 1,2,3,4 ]; |
186 | print ARRAY->[1]; |
187 | ARRAY->[1] = " be changed"; |
188 | print ARRAY->[1]; |
189 | |
190 | Dereferencing constant references incorrectly (such as using an array |
191 | subscript on a constant hash reference, or vice versa) will be trapped at |
192 | compile time. |
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193 | |
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194 | Constants belong to the package they are defined in. To refer to a |
195 | constant defined in another package, specify the full package name, as |
196 | in C<Some::Package::CONSTANT>. Constants may be exported by modules, |
197 | and may also be called as either class or instance methods, that is, |
198 | as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where |
199 | C<$obj> is an instance of C<Some::Package>. Subclasses may define |
200 | their own constants to override those in their base class. |
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201 | |
202 | The use of all caps for constant names is merely a convention, |
203 | although it is recommended in order to make constants stand out |
204 | and to help avoid collisions with other barewords, keywords, and |
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205 | subroutine names. Constant names must begin with a letter or |
206 | underscore. Names beginning with a double underscore are reserved. Some |
207 | poor choices for names will generate warnings, if warnings are enabled at |
208 | compile time. |
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209 | |
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210 | =head2 List constants |
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211 | |
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212 | Constants may be lists of more (or less) than one value. A constant |
213 | with no values evaluates to C<undef> in scalar context. Note that |
214 | constants with more than one value do I<not> return their last value in |
215 | scalar context as one might expect. They currently return the number |
216 | of values, but B<this may change in the future>. Do not use constants |
217 | with multiple values in scalar context. |
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218 | |
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219 | B<NOTE:> This implies that the expression defining the value of a |
220 | constant is evaluated in list context. This may produce surprises: |
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221 | |
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222 | use constant TIMESTAMP => localtime; # WRONG! |
223 | use constant TIMESTAMP => scalar localtime; # right |
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224 | |
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225 | The first line above defines C<TIMESTAMP> as a 9-element list, as |
226 | returned by localtime() in list context. To set it to the string |
227 | returned by localtime() in scalar context, an explicit C<scalar> |
228 | keyword is required. |
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229 | |
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230 | List constants are lists, not arrays. To index or slice them, they |
231 | must be placed in parentheses. |
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232 | |
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233 | my @workdays = WEEKDAYS[1 .. 5]; # WRONG! |
234 | my @workdays = (WEEKDAYS)[1 .. 5]; # right |
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235 | |
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236 | =head2 Defining multiple constants at once |
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237 | |
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238 | Instead of writing multiple C<use constant> statements, you may define |
239 | multiple constants in a single statement by giving, instead of the |
240 | constant name, a reference to a hash where the keys are the names of |
241 | the constants to be defined. Obviously, all constants defined using |
242 | this method must have a single value. |
243 | |
244 | use constant { |
245 | FOO => "A single value", |
246 | BAR => "This", "won't", "work!", # Error! |
247 | }; |
248 | |
249 | This is a fundamental limitation of the way hashes are constructed in |
250 | Perl. The error messages produced when this happens will often be |
251 | quite cryptic -- in the worst case there may be none at all, and |
252 | you'll only later find that something is broken. |
253 | |
254 | When defining multiple constants, you cannot use the values of other |
255 | constants defined in the same declaration. This is because the |
256 | calling package doesn't know about any constant within that group |
257 | until I<after> the C<use> statement is finished. |
258 | |
259 | use constant { |
260 | BITMASK => 0xAFBAEBA8, |
261 | NEGMASK => ~BITMASK, # Error! |
262 | }; |
263 | |
264 | =head2 Magic constants |
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265 | |
266 | Magical values and references can be made into constants at compile |
267 | time, allowing for way cool stuff like this. (These error numbers |
268 | aren't totally portable, alas.) |
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269 | |
270 | use constant E2BIG => ($! = 7); |
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271 | print E2BIG, "\n"; # something like "Arg list too long" |
272 | print 0+E2BIG, "\n"; # "7" |
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273 | |
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274 | You can't produce a tied constant by giving a tied scalar as the |
275 | value. References to tied variables, however, can be used as |
276 | constants without any problems. |
277 | |
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278 | =head1 TECHNICAL NOTES |
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279 | |
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280 | In the current implementation, scalar constants are actually |
281 | inlinable subroutines. As of version 5.004 of Perl, the appropriate |
282 | scalar constant is inserted directly in place of some subroutine |
283 | calls, thereby saving the overhead of a subroutine call. See |
284 | L<perlsub/"Constant Functions"> for details about how and when this |
285 | happens. |
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286 | |
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287 | In the rare case in which you need to discover at run time whether a |
288 | particular constant has been declared via this module, you may use |
289 | this function to examine the hash C<%constant::declared>. If the given |
290 | constant name does not include a package name, the current package is |
291 | used. |
292 | |
293 | sub declared ($) { |
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294 | use constant 1.01; # don't omit this! |
295 | my $name = shift; |
296 | $name =~ s/^::/main::/; |
297 | my $pkg = caller; |
298 | my $full_name = $name =~ /::/ ? $name : "${pkg}::$name"; |
299 | $constant::declared{$full_name}; |
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300 | } |
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301 | |
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302 | =head1 BUGS |
303 | |
304 | In the current version of Perl, list constants are not inlined |
305 | and some symbols may be redefined without generating a warning. |
306 | |
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307 | It is not possible to have a subroutine or a keyword with the same |
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308 | name as a constant in the same package. This is probably a Good Thing. |
309 | |
310 | A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT |
311 | ENV INC SIG> is not allowed anywhere but in package C<main::>, for |
312 | technical reasons. |
313 | |
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314 | Unlike constants in some languages, these cannot be overridden |
315 | on the command line or via environment variables. |
316 | |
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317 | You can get into trouble if you use constants in a context which |
318 | automatically quotes barewords (as is true for any subroutine call). |
319 | For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will |
320 | be interpreted as a string. Use C<$hash{CONSTANT()}> or |
321 | C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from |
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322 | kicking in. Similarly, since the C<< => >> operator quotes a bareword |
323 | immediately to its left, you have to say C<< CONSTANT() => 'value' >> |
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324 | (or simply use a comma in place of the big arrow) instead of |
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325 | C<< CONSTANT => 'value' >>. |
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326 | |
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327 | =head1 AUTHOR |
328 | |
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329 | Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from |
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330 | many other folks. |
331 | |
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332 | Multiple constant declarations at once added by Casey West, |
333 | E<lt>F<casey@geeknest.com>E<gt>. |
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334 | |
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335 | Documentation mostly rewritten by Ilmari Karonen, |
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336 | E<lt>F<perl@itz.pp.sci.fi>E<gt>. |
337 | |
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338 | =head1 COPYRIGHT |
339 | |
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340 | Copyright (C) 1997, 1999 Tom Phoenix |
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341 | |
342 | This module is free software; you can redistribute it or modify it |
343 | under the same terms as Perl itself. |
344 | |
345 | =cut |