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1package constant;
2
83763826 3use strict;
b0d6893f 4use 5.006_00;
d3a7d8c7 5use warnings::register;
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6
7our($VERSION, %declared);
6a761ace 8$VERSION = '1.09';
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9
10#=======================================================================
11
83763826 12# Some names are evil choices.
6a761ace 13my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK UNITCHECK END DESTROY AUTOLOAD };
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14
15my %forced_into_main = map +($_, 1),
16 qw{ STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG };
17
18my %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#=======================================================================
28sub 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;
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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 }
b35226bb 46 $constants = shift;
3cb88d13 47 } else {
b35226bb 48 $constants->{+shift} = undef;
83763826 49 }
3cb88d13 50
b35226bb 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 '$name' is invalid");
83763826 89 } else {
3cb88d13 90 Carp::croak("Constant name looks like boolean value");
83763826 91 }
83763826 92
83763826 93 } else {
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94 # Must have bad characters
95 require Carp;
96 Carp::croak("Constant name '$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;
4764e399 112 &Internals::inc_sub_generation;
3cb88d13 113 } else {
e040ff70 114 *$full_name = sub () { $scalar };
3cb88d13 115 }
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116 } elsif (@_) {
117 my @list = @_;
118 *$full_name = sub () { @list };
119 } else {
120 *$full_name = sub () { };
3cb88d13 121 }
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122 }
123 }
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124}
125
1261;
127
128__END__
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129
130=head1 NAME
131
132constant - Perl pragma to declare constants
133
134=head1 SYNOPSIS
135
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136 use constant PI => 4 * atan2(1, 1);
137 use constant DEBUG => 0;
138
139 print "Pi equals ", PI, "...\n" if DEBUG;
140
3cb88d13 141 use constant {
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142 SEC => 0,
143 MIN => 1,
144 HOUR => 2,
145 MDAY => 3,
146 MON => 4,
147 YEAR => 5,
148 WDAY => 6,
149 YDAY => 7,
150 ISDST => 8,
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151 };
152
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153 use constant WEEKDAYS => qw(
154 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
155 );
156
157 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
158
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159=head1 DESCRIPTION
160
a747501d 161This will declare a symbol to be a constant with the given value.
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162
163When you declare a constant such as C<PI> using the method shown
164above, each machine your script runs upon can have as many digits
165of accuracy as it can use. Also, your program will be easier to
166read, more likely to be maintained (and maintained correctly), and
167far less likely to send a space probe to the wrong planet because
168nobody noticed the one equation in which you wrote C<3.14195>.
169
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170When a constant is used in an expression, perl replaces it with its
171value at compile time, and may then optimize the expression further.
172In particular, any code in an C<if (CONSTANT)> block will be optimized
173away if the constant is false.
174
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175=head1 NOTES
176
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177As with all C<use> directives, defining a constant happens at
178compile time. Thus, it's probably not correct to put a constant
179declaration inside of a conditional statement (like C<if ($foo)
180{ use constant ... }>).
54310121 181
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182Constants defined using this module cannot be interpolated into
183strings like variables. However, concatenation works just fine:
54310121 184
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185 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
186 print "Pi equals ".PI."...\n"; # right
54310121 187
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188Even though a reference may be declared as a constant, the reference may
189point to data which may be changed, as this code shows.
190
191 use constant ARRAY => [ 1,2,3,4 ];
192 print ARRAY->[1];
193 ARRAY->[1] = " be changed";
194 print ARRAY->[1];
195
196Dereferencing constant references incorrectly (such as using an array
197subscript on a constant hash reference, or vice versa) will be trapped at
198compile time.
54310121 199
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200Constants belong to the package they are defined in. To refer to a
201constant defined in another package, specify the full package name, as
202in C<Some::Package::CONSTANT>. Constants may be exported by modules,
203and may also be called as either class or instance methods, that is,
204as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
205C<$obj> is an instance of C<Some::Package>. Subclasses may define
206their own constants to override those in their base class.
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207
208The use of all caps for constant names is merely a convention,
209although it is recommended in order to make constants stand out
210and to help avoid collisions with other barewords, keywords, and
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211subroutine names. Constant names must begin with a letter or
212underscore. Names beginning with a double underscore are reserved. Some
213poor choices for names will generate warnings, if warnings are enabled at
214compile time.
54310121 215
a747501d 216=head2 List constants
54310121 217
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218Constants may be lists of more (or less) than one value. A constant
219with no values evaluates to C<undef> in scalar context. Note that
220constants with more than one value do I<not> return their last value in
221scalar context as one might expect. They currently return the number
222of values, but B<this may change in the future>. Do not use constants
223with multiple values in scalar context.
3cb88d13 224
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225B<NOTE:> This implies that the expression defining the value of a
226constant is evaluated in list context. This may produce surprises:
54310121 227
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228 use constant TIMESTAMP => localtime; # WRONG!
229 use constant TIMESTAMP => scalar localtime; # right
54310121 230
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231The first line above defines C<TIMESTAMP> as a 9-element list, as
232returned by localtime() in list context. To set it to the string
233returned by localtime() in scalar context, an explicit C<scalar>
234keyword is required.
54310121 235
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236List constants are lists, not arrays. To index or slice them, they
237must be placed in parentheses.
54310121 238
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239 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
240 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 241
a747501d 242=head2 Defining multiple constants at once
b0d6893f 243
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244Instead of writing multiple C<use constant> statements, you may define
245multiple constants in a single statement by giving, instead of the
246constant name, a reference to a hash where the keys are the names of
247the constants to be defined. Obviously, all constants defined using
248this method must have a single value.
249
250 use constant {
251 FOO => "A single value",
252 BAR => "This", "won't", "work!", # Error!
253 };
254
255This is a fundamental limitation of the way hashes are constructed in
256Perl. The error messages produced when this happens will often be
257quite cryptic -- in the worst case there may be none at all, and
258you'll only later find that something is broken.
259
260When defining multiple constants, you cannot use the values of other
261constants defined in the same declaration. This is because the
262calling package doesn't know about any constant within that group
263until I<after> the C<use> statement is finished.
264
265 use constant {
266 BITMASK => 0xAFBAEBA8,
267 NEGMASK => ~BITMASK, # Error!
268 };
269
270=head2 Magic constants
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271
272Magical values and references can be made into constants at compile
273time, allowing for way cool stuff like this. (These error numbers
274aren't totally portable, alas.)
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275
276 use constant E2BIG => ($! = 7);
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277 print E2BIG, "\n"; # something like "Arg list too long"
278 print 0+E2BIG, "\n"; # "7"
54310121 279
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280You can't produce a tied constant by giving a tied scalar as the
281value. References to tied variables, however, can be used as
282constants without any problems.
283
a747501d 284=head1 TECHNICAL NOTES
b0d6893f 285
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286In the current implementation, scalar constants are actually
287inlinable subroutines. As of version 5.004 of Perl, the appropriate
288scalar constant is inserted directly in place of some subroutine
289calls, thereby saving the overhead of a subroutine call. See
290L<perlsub/"Constant Functions"> for details about how and when this
291happens.
3cb88d13 292
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293In the rare case in which you need to discover at run time whether a
294particular constant has been declared via this module, you may use
295this function to examine the hash C<%constant::declared>. If the given
296constant name does not include a package name, the current package is
297used.
298
299 sub declared ($) {
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300 use constant 1.01; # don't omit this!
301 my $name = shift;
302 $name =~ s/^::/main::/;
303 my $pkg = caller;
304 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
305 $constant::declared{$full_name};
83763826 306 }
779c5bc9 307
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308=head1 BUGS
309
310In the current version of Perl, list constants are not inlined
311and some symbols may be redefined without generating a warning.
312
a747501d 313It is not possible to have a subroutine or a keyword with the same
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314name as a constant in the same package. This is probably a Good Thing.
315
316A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
317ENV INC SIG> is not allowed anywhere but in package C<main::>, for
318technical reasons.
319
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320Unlike constants in some languages, these cannot be overridden
321on the command line or via environment variables.
322
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323You can get into trouble if you use constants in a context which
324automatically quotes barewords (as is true for any subroutine call).
325For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
326be interpreted as a string. Use C<$hash{CONSTANT()}> or
327C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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328kicking in. Similarly, since the C<< => >> operator quotes a bareword
329immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 330(or simply use a comma in place of the big arrow) instead of
a747501d 331C<< CONSTANT => 'value' >>.
a3cb178b 332
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333=head1 AUTHOR
334
83763826 335Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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336many other folks.
337
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338Multiple constant declarations at once added by Casey West,
339E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 340
a747501d 341Documentation mostly rewritten by Ilmari Karonen,
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342E<lt>F<perl@itz.pp.sci.fi>E<gt>.
343
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344=head1 COPYRIGHT
345
83763826 346Copyright (C) 1997, 1999 Tom Phoenix
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347
348This module is free software; you can redistribute it or modify it
349under the same terms as Perl itself.
350
351=cut