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[perl5.git] / lib / constant.pm
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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($VERSION, %declared);
b0d6893f 8$VERSION = '1.04';
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9
10#=======================================================================
11
83763826 12# Some names are evil choices.
7d30b5c4 13my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
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14
15my %forced_into_main = map +($_, 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;'
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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;
83763826 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");
83763826 88 } else {
3cb88d13 89 Carp::croak("Constant name looks like boolean value");
83763826 90 }
83763826 91
83763826 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
1201;
121
122__END__
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123
124=head1 NAME
125
126constant - 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
3cb88d13 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
a747501d 155This will declare a symbol to be a constant with the given value.
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156
157When you declare a constant such as C<PI> using the method shown
158above, each machine your script runs upon can have as many digits
159of accuracy as it can use. Also, your program will be easier to
160read, more likely to be maintained (and maintained correctly), and
161far less likely to send a space probe to the wrong planet because
162nobody noticed the one equation in which you wrote C<3.14195>.
163
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164When a constant is used in an expression, perl replaces it with its
165value at compile time, and may then optimize the expression further.
166In particular, any code in an C<if (CONSTANT)> block will be optimized
167away if the constant is false.
168
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169=head1 NOTES
170
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171As with all C<use> directives, defining a constant happens at
172compile time. Thus, it's probably not correct to put a constant
173declaration inside of a conditional statement (like C<if ($foo)
174{ use constant ... }>).
54310121 175
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176Constants defined using this module cannot be interpolated into
177strings like variables. However, concatenation works just fine:
54310121 178
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179 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
180 print "Pi equals ".PI."...\n"; # right
54310121 181
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182Even though a reference may be declared as a constant, the reference may
183point 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
190Dereferencing constant references incorrectly (such as using an array
191subscript on a constant hash reference, or vice versa) will be trapped at
192compile time.
54310121 193
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194Constants belong to the package they are defined in. To refer to a
195constant defined in another package, specify the full package name, as
196in C<Some::Package::CONSTANT>. Constants may be exported by modules,
197and may also be called as either class or instance methods, that is,
198as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
199C<$obj> is an instance of C<Some::Package>. Subclasses may define
200their own constants to override those in their base class.
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201
202The use of all caps for constant names is merely a convention,
203although it is recommended in order to make constants stand out
204and to help avoid collisions with other barewords, keywords, and
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205subroutine names. Constant names must begin with a letter or
206underscore. Names beginning with a double underscore are reserved. Some
207poor choices for names will generate warnings, if warnings are enabled at
208compile time.
54310121 209
a747501d 210=head2 List constants
54310121 211
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212Constants may be lists of more (or less) than one value. A constant
213with no values evaluates to C<undef> in scalar context. Note that
214constants with more than one value do I<not> return their last value in
215scalar context as one might expect. They currently return the number
216of values, but B<this may change in the future>. Do not use constants
217with multiple values in scalar context.
3cb88d13 218
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219B<NOTE:> This implies that the expression defining the value of a
220constant is evaluated in list context. This may produce surprises:
54310121 221
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222 use constant TIMESTAMP => localtime; # WRONG!
223 use constant TIMESTAMP => scalar localtime; # right
54310121 224
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225The first line above defines C<TIMESTAMP> as a 9-element list, as
226returned by localtime() in list context. To set it to the string
227returned by localtime() in scalar context, an explicit C<scalar>
228keyword is required.
54310121 229
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230List constants are lists, not arrays. To index or slice them, they
231must be placed in parentheses.
54310121 232
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233 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
234 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 235
a747501d 236=head2 Defining multiple constants at once
b0d6893f 237
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238Instead of writing multiple C<use constant> statements, you may define
239multiple constants in a single statement by giving, instead of the
240constant name, a reference to a hash where the keys are the names of
241the constants to be defined. Obviously, all constants defined using
242this method must have a single value.
243
244 use constant {
245 FOO => "A single value",
246 BAR => "This", "won't", "work!", # Error!
247 };
248
249This is a fundamental limitation of the way hashes are constructed in
250Perl. The error messages produced when this happens will often be
251quite cryptic -- in the worst case there may be none at all, and
252you'll only later find that something is broken.
253
254When defining multiple constants, you cannot use the values of other
255constants defined in the same declaration. This is because the
256calling package doesn't know about any constant within that group
257until 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
266Magical values and references can be made into constants at compile
267time, allowing for way cool stuff like this. (These error numbers
268aren't totally portable, alas.)
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269
270 use constant E2BIG => ($! = 7);
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271 print E2BIG, "\n"; # something like "Arg list too long"
272 print 0+E2BIG, "\n"; # "7"
54310121 273
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274You can't produce a tied constant by giving a tied scalar as the
275value. References to tied variables, however, can be used as
276constants without any problems.
277
a747501d 278=head1 TECHNICAL NOTES
b0d6893f 279
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280In the current implementation, scalar constants are actually
281inlinable subroutines. As of version 5.004 of Perl, the appropriate
282scalar constant is inserted directly in place of some subroutine
283calls, thereby saving the overhead of a subroutine call. See
284L<perlsub/"Constant Functions"> for details about how and when this
285happens.
3cb88d13 286
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287In the rare case in which you need to discover at run time whether a
288particular constant has been declared via this module, you may use
289this function to examine the hash C<%constant::declared>. If the given
290constant name does not include a package name, the current package is
291used.
292
293 sub declared ($) {
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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};
83763826 300 }
779c5bc9 301
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302=head1 BUGS
303
304In the current version of Perl, list constants are not inlined
305and some symbols may be redefined without generating a warning.
306
a747501d 307It is not possible to have a subroutine or a keyword with the same
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308name as a constant in the same package. This is probably a Good Thing.
309
310A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
311ENV INC SIG> is not allowed anywhere but in package C<main::>, for
312technical reasons.
313
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314Unlike constants in some languages, these cannot be overridden
315on the command line or via environment variables.
316
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317You can get into trouble if you use constants in a context which
318automatically quotes barewords (as is true for any subroutine call).
319For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
320be interpreted as a string. Use C<$hash{CONSTANT()}> or
321C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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322kicking in. Similarly, since the C<< => >> operator quotes a bareword
323immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 324(or simply use a comma in place of the big arrow) instead of
a747501d 325C<< CONSTANT => 'value' >>.
a3cb178b 326
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327=head1 AUTHOR
328
83763826 329Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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330many other folks.
331
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332Multiple constant declarations at once added by Casey West,
333E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 334
a747501d 335Documentation mostly rewritten by Ilmari Karonen,
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336E<lt>F<perl@itz.pp.sci.fi>E<gt>.
337
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338=head1 COPYRIGHT
339
83763826 340Copyright (C) 1997, 1999 Tom Phoenix
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341
342This module is free software; you can redistribute it or modify it
343under the same terms as Perl itself.
344
345=cut