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[perl #32400] Unnecessary warning from constant.pm
[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($VERSION, %declared);
5b673cda 8$VERSION = '1.05';
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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 '$name' is " .
73 "forced into package main::");
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74 }
75 }
76
77 # Looks like a boolean
78 # use constant FRED == fred;
79 } elsif ($name =~ /^[01]?\z/) {
80 require Carp;
81 if (@_) {
82 Carp::croak("Constant name '$name' is invalid");
83763826 83 } else {
3cb88d13 84 Carp::croak("Constant name looks like boolean value");
83763826 85 }
83763826 86
83763826 87 } else {
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88 # Must have bad characters
89 require Carp;
90 Carp::croak("Constant name '$name' has invalid characters");
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91 }
92
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93 {
94 no strict 'refs';
95 my $full_name = "${pkg}::$name";
96 $declared{$full_name}++;
97 if ($multiple) {
98 my $scalar = $constants{$name};
99 *$full_name = sub () { $scalar };
100 } else {
101 if (@_ == 1) {
102 my $scalar = $_[0];
103 *$full_name = sub () { $scalar };
104 } elsif (@_) {
105 my @list = @_;
106 *$full_name = sub () { @list };
107 } else {
108 *$full_name = sub () { };
109 }
110 }
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111 }
112 }
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113}
114
1151;
116
117__END__
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118
119=head1 NAME
120
121constant - Perl pragma to declare constants
122
123=head1 SYNOPSIS
124
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125 use constant PI => 4 * atan2(1, 1);
126 use constant DEBUG => 0;
127
128 print "Pi equals ", PI, "...\n" if DEBUG;
129
3cb88d13 130 use constant {
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131 SEC => 0,
132 MIN => 1,
133 HOUR => 2,
134 MDAY => 3,
135 MON => 4,
136 YEAR => 5,
137 WDAY => 6,
138 YDAY => 7,
139 ISDST => 8,
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140 };
141
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142 use constant WEEKDAYS => qw(
143 Sunday Monday Tuesday Wednesday Thursday Friday Saturday
144 );
145
146 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
147
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148=head1 DESCRIPTION
149
a747501d 150This will declare a symbol to be a constant with the given value.
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151
152When you declare a constant such as C<PI> using the method shown
153above, each machine your script runs upon can have as many digits
154of accuracy as it can use. Also, your program will be easier to
155read, more likely to be maintained (and maintained correctly), and
156far less likely to send a space probe to the wrong planet because
157nobody noticed the one equation in which you wrote C<3.14195>.
158
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159When a constant is used in an expression, perl replaces it with its
160value at compile time, and may then optimize the expression further.
161In particular, any code in an C<if (CONSTANT)> block will be optimized
162away if the constant is false.
163
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164=head1 NOTES
165
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166As with all C<use> directives, defining a constant happens at
167compile time. Thus, it's probably not correct to put a constant
168declaration inside of a conditional statement (like C<if ($foo)
169{ use constant ... }>).
54310121 170
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171Constants defined using this module cannot be interpolated into
172strings like variables. However, concatenation works just fine:
54310121 173
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174 print "Pi equals PI...\n"; # WRONG: does not expand "PI"
175 print "Pi equals ".PI."...\n"; # right
54310121 176
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177Even though a reference may be declared as a constant, the reference may
178point to data which may be changed, as this code shows.
179
180 use constant ARRAY => [ 1,2,3,4 ];
181 print ARRAY->[1];
182 ARRAY->[1] = " be changed";
183 print ARRAY->[1];
184
185Dereferencing constant references incorrectly (such as using an array
186subscript on a constant hash reference, or vice versa) will be trapped at
187compile time.
54310121 188
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189Constants belong to the package they are defined in. To refer to a
190constant defined in another package, specify the full package name, as
191in C<Some::Package::CONSTANT>. Constants may be exported by modules,
192and may also be called as either class or instance methods, that is,
193as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
194C<$obj> is an instance of C<Some::Package>. Subclasses may define
195their own constants to override those in their base class.
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196
197The use of all caps for constant names is merely a convention,
198although it is recommended in order to make constants stand out
199and to help avoid collisions with other barewords, keywords, and
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200subroutine names. Constant names must begin with a letter or
201underscore. Names beginning with a double underscore are reserved. Some
202poor choices for names will generate warnings, if warnings are enabled at
203compile time.
54310121 204
a747501d 205=head2 List constants
54310121 206
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207Constants may be lists of more (or less) than one value. A constant
208with no values evaluates to C<undef> in scalar context. Note that
209constants with more than one value do I<not> return their last value in
210scalar context as one might expect. They currently return the number
211of values, but B<this may change in the future>. Do not use constants
212with multiple values in scalar context.
3cb88d13 213
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214B<NOTE:> This implies that the expression defining the value of a
215constant is evaluated in list context. This may produce surprises:
54310121 216
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217 use constant TIMESTAMP => localtime; # WRONG!
218 use constant TIMESTAMP => scalar localtime; # right
54310121 219
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220The first line above defines C<TIMESTAMP> as a 9-element list, as
221returned by localtime() in list context. To set it to the string
222returned by localtime() in scalar context, an explicit C<scalar>
223keyword is required.
54310121 224
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225List constants are lists, not arrays. To index or slice them, they
226must be placed in parentheses.
54310121 227
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228 my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
229 my @workdays = (WEEKDAYS)[1 .. 5]; # right
b0d6893f 230
a747501d 231=head2 Defining multiple constants at once
b0d6893f 232
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233Instead of writing multiple C<use constant> statements, you may define
234multiple constants in a single statement by giving, instead of the
235constant name, a reference to a hash where the keys are the names of
236the constants to be defined. Obviously, all constants defined using
237this method must have a single value.
238
239 use constant {
240 FOO => "A single value",
241 BAR => "This", "won't", "work!", # Error!
242 };
243
244This is a fundamental limitation of the way hashes are constructed in
245Perl. The error messages produced when this happens will often be
246quite cryptic -- in the worst case there may be none at all, and
247you'll only later find that something is broken.
248
249When defining multiple constants, you cannot use the values of other
250constants defined in the same declaration. This is because the
251calling package doesn't know about any constant within that group
252until I<after> the C<use> statement is finished.
253
254 use constant {
255 BITMASK => 0xAFBAEBA8,
256 NEGMASK => ~BITMASK, # Error!
257 };
258
259=head2 Magic constants
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260
261Magical values and references can be made into constants at compile
262time, allowing for way cool stuff like this. (These error numbers
263aren't totally portable, alas.)
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264
265 use constant E2BIG => ($! = 7);
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266 print E2BIG, "\n"; # something like "Arg list too long"
267 print 0+E2BIG, "\n"; # "7"
54310121 268
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269You can't produce a tied constant by giving a tied scalar as the
270value. References to tied variables, however, can be used as
271constants without any problems.
272
a747501d 273=head1 TECHNICAL NOTES
b0d6893f 274
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275In the current implementation, scalar constants are actually
276inlinable subroutines. As of version 5.004 of Perl, the appropriate
277scalar constant is inserted directly in place of some subroutine
278calls, thereby saving the overhead of a subroutine call. See
279L<perlsub/"Constant Functions"> for details about how and when this
280happens.
3cb88d13 281
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282In the rare case in which you need to discover at run time whether a
283particular constant has been declared via this module, you may use
284this function to examine the hash C<%constant::declared>. If the given
285constant name does not include a package name, the current package is
286used.
287
288 sub declared ($) {
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289 use constant 1.01; # don't omit this!
290 my $name = shift;
291 $name =~ s/^::/main::/;
292 my $pkg = caller;
293 my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
294 $constant::declared{$full_name};
83763826 295 }
779c5bc9 296
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297=head1 BUGS
298
299In the current version of Perl, list constants are not inlined
300and some symbols may be redefined without generating a warning.
301
a747501d 302It is not possible to have a subroutine or a keyword with the same
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303name as a constant in the same package. This is probably a Good Thing.
304
305A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
306ENV INC SIG> is not allowed anywhere but in package C<main::>, for
307technical reasons.
308
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309Unlike constants in some languages, these cannot be overridden
310on the command line or via environment variables.
311
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312You can get into trouble if you use constants in a context which
313automatically quotes barewords (as is true for any subroutine call).
314For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
315be interpreted as a string. Use C<$hash{CONSTANT()}> or
316C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
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317kicking in. Similarly, since the C<< => >> operator quotes a bareword
318immediately to its left, you have to say C<< CONSTANT() => 'value' >>
83763826 319(or simply use a comma in place of the big arrow) instead of
a747501d 320C<< CONSTANT => 'value' >>.
a3cb178b 321
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322=head1 AUTHOR
323
83763826 324Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
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325many other folks.
326
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327Multiple constant declarations at once added by Casey West,
328E<lt>F<casey@geeknest.com>E<gt>.
3cb88d13 329
a747501d 330Documentation mostly rewritten by Ilmari Karonen,
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331E<lt>F<perl@itz.pp.sci.fi>E<gt>.
332
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333=head1 COPYRIGHT
334
83763826 335Copyright (C) 1997, 1999 Tom Phoenix
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336
337This module is free software; you can redistribute it or modify it
338under the same terms as Perl itself.
339
340=cut