lib/perl5/5.8.8/constant.pm

   1 package constant;
   2
   3 use strict;
   4 use 5.006_00;
   5 use warnings::register;
   6
   7 our($VERSION, %declared);
   8 $VERSION = '1.05';
   9
  10 #=======================================================================
  11
  12 # Some names are evil choices.
  13 my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
  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;'
  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;
  42     }
  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 '$name' is " .
  73                         "forced into package main::");
  74                 }
  75             }
  76
  77         # Looks like a boolean
  78         # use constant FRED == fred;
  79         } elsif ($name =~ /^[01]?\z/) {
  80             require Carp;
  81             if (@_) {
  82                 Carp::croak("Constant name '$name' is invalid");
  83             } else {
  84                 Carp::croak("Constant name looks like boolean value");
  85             }
  86
  87         } else {
  88            # Must have bad characters
  89             require Carp;
  90             Carp::croak("Constant name '$name' has invalid characters");
  91         }
  92
  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                     *$full_name = sub () { };
 109                 }
 110             }
 111         }
 112     }
 113 }
 114
 115 1;
 116
 117 __END__
 118
 119 =head1 NAME
 120
 121 constant - Perl pragma to declare constants
 122
 123 =head1 SYNOPSIS
 124
 125     use constant PI    => 4 * atan2(1, 1);
 126     use constant DEBUG => 0;
 127
 128     print "Pi equals ", PI, "...\n" if DEBUG;
 129
 130     use constant {
 131         SEC   => 0,
 132         MIN   => 1,
 133         HOUR  => 2,
 134         MDAY  => 3,
 135         MON   => 4,
 136         YEAR  => 5,
 137         WDAY  => 6,
 138         YDAY  => 7,
 139         ISDST => 8,
 140     };
 141
 142     use constant WEEKDAYS => qw(
 143         Sunday Monday Tuesday Wednesday Thursday Friday Saturday
 144     );
 145
 146     print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
 147
 148 =head1 DESCRIPTION
 149
 150 This will declare a symbol to be a constant with the given value.
 151
 152 When you declare a constant such as C<PI> using the method shown
 153 above, each machine your script runs upon can have as many digits
 154 of accuracy as it can use. Also, your program will be easier to
 155 read, more likely to be maintained (and maintained correctly), and
 156 far less likely to send a space probe to the wrong planet because
 157 nobody noticed the one equation in which you wrote C<3.14195>.
 158
 159 When a constant is used in an expression, perl replaces it with its
 160 value at compile time, and may then optimize the expression further.
 161 In particular, any code in an C<if (CONSTANT)> block will be optimized
 162 away if the constant is false.
 163
 164 =head1 NOTES
 165
 166 As with all C<use> directives, defining a constant happens at
 167 compile time. Thus, it's probably not correct to put a constant
 168 declaration inside of a conditional statement (like C<if ($foo)
 169 { use constant ... }>).
 170
 171 Constants defined using this module cannot be interpolated into
 172 strings like variables.  However, concatenation works just fine:
 173
 174     print "Pi equals PI...\n";        # WRONG: does not expand "PI"
 175     print "Pi equals ".PI."...\n";    # right
 176
 177 Even though a reference may be declared as a constant, the reference may
 178 point 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
 185 Dereferencing constant references incorrectly (such as using an array
 186 subscript on a constant hash reference, or vice versa) will be trapped at
 187 compile time.
 188
 189 Constants belong to the package they are defined in.  To refer to a
 190 constant defined in another package, specify the full package name, as
 191 in C<Some::Package::CONSTANT>.  Constants may be exported by modules,
 192 and may also be called as either class or instance methods, that is,
 193 as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
 194 C<$obj> is an instance of C<Some::Package>.  Subclasses may define
 195 their own constants to override those in their base class.
 196
 197 The use of all caps for constant names is merely a convention,
 198 although it is recommended in order to make constants stand out
 199 and to help avoid collisions with other barewords, keywords, and
 200 subroutine names. Constant names must begin with a letter or
 201 underscore. Names beginning with a double underscore are reserved. Some
 202 poor choices for names will generate warnings, if warnings are enabled at
 203 compile time.
 204
 205 =head2 List constants
 206
 207 Constants may be lists of more (or less) than one value.  A constant
 208 with no values evaluates to C<undef> in scalar context.  Note that
 209 constants with more than one value do I<not> return their last value in
 210 scalar context as one might expect.  They currently return the number
 211 of values, but B<this may change in the future>.  Do not use constants
 212 with multiple values in scalar context.
 213
 214 B<NOTE:> This implies that the expression defining the value of a
 215 constant is evaluated in list context.  This may produce surprises:
 216
 217     use constant TIMESTAMP => localtime;                # WRONG!
 218     use constant TIMESTAMP => scalar localtime;         # right
 219
 220 The first line above defines C<TIMESTAMP> as a 9-element list, as
 221 returned by localtime() in list context.  To set it to the string
 222 returned by localtime() in scalar context, an explicit C<scalar>
 223 keyword is required.
 224
 225 List constants are lists, not arrays.  To index or slice them, they
 226 must be placed in parentheses.
 227
 228     my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
 229     my @workdays = (WEEKDAYS)[1 .. 5];          # right
 230
 231 =head2 Defining multiple constants at once
 232
 233 Instead of writing multiple C<use constant> statements, you may define
 234 multiple constants in a single statement by giving, instead of the
 235 constant name, a reference to a hash where the keys are the names of
 236 the constants to be defined.  Obviously, all constants defined using
 237 this method must have a single value.
 238
 239     use constant {
 240         FOO => "A single value",
 241         BAR => "This", "won't", "work!",        # Error!
 242     };
 243
 244 This is a fundamental limitation of the way hashes are constructed in
 245 Perl.  The error messages produced when this happens will often be
 246 quite cryptic -- in the worst case there may be none at all, and
 247 you'll only later find that something is broken.
 248
 249 When defining multiple constants, you cannot use the values of other
 250 constants defined in the same declaration.  This is because the
 251 calling package doesn't know about any constant within that group
 252 until 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
 260
 261 Magical values and references can be made into constants at compile
 262 time, allowing for way cool stuff like this.  (These error numbers
 263 aren't totally portable, alas.)
 264
 265     use constant E2BIG => ($! = 7);
 266     print   E2BIG, "\n";        # something like "Arg list too long"
 267     print 0+E2BIG, "\n";        # "7"
 268
 269 You can't produce a tied constant by giving a tied scalar as the
 270 value.  References to tied variables, however, can be used as
 271 constants without any problems.
 272
 273 =head1 TECHNICAL NOTES
 274
 275 In the current implementation, scalar constants are actually
 276 inlinable subroutines. As of version 5.004 of Perl, the appropriate
 277 scalar constant is inserted directly in place of some subroutine
 278 calls, thereby saving the overhead of a subroutine call. See
 279 L<perlsub/"Constant Functions"> for details about how and when this
 280 happens.
 281
 282 In the rare case in which you need to discover at run time whether a
 283 particular constant has been declared via this module, you may use
 284 this function to examine the hash C<%constant::declared>. If the given
 285 constant name does not include a package name, the current package is
 286 used.
 287
 288     sub declared ($) {
 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};
 295     }
 296
 297 =head1 BUGS
 298
 299 In the current version of Perl, list constants are not inlined
 300 and some symbols may be redefined without generating a warning.
 301
 302 It is not possible to have a subroutine or a keyword with the same
 303 name as a constant in the same package. This is probably a Good Thing.
 304
 305 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
 306 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
 307 technical reasons.
 308
 309 Unlike constants in some languages, these cannot be overridden
 310 on the command line or via environment variables.
 311
 312 You can get into trouble if you use constants in a context which
 313 automatically quotes barewords (as is true for any subroutine call).
 314 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
 315 be interpreted as a string.  Use C<$hash{CONSTANT()}> or
 316 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
 317 kicking in.  Similarly, since the C<< => >> operator quotes a bareword
 318 immediately to its left, you have to say C<< CONSTANT() => 'value' >>
 319 (or simply use a comma in place of the big arrow) instead of
 320 C<< CONSTANT => 'value' >>.
 321
 322 =head1 AUTHOR
 323
 324 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
 325 many other folks.
 326
 327 Multiple constant declarations at once added by Casey West,
 328 E<lt>F<casey@geeknest.com>E<gt>.
 329
 330 Documentation mostly rewritten by Ilmari Karonen,
 331 E<lt>F<perl@itz.pp.sci.fi>E<gt>.
 332
 333 =head1 COPYRIGHT
 334
 335 Copyright (C) 1997, 1999 Tom Phoenix
 336
 337 This module is free software; you can redistribute it or modify it
 338 under the same terms as Perl itself.
 339
 340 =cut