lib/perl5/5.6.1/constant.pm

   1 package constant;
   2
   3 use strict;
   4 use 5.005_64;
   5 use warnings::register;
   6
   7 our($VERSION, %declared);
   8 $VERSION = '1.02';
   9
  10 #=======================================================================
  11
  12 # Some names are evil choices.
  13 my %keywords = map +($_, 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 $name = shift;
  32     unless (defined $name) {
  33         require Carp;
  34         Carp::croak("Can't use undef as constant name");
  35     }
  36     my $pkg = caller;
  37
  38     # Normal constant name
  39     if ($name =~ /^_?[^\W_0-9]\w*\z/ and !$forbidden{$name}) {
  40         # Everything is okay
  41
  42     # Name forced into main, but we're not in main. Fatal.
  43     } elsif ($forced_into_main{$name} and $pkg ne 'main') {
  44         require Carp;
  45         Carp::croak("Constant name '$name' is forced into main::");
  46
  47     # Starts with double underscore. Fatal.
  48     } elsif ($name =~ /^__/) {
  49         require Carp;
  50         Carp::croak("Constant name '$name' begins with '__'");
  51
  52     # Maybe the name is tolerable
  53     } elsif ($name =~ /^[A-Za-z_]\w*\z/) {
  54         # Then we'll warn only if you've asked for warnings
  55         if (warnings::enabled()) {
  56             if ($keywords{$name}) {
  57                 warnings::warn("Constant name '$name' is a Perl keyword");
  58             } elsif ($forced_into_main{$name}) {
  59                 warnings::warn("Constant name '$name' is " .
  60                     "forced into package main::");
  61             } else {
  62                 # Catch-all - what did I miss? If you get this error,
  63                 # please let me know what your constant's name was.
  64                 # Write to <rootbeer@redcat.com>. Thanks!
  65                 warnings::warn("Constant name '$name' has unknown problems");
  66             }
  67         }
  68
  69     # Looks like a boolean
  70     #           use constant FRED == fred;
  71     } elsif ($name =~ /^[01]?\z/) {
  72         require Carp;
  73         if (@_) {
  74             Carp::croak("Constant name '$name' is invalid");
  75         } else {
  76             Carp::croak("Constant name looks like boolean value");
  77         }
  78
  79     } else {
  80         # Must have bad characters
  81         require Carp;
  82         Carp::croak("Constant name '$name' has invalid characters");
  83     }
  84
  85     {
  86         no strict 'refs';
  87         my $full_name = "${pkg}::$name";
  88         $declared{$full_name}++;
  89         if (@_ == 1) {
  90             my $scalar = $_[0];
  91             *$full_name = sub () { $scalar };
  92         } elsif (@_) {
  93             my @list = @_;
  94             *$full_name = sub () { @list };
  95         } else {
  96             *$full_name = sub () { };
  97         }
  98     }
  99
 100 }
 101
 102 1;
 103
 104 __END__
 105
 106 =head1 NAME
 107
 108 constant - Perl pragma to declare constants
 109
 110 =head1 SYNOPSIS
 111
 112     use constant BUFFER_SIZE    => 4096;
 113     use constant ONE_YEAR       => 365.2425 * 24 * 60 * 60;
 114     use constant PI             => 4 * atan2 1, 1;
 115     use constant DEBUGGING      => 0;
 116     use constant ORACLE         => 'oracle@cs.indiana.edu';
 117     use constant USERNAME       => scalar getpwuid($<);
 118     use constant USERINFO       => getpwuid($<);
 119
 120     sub deg2rad { PI * $_[0] / 180 }
 121
 122     print "This line does nothing"              unless DEBUGGING;
 123
 124     # references can be constants
 125     use constant CHASH          => { foo => 42 };
 126     use constant CARRAY         => [ 1,2,3,4 ];
 127     use constant CPSEUDOHASH    => [ { foo => 1}, 42 ];
 128     use constant CCODE          => sub { "bite $_[0]\n" };
 129
 130     print CHASH->{foo};
 131     print CARRAY->[$i];
 132     print CPSEUDOHASH->{foo};
 133     print CCODE->("me");
 134     print CHASH->[10];                  # compile-time error
 135
 136 =head1 DESCRIPTION
 137
 138 This will declare a symbol to be a constant with the given scalar
 139 or list value.
 140
 141 When you declare a constant such as C<PI> using the method shown
 142 above, each machine your script runs upon can have as many digits
 143 of accuracy as it can use. Also, your program will be easier to
 144 read, more likely to be maintained (and maintained correctly), and
 145 far less likely to send a space probe to the wrong planet because
 146 nobody noticed the one equation in which you wrote C<3.14195>.
 147
 148 =head1 NOTES
 149
 150 The value or values are evaluated in a list context. You may override
 151 this with C<scalar> as shown above.
 152
 153 These constants do not directly interpolate into double-quotish
 154 strings, although you may do so indirectly. (See L<perlref> for
 155 details about how this works.)
 156
 157     print "The value of PI is @{[ PI ]}.\n";
 158
 159 List constants are returned as lists, not as arrays.
 160
 161     $homedir = USERINFO[7];             # WRONG
 162     $homedir = (USERINFO)[7];           # Right
 163
 164 The use of all caps for constant names is merely a convention,
 165 although it is recommended in order to make constants stand out
 166 and to help avoid collisions with other barewords, keywords, and
 167 subroutine names. Constant names must begin with a letter or
 168 underscore. Names beginning with a double underscore are reserved. Some
 169 poor choices for names will generate warnings, if warnings are enabled at
 170 compile time.
 171
 172 Constant symbols are package scoped (rather than block scoped, as
 173 C<use strict> is). That is, you can refer to a constant from package
 174 Other as C<Other::CONST>.
 175
 176 As with all C<use> directives, defining a constant happens at
 177 compile time. Thus, it's probably not correct to put a constant
 178 declaration inside of a conditional statement (like C<if ($foo)
 179 { use constant ... }>).
 180
 181 Omitting the value for a symbol gives it the value of C<undef> in
 182 a scalar context or the empty list, C<()>, in a list context. This
 183 isn't so nice as it may sound, though, because in this case you
 184 must either quote the symbol name, or use a big arrow, (C<=E<gt>>),
 185 with nothing to point to. It is probably best to declare these
 186 explicitly.
 187
 188     use constant UNICORNS       => ();
 189     use constant LOGFILE        => undef;
 190
 191 The result from evaluating a list constant in a scalar context is
 192 not documented, and is B<not> guaranteed to be any particular value
 193 in the future. In particular, you should not rely upon it being
 194 the number of elements in the list, especially since it is not
 195 B<necessarily> that value in the current implementation.
 196
 197 Magical values, tied values, and references can be made into
 198 constants at compile time, allowing for way cool stuff like this.
 199 (These error numbers aren't totally portable, alas.)
 200
 201     use constant E2BIG => ($! = 7);
 202     print   E2BIG, "\n";        # something like "Arg list too long"
 203     print 0+E2BIG, "\n";        # "7"
 204
 205 Dereferencing constant references incorrectly (such as using an array
 206 subscript on a constant hash reference, or vice versa) will be trapped at
 207 compile time.
 208
 209 In the rare case in which you need to discover at run time whether a
 210 particular constant has been declared via this module, you may use
 211 this function to examine the hash C<%constant::declared>. If the given
 212 constant name does not include a package name, the current package is
 213 used.
 214
 215     sub declared ($) {
 216         use constant 1.01;              # don't omit this!
 217         my $name = shift;
 218         $name =~ s/^::/main::/;
 219         my $pkg = caller;
 220         my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
 221         $constant::declared{$full_name};
 222     }
 223
 224 =head1 TECHNICAL NOTE
 225
 226 In the current implementation, scalar constants are actually
 227 inlinable subroutines. As of version 5.004 of Perl, the appropriate
 228 scalar constant is inserted directly in place of some subroutine
 229 calls, thereby saving the overhead of a subroutine call. See
 230 L<perlsub/"Constant Functions"> for details about how and when this
 231 happens.
 232
 233 =head1 BUGS
 234
 235 In the current version of Perl, list constants are not inlined
 236 and some symbols may be redefined without generating a warning.
 237
 238 It is not possible to have a subroutine or keyword with the same
 239 name as a constant in the same package. This is probably a Good Thing.
 240
 241 A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
 242 ENV INC SIG> is not allowed anywhere but in package C<main::>, for
 243 technical reasons.
 244
 245 Even though a reference may be declared as a constant, the reference may
 246 point to data which may be changed, as this code shows.
 247
 248     use constant CARRAY         => [ 1,2,3,4 ];
 249     print CARRAY->[1];
 250     CARRAY->[1] = " be changed";
 251     print CARRAY->[1];
 252
 253 Unlike constants in some languages, these cannot be overridden
 254 on the command line or via environment variables.
 255
 256 You can get into trouble if you use constants in a context which
 257 automatically quotes barewords (as is true for any subroutine call).
 258 For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
 259 be interpreted as a string.  Use C<$hash{CONSTANT()}> or
 260 C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
 261 kicking in.  Similarly, since the C<=E<gt>> operator quotes a bareword
 262 immediately to its left, you have to say C<CONSTANT() =E<gt> 'value'>
 263 (or simply use a comma in place of the big arrow) instead of
 264 C<CONSTANT =E<gt> 'value'>.
 265
 266 =head1 AUTHOR
 267
 268 Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
 269 many other folks.
 270
 271 =head1 COPYRIGHT
 272
 273 Copyright (C) 1997, 1999 Tom Phoenix
 274
 275 This module is free software; you can redistribute it or modify it
 276 under the same terms as Perl itself.
 277
 278 =cut