1package constant; 2 3use strict; 4use 5.006_00; 5use warnings::register; 6 7our($VERSION, %declared); 8$VERSION = '1.05'; 9 10#======================================================================= 11 12# Some names are evil choices. 13my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD }; 14 15my %forced_into_main = map +($_, 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;' 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 *$full_name = sub () { }; 109 } 110 } 111 } 112 } 113} 114 1151; 116 117__END__ 118 119=head1 NAME 120 121constant - Perl pragma to declare constants 122 123=head1 SYNOPSIS 124 125 use constant PI => 4 * atan2(1, 1); 126 use constant DEBUG => 0; 127 128 print "Pi equals ", PI, "...\n" if DEBUG; 129 130 use constant { 131 SEC => 0, 132 MIN => 1, 133 HOUR => 2, 134 MDAY => 3, 135 MON => 4, 136 YEAR => 5, 137 WDAY => 6, 138 YDAY => 7, 139 ISDST => 8, 140 }; 141 142 use constant WEEKDAYS => qw( 143 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 144 ); 145 146 print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n"; 147 148=head1 DESCRIPTION 149 150This will declare a symbol to be a constant with the given value. 151 152When you declare a constant such as C<PI> using the method shown 153above, each machine your script runs upon can have as many digits 154of accuracy as it can use. Also, your program will be easier to 155read, more likely to be maintained (and maintained correctly), and 156far less likely to send a space probe to the wrong planet because 157nobody noticed the one equation in which you wrote C<3.14195>. 158 159When a constant is used in an expression, perl replaces it with its 160value at compile time, and may then optimize the expression further. 161In particular, any code in an C<if (CONSTANT)> block will be optimized 162away if the constant is false. 163 164=head1 NOTES 165 166As with all C<use> directives, defining a constant happens at 167compile time. Thus, it's probably not correct to put a constant 168declaration inside of a conditional statement (like C<if ($foo) 169{ use constant ... }>). 170 171Constants defined using this module cannot be interpolated into 172strings 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 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. 188 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. 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 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. 204 205=head2 List constants 206 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. 213 214B<NOTE:> This implies that the expression defining the value of a 215constant is evaluated in list context. This may produce surprises: 216 217 use constant TIMESTAMP => localtime; # WRONG! 218 use constant TIMESTAMP => scalar localtime; # right 219 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. 224 225List constants are lists, not arrays. To index or slice them, they 226must 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 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 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.) 264 265 use constant E2BIG => ($! = 7); 266 print E2BIG, "\n"; # something like "Arg list too long" 267 print 0+E2BIG, "\n"; # "7" 268 269You can't produce a tied constant by giving a tied scalar as the 270value. References to tied variables, however, can be used as 271constants without any problems. 272 273=head1 TECHNICAL NOTES 274 275In the current implementation, scalar constants are actually 276inlinable subroutines. As of version 5.004 of Perl, the appropriate 277scalar constant is inserted directly in place of some subroutine 278calls, thereby saving the overhead of a subroutine call. See 279L<perlsub/"Constant Functions"> for details about how and when this 280happens. 281 282In the rare case in which you need to discover at run time whether a 283particular constant has been declared via this module, you may use 284this function to examine the hash C<%constant::declared>. If the given 285constant name does not include a package name, the current package is 286used. 287 288 sub declared ($) { 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 299In the current version of Perl, list constants are not inlined 300and some symbols may be redefined without generating a warning. 301 302It is not possible to have a subroutine or a keyword with the same 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 309Unlike constants in some languages, these cannot be overridden 310on the command line or via environment variables. 311 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 317kicking in. Similarly, since the C<< => >> operator quotes a bareword 318immediately to its left, you have to say C<< CONSTANT() => 'value' >> 319(or simply use a comma in place of the big arrow) instead of 320C<< CONSTANT => 'value' >>. 321 322=head1 AUTHOR 323 324Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from 325many other folks. 326 327Multiple constant declarations at once added by Casey West, 328E<lt>F<casey@geeknest.com>E<gt>. 329 330Documentation mostly rewritten by Ilmari Karonen, 331E<lt>F<perl@itz.pp.sci.fi>E<gt>. 332 333=head1 COPYRIGHT 334 335Copyright (C) 1997, 1999 Tom Phoenix 336 337This module is free software; you can redistribute it or modify it 338under the same terms as Perl itself. 339 340=cut 341