[Summary view] [Print] [Text view]

   1  package constant;
   2  use 5.005;
   3  use strict;
   4  use warnings::register;
   5  
   6  use vars qw($VERSION %declared);
   7  $VERSION = '1.13';
   8  
   9  #=======================================================================
  10  
  11  # Some names are evil choices.
  12  my %keywords = map +($_, 1), qw{ BEGIN INIT CHECK END DESTROY AUTOLOAD };
  13  $keywords{UNITCHECK}++ if $] > 5.009;
  14  
  15  my %forced_into_main = map +($_, 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      my $pkg = caller;
  34      my $symtab;
  35      my $str_end = $] >= 5.006 ? "\\z" : "\\Z";
  36  
  37      if ($] > 5.009002) {
  38      no strict 'refs';
  39      $symtab = \%{$pkg . '::'};
  40      };
  41  
  42      if ( $multiple ) {
  43      if (ref $_[0] ne 'HASH') {
  44          require Carp;
  45          Carp::croak("Invalid reference type '".ref(shift)."' not 'HASH'");
  46      }
  47      $constants = shift;
  48      } else {
  49      $constants->{+shift} = undef;
  50      }
  51  
  52      foreach my $name ( keys %$constants ) {
  53      unless (defined $name) {
  54          require Carp;
  55          Carp::croak("Can't use undef as constant name");
  56      }
  57  
  58      # Normal constant name
  59      if ($name =~ /^_?[^\W_0-9]\w*$str_end/ and !$forbidden{$name}) {
  60          # Everything is okay
  61  
  62      # Name forced into main, but we're not in main. Fatal.
  63      } elsif ($forced_into_main{$name} and $pkg ne 'main') {
  64          require Carp;
  65          Carp::croak("Constant name '$name' is forced into main::");
  66  
  67      # Starts with double underscore. Fatal.
  68      } elsif ($name =~ /^__/) {
  69          require Carp;
  70          Carp::croak("Constant name '$name' begins with '__'");
  71  
  72      # Maybe the name is tolerable
  73      } elsif ($name =~ /^[A-Za-z_]\w*$str_end/) {
  74          # Then we'll warn only if you've asked for warnings
  75          if (warnings::enabled()) {
  76          if ($keywords{$name}) {
  77              warnings::warn("Constant name '$name' is a Perl keyword");
  78          } elsif ($forced_into_main{$name}) {
  79              warnings::warn("Constant name '$name' is " .
  80              "forced into package main::");
  81          }
  82          }
  83  
  84      # Looks like a boolean
  85      # use constant FRED == fred;
  86      } elsif ($name =~ /^[01]?$str_end/) {
  87              require Carp;
  88          if (@_) {
  89          Carp::croak("Constant name '$name' is invalid");
  90          } else {
  91          Carp::croak("Constant name looks like boolean value");
  92          }
  93  
  94      } else {
  95         # Must have bad characters
  96              require Carp;
  97          Carp::croak("Constant name '$name' has invalid characters");
  98      }
  99  
 100      {
 101          no strict 'refs';
 102          my $full_name = "$pkg}::$name";
 103          $declared{$full_name}++;
 104          if ($multiple || @_ == 1) {
 105          my $scalar = $multiple ? $constants->{$name} : $_[0];
 106          if ($symtab && !exists $symtab->{$name}) {
 107              # No typeglob yet, so we can use a reference as space-
 108              # efficient proxy for a constant subroutine
 109              # The check in Perl_ck_rvconst knows that inlinable
 110              # constants from cv_const_sv are read only. So we have to:
 111              Internals::SvREADONLY($scalar, 1);
 112              $symtab->{$name} = \$scalar;
 113              mro::method_changed_in($pkg);
 114          } else {
 115              *$full_name = sub () { $scalar };
 116          }
 117          } elsif (@_) {
 118          my @list = @_;
 119          *$full_name = sub () { @list };
 120          } else {
 121          *$full_name = sub () { };
 122          }
 123      }
 124      }
 125  }
 126  
 127  1;
 128  
 129  __END__
 130  
 131  =head1 NAME
 132  
 133  constant - Perl pragma to declare constants
 134  
 135  =head1 SYNOPSIS
 136  
 137      use constant PI    => 4 * atan2(1, 1);
 138      use constant DEBUG => 0;
 139  
 140      print "Pi equals ", PI, "...\n" if DEBUG;
 141  
 142      use constant {
 143          SEC   => 0,
 144          MIN   => 1,
 145          HOUR  => 2,
 146          MDAY  => 3,
 147          MON   => 4,
 148          YEAR  => 5,
 149          WDAY  => 6,
 150          YDAY  => 7,
 151          ISDST => 8,
 152      };
 153  
 154      use constant WEEKDAYS => qw(
 155          Sunday Monday Tuesday Wednesday Thursday Friday Saturday
 156      );
 157  
 158      print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";
 159  
 160  =head1 DESCRIPTION
 161  
 162  This pragma allows you to declare constants at compile-time.
 163  
 164  When you declare a constant such as C<PI> using the method shown
 165  above, each machine your script runs upon can have as many digits
 166  of accuracy as it can use. Also, your program will be easier to
 167  read, more likely to be maintained (and maintained correctly), and
 168  far less likely to send a space probe to the wrong planet because
 169  nobody noticed the one equation in which you wrote C<3.14195>.
 170  
 171  When a constant is used in an expression, perl replaces it with its
 172  value at compile time, and may then optimize the expression further.
 173  In particular, any code in an C<if (CONSTANT)> block will be optimized
 174  away if the constant is false.
 175  
 176  =head1 NOTES
 177  
 178  As with all C<use> directives, defining a constant happens at
 179  compile time. Thus, it's probably not correct to put a constant
 180  declaration inside of a conditional statement (like C<if ($foo)
 181  { use constant ... }>).
 182  
 183  Constants defined using this module cannot be interpolated into
 184  strings like variables.  However, concatenation works just fine:
 185  
 186      print "Pi equals PI...\n";        # WRONG: does not expand "PI"
 187      print "Pi equals ".PI."...\n";    # right
 188  
 189  Even though a reference may be declared as a constant, the reference may
 190  point to data which may be changed, as this code shows.
 191  
 192      use constant ARRAY => [ 1,2,3,4 ];
 193      print ARRAY->[1];
 194      ARRAY->[1] = " be changed";
 195      print ARRAY->[1];
 196  
 197  Dereferencing constant references incorrectly (such as using an array
 198  subscript on a constant hash reference, or vice versa) will be trapped at
 199  compile time.
 200  
 201  Constants belong to the package they are defined in.  To refer to a
 202  constant defined in another package, specify the full package name, as
 203  in C<Some::Package::CONSTANT>.  Constants may be exported by modules,
 204  and may also be called as either class or instance methods, that is,
 205  as C<< Some::Package->CONSTANT >> or as C<< $obj->CONSTANT >> where
 206  C<$obj> is an instance of C<Some::Package>.  Subclasses may define
 207  their own constants to override those in their base class.
 208  
 209  The use of all caps for constant names is merely a convention,
 210  although it is recommended in order to make constants stand out
 211  and to help avoid collisions with other barewords, keywords, and
 212  subroutine names. Constant names must begin with a letter or
 213  underscore. Names beginning with a double underscore are reserved. Some
 214  poor choices for names will generate warnings, if warnings are enabled at
 215  compile time.
 216  
 217  =head2 List constants
 218  
 219  Constants may be lists of more (or less) than one value.  A constant
 220  with no values evaluates to C<undef> in scalar context.  Note that
 221  constants with more than one value do I<not> return their last value in
 222  scalar context as one might expect.  They currently return the number
 223  of values, but B<this may change in the future>.  Do not use constants
 224  with multiple values in scalar context.
 225  
 226  B<NOTE:> This implies that the expression defining the value of a
 227  constant is evaluated in list context.  This may produce surprises:
 228  
 229      use constant TIMESTAMP => localtime;                # WRONG!
 230      use constant TIMESTAMP => scalar localtime;         # right
 231  
 232  The first line above defines C<TIMESTAMP> as a 9-element list, as
 233  returned by C<localtime()> in list context.  To set it to the string
 234  returned by C<localtime()> in scalar context, an explicit C<scalar>
 235  keyword is required.
 236  
 237  List constants are lists, not arrays.  To index or slice them, they
 238  must be placed in parentheses.
 239  
 240      my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
 241      my @workdays = (WEEKDAYS)[1 .. 5];          # right
 242  
 243  =head2 Defining multiple constants at once
 244  
 245  Instead of writing multiple C<use constant> statements, you may define
 246  multiple constants in a single statement by giving, instead of the
 247  constant name, a reference to a hash where the keys are the names of
 248  the constants to be defined.  Obviously, all constants defined using
 249  this method must have a single value.
 250  
 251      use constant {
 252          FOO => "A single value",
 253          BAR => "This", "won't", "work!",        # Error!
 254      };
 255  
 256  This is a fundamental limitation of the way hashes are constructed in
 257  Perl.  The error messages produced when this happens will often be
 258  quite cryptic -- in the worst case there may be none at all, and
 259  you'll only later find that something is broken.
 260  
 261  When defining multiple constants, you cannot use the values of other
 262  constants defined in the same declaration.  This is because the
 263  calling package doesn't know about any constant within that group
 264  until I<after> the C<use> statement is finished.
 265  
 266      use constant {
 267          BITMASK => 0xAFBAEBA8,
 268          NEGMASK => ~BITMASK,                    # Error!
 269      };
 270  
 271  =head2 Magic constants
 272  
 273  Magical values and references can be made into constants at compile
 274  time, allowing for way cool stuff like this.  (These error numbers
 275  aren't totally portable, alas.)
 276  
 277      use constant E2BIG => ($! = 7);
 278      print   E2BIG, "\n";        # something like "Arg list too long"
 279      print 0+E2BIG, "\n";        # "7"
 280  
 281  You can't produce a tied constant by giving a tied scalar as the
 282  value.  References to tied variables, however, can be used as
 283  constants without any problems.
 284  
 285  =head1 TECHNICAL NOTES
 286  
 287  In the current implementation, scalar constants are actually
 288  inlinable subroutines. As of version 5.004 of Perl, the appropriate
 289  scalar constant is inserted directly in place of some subroutine
 290  calls, thereby saving the overhead of a subroutine call. See
 291  L<perlsub/"Constant Functions"> for details about how and when this
 292  happens.
 293  
 294  In the rare case in which you need to discover at run time whether a
 295  particular constant has been declared via this module, you may use
 296  this function to examine the hash C<%constant::declared>. If the given
 297  constant name does not include a package name, the current package is
 298  used.
 299  
 300      sub declared ($) {
 301          use constant 1.01;              # don't omit this!
 302          my $name = shift;
 303          $name =~ s/^::/main::/;
 304          my $pkg = caller;
 305          my $full_name = $name =~ /::/ ? $name : "$pkg}::$name";
 306          $constant::declared{$full_name};
 307      }
 308  
 309  =head1 CAVEATS
 310  
 311  In the current version of Perl, list constants are not inlined
 312  and some symbols may be redefined without generating a warning.
 313  
 314  It is not possible to have a subroutine or a keyword with the same
 315  name as a constant in the same package. This is probably a Good Thing.
 316  
 317  A constant with a name in the list C<STDIN STDOUT STDERR ARGV ARGVOUT
 318  ENV INC SIG> is not allowed anywhere but in package C<main::>, for
 319  technical reasons. 
 320  
 321  Unlike constants in some languages, these cannot be overridden
 322  on the command line or via environment variables.
 323  
 324  You can get into trouble if you use constants in a context which
 325  automatically quotes barewords (as is true for any subroutine call).
 326  For example, you can't say C<$hash{CONSTANT}> because C<CONSTANT> will
 327  be interpreted as a string.  Use C<$hash{CONSTANT()}> or
 328  C<$hash{+CONSTANT}> to prevent the bareword quoting mechanism from
 329  kicking in.  Similarly, since the C<< => >> operator quotes a bareword
 330  immediately to its left, you have to say C<< CONSTANT() => 'value' >>
 331  (or simply use a comma in place of the big arrow) instead of
 332  C<< CONSTANT => 'value' >>.
 333  
 334  =head1 BUGS
 335  
 336  Please report any bugs or feature requests via the perlbug(1) utility.
 337  
 338  =head1 AUTHORS
 339  
 340  Tom Phoenix, E<lt>F<rootbeer@redcat.com>E<gt>, with help from
 341  many other folks.
 342  
 343  Multiple constant declarations at once added by Casey West,
 344  E<lt>F<casey@geeknest.com>E<gt>.
 345  
 346  Documentation mostly rewritten by Ilmari Karonen,
 347  E<lt>F<perl@itz.pp.sci.fi>E<gt>.
 348  
 349  This program is maintained by the Perl 5 Porters. 
 350  The CPAN distribution is maintained by SE<eacute>bastien Aperghis-Tramoni
 351  E<lt>F<sebastien@aperghis.net>E<gt>.
 352  
 353  =head1 COPYRIGHT
 354  
 355  Copyright (C) 1997, 1999 Tom Phoenix
 356  
 357  This module is free software; you can redistribute it or modify it
 358  under the same terms as Perl itself.
 359  
 360  =cut