NAME

       constant - Perl pragma to declare constants

SYNOPSIS

           use constant PI    => 4 * atan2(1, 1);
           use constant DEBUG => 0;

           print "Pi equals ", PI, "...\n" if DEBUG;

           use constant {
               SEC   => 0,
               MIN   => 1,
               HOUR  => 2,
               MDAY  => 3,
               MON   => 4,
               YEAR  => 5,
               WDAY  => 6,
               YDAY  => 7,
               ISDST => 8,
           };

           use constant WEEKDAYS => qw(
               Sunday Monday Tuesday Wednesday Thursday Friday Saturday
           );

           print "Today is ", (WEEKDAYS)[ (localtime)[WDAY] ], ".\n";

DESCRIPTION

       This pragma allows you to declare constants at compile-time.

       When you declare a constant such as "PI" using the method shown above, each machine your
       script runs upon can have as many digits of accuracy as it can use.  Also, your program
       will be easier to read, more likely to be maintained (and maintained correctly), and far
       less likely to send a space probe to the wrong planet because nobody noticed the one
       equation in which you wrote 3.14195.

       When a constant is used in an expression, Perl replaces it with its value at compile time,
       and may then optimize the expression further.  In particular, any code in an "if
       (CONSTANT)" block will be optimized away if the constant is false.

NOTES

       As with all "use" directives, defining a constant happens at compile time.  Thus, it's
       probably not correct to put a constant declaration inside of a conditional statement (like
       "if ($foo) { use constant ... }").

       Constants defined using this module cannot be interpolated into strings like variables.
       However, concatenation works just fine:

           print "Pi equals PI...\n";        # WRONG: does not expand "PI"
           print "Pi equals ".PI."...\n";    # right

       Even though a reference may be declared as a constant, the reference may point to data
       which may be changed, as this code shows.

           use constant ARRAY => [ 1,2,3,4 ];
           print ARRAY->[1];
           ARRAY->[1] = " be changed";
           print ARRAY->[1];

       Constants belong to the package they are defined in.  To refer to a constant defined in
       another package, specify the full package name, as in "Some::Package::CONSTANT".
       Constants may be exported by modules, and may also be called as either class or instance
       methods, that is, as "Some::Package->CONSTANT" or as "$obj->CONSTANT" where $obj is an
       instance of "Some::Package".  Subclasses may define their own constants to override those
       in their base class.

       As of version 1.32 of this module, constants can be defined in packages other than the
       caller, by including the package name in the name of the constant:

           use constant "OtherPackage::FWIBBLE" => 7865;
           constant->import("Other::FWOBBLE",$value); # dynamically at run time

       The use of all caps for constant names is merely a convention, although it is recommended
       in order to make constants stand out and to help avoid collisions with other barewords,
       keywords, and subroutine names.  Constant names must begin with a letter or underscore.
       Names beginning with a double underscore are reserved.  Some poor choices for names will
       generate warnings, if warnings are enabled at compile time.

   List constants
       Constants may be lists of more (or less) than one value.  A constant with no values
       evaluates to "undef" in scalar context.  Note that constants with more than one value do
       not return their last value in scalar context as one might expect.  They currently return
       the number of values, but this may change in the future.  Do not use constants with
       multiple values in scalar context.

       NOTE: This implies that the expression defining the value of a constant is evaluated in
       list context.  This may produce surprises:

           use constant TIMESTAMP => localtime;                # WRONG!
           use constant TIMESTAMP => scalar localtime;         # right

       The first line above defines "TIMESTAMP" as a 9-element list, as returned by "localtime()"
       in list context.  To set it to the string returned by "localtime()" in scalar context, an
       explicit "scalar" keyword is required.

       List constants are lists, not arrays.  To index or slice them, they must be placed in
       parentheses.

           my @workdays = WEEKDAYS[1 .. 5];            # WRONG!
           my @workdays = (WEEKDAYS)[1 .. 5];          # right

   Defining multiple constants at once
       Instead of writing multiple "use constant" statements, you may define multiple constants
       in a single statement by giving, instead of the constant name, a reference to a hash where
       the keys are the names of the constants to be defined.  Obviously, all constants defined
       using this method must have a single value.

           use constant {
               FOO => "A single value",
               BAR => "This", "won't", "work!",        # Error!
           };

       This is a fundamental limitation of the way hashes are constructed in Perl.  The error
       messages produced when this happens will often be quite cryptic -- in the worst case there
       may be none at all, and you'll only later find that something is broken.

       When defining multiple constants, you cannot use the values of other constants defined in
       the same declaration.  This is because the calling package doesn't know about any constant
       within that group until after the "use" statement is finished.

           use constant {
               BITMASK => 0xAFBAEBA8,
               NEGMASK => ~BITMASK,                    # Error!
           };

   Magic constants
       Magical values and references can be made into constants at compile time, allowing for way
       cool stuff like this.  (These error numbers aren't totally portable, alas.)

           use constant E2BIG => ($! = 7);
           print   E2BIG, "\n";        # something like "Arg list too long"
           print 0+E2BIG, "\n";        # "7"

       You can't produce a tied constant by giving a tied scalar as the value.  References to
       tied variables, however, can be used as constants without any problems.

TECHNICAL NOTES

       In the current implementation, scalar constants are actually inlinable subroutines.  As of
       version 5.004 of Perl, the appropriate scalar constant is inserted directly in place of
       some subroutine calls, thereby saving the overhead of a subroutine call.  See "Constant
       Functions" in perlsub for details about how and when this happens.

       In the rare case in which you need to discover at run time whether a particular constant
       has been declared via this module, you may use this function to examine the hash
       %constant::declared.  If the given constant name does not include a package name, the
       current package is used.

           sub declared ($) {
               use constant 1.01;              # don't omit this!
               my $name = shift;
               $name =~ s/^::/main::/;
               my $pkg = caller;
               my $full_name = $name =~ /::/ ? $name : "${pkg}::$name";
               $constant::declared{$full_name};
           }

CAVEATS

       List constants are not inlined unless you are using Perl v5.20 or higher.  In v5.20 or
       higher, they are still not read-only, but that may change in future versions.

       It is not possible to have a subroutine or a keyword with the same name as a constant in
       the same package.  This is probably a Good Thing.

       A constant with a name in the list "STDIN STDOUT STDERR ARGV ARGVOUT ENV INC SIG" is not
       allowed anywhere but in package "main::", for technical reasons.

       Unlike constants in some languages, these cannot be overridden on the command line or via
       environment variables.

       You can get into trouble if you use constants in a context which automatically quotes
       barewords (as is true for any subroutine call).  For example, you can't say
       $hash{CONSTANT} because "CONSTANT" will be interpreted as a string.  Use $hash{CONSTANT()}
       or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in.  Similarly,
       since the "=>" operator quotes a bareword immediately to its left, you have to say
       "CONSTANT() => 'value'" (or simply use a comma in place of the big arrow) instead of
       "CONSTANT => 'value'".

SEE ALSO

       Readonly - Facility for creating read-only scalars, arrays, hashes.

       Attribute::Constant - Make read-only variables via attribute

       Scalar::Readonly - Perl extension to the "SvREADONLY" scalar flag

       Hash::Util - A selection of general-utility hash subroutines (mostly to lock/unlock keys
       and values)

BUGS

       Please report any bugs or feature requests via the perlbug(1) utility.

AUTHORS

       Tom Phoenix, <rootbeer@redcat.com>, with help from many other folks.

       Multiple constant declarations at once added by Casey West, <casey@geeknest.com>.

       Documentation mostly rewritten by Ilmari Karonen, <perl@itz.pp.sci.fi>.

       This program is maintained by the Perl 5 Porters.  The CPAN distribution is maintained by
       Sébastien Aperghis-Tramoni <sebastien@aperghis.net>.

COPYRIGHT & LICENSE

       Copyright (C) 1997, 1999 Tom Phoenix

       This module is free software; you can redistribute it or modify it under the same terms as
       Perl itself.