MirOS Manual: constant(3p)


constant(3p)    Perl Programmers Reference Guide     constant(3p)

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 will declare a symbol to be a constant with the given
     value.

     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 (CON-
     STANT)" 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 con-
     stant declaration inside of a conditional statement (like
     "if ($foo) { use constant ... }").

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     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];

     Dereferencing constant references incorrectly (such as using
     an array subscript on a constant hash reference, or vice
     versa) will be trapped at compile time.

     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".  Con-
     stants 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.

     The use of all caps for constant names is merely a conven-
     tion, 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 con-
     text.  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:

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         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 giv-
     ing, 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 con-
     structed 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 fin-
     ished.

         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.)

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         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 sub-
     routine 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};
         }

BUGS

     In the current version of Perl, list constants are not
     inlined and some symbols may be redefined without generating
     a warning.

     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 overrid-
     den 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

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     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'".

AUTHOR

     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>.

COPYRIGHT

     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.

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