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constant(3)							   constant(3)

NAME    [Toc]    [Back]

     constant -	Perl pragma to declare constants

SYNOPSIS    [Toc]    [Back]

	 use constant BUFFER_SIZE    =>	4096;
	 use constant ONE_YEAR	     =>	365.2425 * 24 *	60 * 60;
	 use constant PI	     =>	4 * atan2 1, 1;
	 use constant DEBUGGING	     =>	0;
	 use constant ORACLE	     =>	'oracle@cs.indiana.edu';
	 use constant USERNAME	     =>	scalar getpwuid($<);
	 use constant USERINFO	     =>	getpwuid($<);

	 sub deg2rad { PI * $_[0] / 180	}

	 print "This line does nothing"		     unless DEBUGGING;

DESCRIPTION    [Toc]    [Back]

     This will declare a symbol	to be a	constant with the given	scalar or list

     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.

NOTES    [Toc]    [Back]

     The value or values are evaluated in a list context. You may override
     this with scalar as shown above.

     These constants do	not directly interpolate into double-quotish strings,
     although you may do so indirectly.	(See the perlref manpage for details
     about how this works.)

	 print "The value of PI	is @{[ PI ]}.\n";

     List constants are	returned as lists, not as arrays.

	 $homedir = USERINFO[7];	     # WRONG
	 $homedir = (USERINFO)[7];	     # Right

     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.

     Constant symbols are package scoped (rather than block scoped, as use
     strict is). That is, you can refer	to a constant from package Other as

									Page 1

constant(3)							   constant(3)

     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 ... }).

     Omitting the value	for a symbol gives it the value	of undef in a scalar
     context or	the empty list,	(), in a list context. This isn't so nice as
     it	may sound, though, because in this case	you must either	quote the
     symbol name, or use a big arrow, (=>), with nothing to point to. It is
     probably best to declare these explicitly.

	 use constant UNICORNS	     =>	();
	 use constant LOGFILE	     =>	undef;

     The result	from evaluating	a list constant	in a scalar context is not
     documented, and is	not guaranteed to be any particular value in the
     future. In	particular, you	should not rely	upon it	being the number of
     elements in the list, especially since it is not necessarily that value
     in	the current implementation.

     Magical values, tied 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"

TECHNICAL NOTE    [Toc]    [Back]

     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 the	section	on Constant Functions
     in	the perlsub manpage for	details	about how and when this	happens.

BUGS    [Toc]    [Back]

     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	keyword	with the same name as
     a constant. This is probably a Good Thing.

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

AUTHOR    [Toc]    [Back]

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

COPYRIGHT    [Toc]    [Back]

     Copyright (C) 1997, Tom Phoenix

									Page 2

constant(3)							   constant(3)

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

									PPPPaaaaggggeeee 3333
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