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perlman:lib:constant

by root (Scribe)
on Dec 23, 1999 at 00:49 UTC ( #1136=perlfunc: print w/ replies, xml ) Need Help??

lib:constant

See the current Perl documentation for lib:constant.

Here is our local, out-dated (pre-5.6) version:


constant - Perl pragma to declare constants



    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         => 'orac
    

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


NOTES

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 perlref 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 Other::CONST.

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

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 perlsub for details about how and when this happens.


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 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.

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' instead of CONSTANT => 'value'.


AUTHOR

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


COPYRIGHT

Copyright (C) 1997, 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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