IRIX Base Documentation 2001 May/SGI IRIX Base Documentation 2001 May.iso/usr/share/catman/p_man/cat3/perl5/constant.z/constant

Text File | 1998-10-30 | 5.9 KB | 199 lines

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     constant - Perl pragma to declare constants

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         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         => '[e-mail]';
         use constant USERNAME       => scalar getpwuid($<);
         use constant USERINFO       => getpwuid($<);

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

         print "This line does nothing"              unless DEBUGGING;


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     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.

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     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 _p_e_r_l_r_e_f 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
     Other::CONST.




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ccccoooonnnnssssttttaaaannnntttt((((3333))))                                                        ccccoooonnnnssssttttaaaannnntttt((((3333))))



     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 nnnnooootttt 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 nnnneeeecccceeeessssssssaaaarrrriiiillllyyyy 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"


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     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 _C_o_n_s_t_a_n_t _F_u_n_c_t_i_o_n_s
     in the _p_e_r_l_s_u_b manpage for details about how and when this happens.

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     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.

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     Tom Phoenix, <_r_o_o_t_b_e_e_r@_t_e_l_e_p_o_r_t._c_o_m>, with help from many other folks.

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     Copyright (C) 1997, Tom Phoenix





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ccccoooonnnnssssttttaaaannnntttt((((3333))))                                                        ccccoooonnnnssssttttaaaannnntttt((((3333))))



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





















































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