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B.6. Pragmas
Pragmas
are special modules that come with each release of Perl and tell
Perl''s internal compiler something about your code.
You''ve already used the strict pragma. The
pragmas available for your release of Perl should be listed in the
perlmodlib
manpage.
You use pragmas much like you''d use ordinary modules, with a
use
directive. Some pragmas are lexically scoped, like lexical
("my") variables are, and they
therefore apply to the smallest enclosing block or file. Others may
apply to the entire program or to the current package. (If you
don''t use any packages, the pragmas apply to your entire
program.) Pragmas should generally appear near the top of your source
code. The documentation for each pragma should tell you how
it''s scoped.
B.6.1. The constant Pragma
If you''ve used other languages, you''ve probably seen the
ability to declare constants in one way or another. Constants are
handy for making a setting just once, near the beginning of a
program, but that can be easily updated if the need arises. Perl can
do this with the package-scoped
constant pragma, which tells the compiler that a
given identifier has a constant value, which may thus be optimized
wherever it appears. For example:
use constant DEBUGGING => 0;
use constant ONE_YEAR => 365.2425 * 24 * 60 * 60;
if (DEBUGGING) {
# This code will be optimized away unless DEBUGGING is turned on
...
}
B.6.2. The diagnostics Pragma
Perl''s diagnostic messages often seem somewhat cryptic, at
least the first time you see them. But you can always look them up in
the perldiag manpage to find out what they mean,
and often a little about what''s likely to be the problem and
how to fix it. But you can save yourself the trouble of searching
that manpage if you use the
diagnostics pragma, which tells Perl to track down
and print out the related information for any message. Unlike most
pragmas, though, this one is not intended for
everyday use, as it makes your program read the entire
perldiag manpage just to get started. (This is
potentially a significant amount of overhead, both in terms of time
and memory.) Use this pragma only when you''re debugging
and expecting to get error message you
don''t yet understand. It affects your entire program. The
syntax is:
use diagnostics;
B.6.3. The lib Pragma
It''s nearly always best to install modules in the standard
directories, so that they''re available for everyone, but only
the system administrator can do that. If you install your own
modules, you''ll have to store them in your own
directories -- so, how will Perl know where to find them?
That''s what the lib pragma is all about. It tells Perl that
the given directory is the first place to look for
modules. (That means that it''s also
useful for trying out a new release of a given module.) It affects
all modules loaded from this point on. The syntax is:
use lib ''/home/rootbeer/experimental'';
Be sure to use a nonrelative pathname as the argument, since
there''s no telling what will be the current working directory
when your program is run. This is especially important for CGI
programs (that is, programs run by a web server).
B.6.4. The strict Pragma
You''ve been using use
strict for a while already without having to
understand that it''s a pragma. It''s lexically scoped, and
it enforces some good programming rules. See its documentation to
learn what restrictions are available in your release of Perl.
B.6.5. The vars Pragma
In the rare case that you truly need a global variable while
use strict is in effect, you may declare it with
the vars pragma.[407] This package-scoped pragma tells
Perl that you are intentionally using one or more global variables:
[407]If your program
will never be used with a version of Perl prior to 5.6, you should
use the our keyword instead of the vars
pragma.
use strict;
use vars qw/ $fred $barney /;
$fred = "This is a global variable, but that''s all right.\n";
B.6.6. The warnings Pragma
Starting in Perl version 5.6, you may choose to have lexically scoped
warnings with the
warnings pragma.[408] That is,
rather than using the -w option crudely to turn warnings on or
off for the entire program at once, you may specify that you want no
warnings about undefined values in just one section of code, while
other warnings should be available. This also serves as a signal to
the maintenance programmer that says, "I know that this code
would produce warnings, but I know what I''m doing
anyway." See the documentation for this pragma to learn about
the categories of warnings available in your release of
Perl.
[408]If your program
may be used with a version of Perl prior to 5.6, you should not use
the warnings pragma.
