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 (CONSTANT)
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 constant
declaration inside of a conditional statement (like if ($foo)
{ use constant ... }
).
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
. Constants 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 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 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 context. 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:
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 giving, 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 constructed 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 finished.
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.)
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 subroutine 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 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'
(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.
