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constant - Perl pragma to declare constants
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";
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.
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 $obj is an instance of "Some::Package". Subclasses may define their own constants to override those in their base class.
The use of all caps for constant names is merely a convention, although it is recommended in order to make constants stand out and to help avoid collisions with other barewords, keywords, and subroutine names. Constant names must begin with a letter or underscore. Names beginning with a double underscore are reserved. Some poor choices for names will generate warnings, if warnings are enabled at compile time.
NOTE: This implies that the expression defining the value of a constant is evaluated in list context. This may produce surprises:
use constant TIMESTAMP => localtime; # WRONG!
use constant TIMESTAMP => scalar localtime; # right
The first line above defines "TIMESTAMP" as a 9-element list, as returned by localtime() in list context. To set it to the string returned by localtime() in scalar context, an explicit "scalar" keyword is required.
List constants are lists, not arrays. To index or slice them, they must be placed in parentheses.
my @workdays = WEEKDAYS[1 .. 5]; # WRONG!
my @workdays = (WEEKDAYS)[1 .. 5]; # right
use constant {
FOO => "A single value",
BAR => "This", "won't", "work!", # Error!
};
This is a fundamental limitation of the way hashes are constructed in Perl. The error messages produced when this happens will often be quite cryptic --- in the worst case there may be none at all, and you'll only later find that something is broken.
When defining multiple constants, you cannot use the values of other constants defined in the same declaration. This is because the calling package doesn't know about any constant within that group until after the "use" statement is finished.
use constant {
BITMASK => 0xAFBAEBA8,
NEGMASK => ~BITMASK, # Error!
};
use constant E2BIG => ($! = 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.
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};
}
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'".
Multiple constant declarations at once added by Casey West, <casey@geeknest.com>.
Documentation mostly rewritten by Ilmari Karonen, <perl@itz.pp.sci.fi>.
This module is free software; you can redistribute it or modify it under the same terms as Perl itself.
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