As the Perl compiler needs to know about which barewords are keywords, constants have to defined in
"BEGIN" section. Usually, this is not a problem, as "use" statement is automatically put in implicit
"BEGIN" section, but that also means you cannot dynamically create constants. For example, in the
example below, the "DEBUG" constant is always created, with value 1, as "use" is processed when Perl
parser sees it.
if ($ENV{DEBUG}) {
use Acme::constant DEBUG => 1; # WRONG!
}
It's possible to dynamically use this module using if module, however, this is likely to cause problems
when trying to use constant that doesn't exist.
use if $ENV{DEBUG}, Acme::constant => DEBUG => 1;
You can also use directly use "import" method, in order to conditionally load constant.
BEGIN {
require Acme::constant;
Acme::constant->import(DEBUG => 1) if $ENV{DEBUG};
}
However, usually the good idea to declare constant anyway, as using undefined constants in strict mode
causes Perl errors (and sometimes could be parsed incorrectly).
use Acme::constant DEBUG => $ENV{DEBUG};
Constants belong to the package they were defined in. When you declare constant in some module, the
constant is subroutine declared in it. However, it's possible to export constants with module such as
Exporter, just as you would export standard subroutine.
package Some::Package;
use Acme::constant MAGIC => "Hello, world!\n";
package Some::Other::Package;
print Some::Package::MAGIC; # MAGIC directly won't work.
Listconstants
Just like standard constant module, you can use lists with this module. However, there are few catches
you should be aware of.
To begin with, you cannot use list constants in scalar context. While constant module lets you do this, I
believe allowing something like this can open can of worms, because constant with one element is just as
valid constant (that wouldn't return 1). Something like this won't work.
use Acme::constant NUMBERS => 1..6;
print 'Found ', scalar NUMBERS, " numbers in NUMBERS.\n"; # WRONG!
Instead, to count number of elements in the constant, you can use the "() =" trick, that lets you count
elements in any sort of list.
use Acme::constant NUMBERS => 1..6;
print 'Found ', scalar(() = NUMBERS), " numbers in NUMBERS.\n";
Also, as "use" statement arguments are always parsed in the list context, sometimes you could be
surprised with argument being executed in list context, instead of scalar context.
use Acme::constant TIMESTAMP => localtime; # WRONG!
Usually, when this happens, it's possible to use "scalar" operator in order to force interpretation of
code in scalar context.
use Acme::constant TIMESTAMP => scalar localtime;
Constants return lists, not arrays (you don't use "@" syntax, do you?), so in order to get single
element, you will need to put a constant in parenthesis.
use Acme::constant NUMBERS => 1..6;
print join(" ", (NUMBERS)[2..4]), "\n";
Assignments
The assignments are done using standard "=" operator.
use Acme::constant SOMETHING => 1;
SOMETHING = 2;
print "Something is ", SOMETHING, ".\n";
use Acme::constant ARRAY => 1, 2, 3;
my $four = 7;
($four, ARRAY) = (4, 5, 6);
print "Something is ", join(", ", ARRAY), ", and four is $four.\n";
There are also catches about assignments. Perl normally runs the part after "=" operator in scalar
context, unless leftside argument is a list or array. As inconstant constant is neither a list or array,
the argument on right side is ran in scalar context. For example, following code will only save 2, as
comma operator is ran in scalar context.
use Acme::constant SOMETHING => 0;
SOMETHING = (1, 2); # WRONG!
print "Something is ", join(", ", SOMETHING), ".\n";
In order to force list interpretation, you need to put constant in the parenthesis.
use Acme::constant SOMETHING => 0;
(SOMETHING) = (1, 2);
print "Something is ", join(", ", SOMETHING), ".\n";
Similarly, you cannot modify list constant in scalar context, as Perl expects you put a list, not a
single value.
use Acme::constant SOMETHING => (1, 2);
SOMETHING = 3; # WRONG!
print "Something is ", SOMETHING, ".\n";
To fix that, you need to put constant in parenthesis. This is only needed when constant has different
number of elements than one, so after such assignment, you can use normal assignment, without
parenthesis.
use Acme::constant SOMETHING => (1, 2);
(SOMETHING) = 3;
print "Something is ", SOMETHING, ".\n";
SOMETHING = 4;
print "Something is now ", SOMETHING, ".\n";
Also, the localization of Acme constants is broken, and while it will change the value, it won't change
value back after leaving the block. This is related to that you cannot localize lexicals and references
in Perl 5.
use Acme::constant PI => 4 * atan2 1, 1;
{
local PI = 3;
print "PI = ", PI, "\n";
}
print "PI = ", PI, "\n";
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