Stewart Anderson wrote:
>> From: mrstevegross [mailto:[EMAIL PROTECTED]
>>
>> I have a package named "Foo" in which I want to define some package-
>> level constants (such as $VAR="soemval"). I want those constants
>> available to users of package Foo, so the following code would work:
>>
>> === foo.pl ===
>> package foo;
>> use constant VAR => "someval";
>>
>> === bar.pl ===
>> use foo;
>> print $foo::VAR;
>>
>> It doesn't appear to be working; it compiles ok, but it prints
>> nothing. I thought it would print "someval".
>>
> That looks interesting. How do people use that kind of constant
> assignment. I can see uses for it, but would be interested to hear what
> others use this technique for.
Take a look at
perldoc constant
What the pragma creates is a prototyped subroutine with exactly zero parameters,
so writing
use constant PI => 3.14159265359;
is equivalent to a subroutine
sub PI() {
3.14159265359;
}
However it has a few of advantages over just writing this directly:
- It is self-documenting, i.e. it is clear that a named constant is being
defined.
- The Perl compiler has a chance to optimise out the subroutine definition and
call
- The implementation could change to something more optimal in the future
without needing to alter any code that uses the pragma
I hope this helps,
Rob
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