Constant name stored in a string

SBECK has asked for the wisdom of the Perl Monks concerning the following question:

With constants, I can test that one is defined, or use it trivially:

   use constant FOO => 42;
   print defined(FOO);
   print FOO;

Now, if I have the name of the constant stored in a string:

   $var = "FOO";

how can I operate on $var to find out if the constant stored in it's name is defined, and what it's value is?

If I'm understanding what I've read, it may not be possible... but if it is, I'd like to know how.

Thanks

Comment on Constant name stored in a string

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Re: Constant name stored in a string by WizardOfUz (Friar) on Jan 28, 2010 at 14:25 UTC
`use constant FOO => 42; my $var = 'FOO'; print &{$var} . "\n"; print defined( &{$var} ) ? "true\n" : "false\n";` [download] Result: `# 42 # true` [download] But this is a bad idea, on many levels ...	[reply] [d/l] [select]
Re: Constant name stored in a string by umasuresh (Hermit) on Jan 28, 2010 at 15:17 UTC
According to Damian Conway using Readonly module is a better way of defining constants. See the following post by Randal Schwartz. `http://www.stonehenge.com/merlyn/UnixReview/col65.html`	[reply] [d/l]
Re: Constant name stored in a string by JavaFan (Canon) on Jan 28, 2010 at 15:33 UTC
That's not trivial. Defining a constant using the constant pragma means a subroutine with that name is created - a subroutine without a prototype. Now, it's fairly easy to test if there's a subroutine defined given a name, and it's also fairly easy to see whether it has an empty prototype. But not every function with an empty prototype is a constant. So, you may have to resort to introspection tools like `Devel::Peek`. It's `Dump` function can tell whether a subroutine is a constant - look for the `CONST` flag. `Dump` writes to STDERR, so if you want to do this from within your program, you may want to redirect STDERR and capture it yourself.	[reply] [d/l] [select]
Re^2: Constant name stored in a string by WizardOfUz (Friar) on Jan 28, 2010 at 16:54 UTC
Or you could examine `%constant::declared`.	[reply] [d/l]