From: Ben Giddings <bg-rubytalk@...>
Date: 2004-06-15T00:17:55+09:00
Subject: Re: undefine

tony summerfelt wrote:
> in my particular case i'm thinking black box approach. i don't want
> the variable to 'exist' any more.
> 
> i don't know if that's how it's thought of in perl, but when i
> undef($x), $x is no longer there for me to use unless i assign it
> something, and i can test that it isn't with:
> 
>  if ! defined($x) # do something.
> 
> as long as ruby 'acts' the same way, it's all i need.

I think the best way to get the behaviour you want is to use a hash. 
Instead of assigning the value to a variable, you just assign the value 
to a hash key.

$h = Hash.new

def test_key(sym)
   if $h.has_key?(sym)
     ...
   else
     ...
   end
end

test_key(:foo)

$h[:foo] = "Bar"

test_key(:foo)

$h.delete(:foo)

test_key(:foo)

Now if you don't use .has_key? and you just try to access the hash 
entry, you'll just get 'nil' back (by default at least).  If you want to 
  have an exception raised instead, that's doable too.

Ben