From: Glenn Parker <glenn.parker@...>
Date: 2005-06-02T21:33:29+09:00
Subject: Re: binding, ObjectSpace._id2ref [WAS] Re: [ANN] parseargs-0.0.0

Zev Blut wrote:
> 
> If you cannot access a in your binding, why would eval add a to the
> local_variables?  Can anyone explain this?  I would like to know.

It is a curious side effect of the way Ruby is interpreted.  Perhaps it 
is a performance optimization?  It doesn't really have to be this way, 
but it won't affect you (much) in practical terms.

Variables that are "visible" when a method is compiled are added to a 
table somewhere in the AST, and only those variables can be accessed 
directly in that method.  Clearly, this table is not the same one that 
"eval" consults so we have, to put it diplomatically, a little 
disconnect.  Eval can find the variable when asked, but the interpreter 
simply refuses to go looking for it in the first place.

Now, ask yourself if you really care.  Are you going to be writing code 
where you know the name of a variable ahead of time, but only initialize 
it inside an eval statement?  If so, there are two simple workarounds:

  1. Initialize the variable before the eval.

     x = nil       # make "x" visible beforehand
     eval "x = 1"
     print "#{x}\n"

  2. Assign the eval-initialized variable to a "visible" variable.

     eval "y = 2"
     y = eval "y"  # make "y" visible afterwards
     print "#{y}\n"

-- 
Glenn Parker | glenn.parker-AT-comcast.net | <http://www.tetrafoil.com/>