From: David Whetstone <rubyforum@...>
Date: 2009-08-11T07:16:57+09:00
Subject: Re: instance_variables vs. local_variables

Joel VanderWerf wrote:
> def get_empty_binding
>    binding
> end
> b = get_empty_binding
> 
> eval("a=1", b)
> p eval("a", b) # ==> 1
> 
> Not sure this is what you're looking for.

This clears up a misperception I had when trying similar examples.  For 
example, while your example works, the following does not:

  eval("a=1")
  p eval("a") # ==> NameError: undefined local variable or method ‘a’ 
for main:Object

My incorrect assumption was that the object returned by Kernel::binding 
(implicitly called by eval when no binding is specified) represented the 
_actual_ bindings of the current context.  But it's really only a copy, 
with each call to eval retrieving a fresh copy.  In practice, this:

  eval("a=1")

is roughly equivalent to this:

  do
    a = 1
  end

in that local symbols introduced inside are not accessible outside.  So, 
there really is no way to affect the bindings of an existing context, 
since only a copy of the current bindings can ever be acquired.  Looks 
like I'm sticking with method_missing.

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