From: David Whetstone <rubyforum@...>
Date: 2009-08-11T22:42:50+09:00
Subject: Re: instance_variables vs. local_variables

Brian Candler wrote:
> As far as I understand, it's not exactly a "copy". Rather, each binding 
> is a linked list of frames, and if you create a new binding it's the 
> same linked list but with an empty frame on top. So if you create a new 
> variable it goes in the top frame, but if you are searching for an 
> existing variable it hunts back along the list.

That makes sense.

>> 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.
> 
> No, because you can pass a binding around as an object, which means you 
> can manipulate it even when it isn't the 'current' binding:
> 
>   def define_a(b)
>     eval "a=1", b
>   end
> 
>   define_a(binding)
>   puts local_variables.inspect   # prints ["a"]
> 
> You can also implicitly do this using the binding of a block:
> 
>   def another_a(&blk)
>     eval "a=1", blk.binding
>     yield
>   end
> 
>   another_a do
>     puts local_variables.inspect  # prints ["a"]
>   end

But these examples completely contradict the previous discussion.  I was 
surprised by them, because if they worked, I would have never made my 
original post.  But it turns out you are right, they do work ... in Ruby 
1.8.  I should have prefaced that I am using Ruby 1.9.  Apparently, 
there must have been a change to how bindings work between the two 
versions, because neither example works in Ruby 1.9.

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