From: Joel VanderWerf <vjoel@...>
Date: 2003-06-24T14:28:33+09:00
Subject: Re: Block Scope Statement in the Pickaxe

Robert Klemme wrote:
> Hi,
> 
> I stumbled across this quote in chapter 15, "Ruby Tk", which confused me a
> bit:
> 
> "One small caution when using the code block form: the scope of variables is
> not what you think it is. The block is actually evaluated in the context of
> the widget's object, not the caller's. This means that the caller's instance
> variables will not be available in the block, but local variables from the
> enclosing scope and globals (not that you ever use those) will be."
> 
> How can this be?  I mean, AFAIK the block's binding refers to client code
> and cannot be changed afterwards.  Or did I miss something?  Is this some
> kind of implementation trick that can be done with a Ruby extension?

I think this is just the distinction between #instance_eval and calling 
a block directly. With instance_eval, the binding of self in the block 
is changed to the receiver of instance_eval. Hope this example says it 
better:

class A
   attr_reader :x
   def initialize
     @x = 1
   end
   def twiddle(&block)
     block.call
   end
   def frob(&block)
     instance_eval(&block)
   end
end

a = A.new

some_local_var = 2
@x = 10

a.twiddle do
   some_local_var += 1
   @x += 1
end

p some_local_var, @x, a.x  # => 3, 11, 1

a.frob do
   some_local_var += 1
   @x += 1
end

p some_local_var, @x, a.x  # => 4, 11, 2