From: Stefano Crocco <stefano.crocco@...>
Date: 2008-08-12T00:03:41+09:00
Subject: Re: Bindings and Context

On Monday 11 August 2008, Patrick Li wrote:
> Hi,
> I'm still really confused about Ruby Closures.
>
> At first I thought closures are just blocks who "remember" the context
> that they were defined in (so that they can refer to local variables and
> methods)
>
> And then I discovered instance_eval (which changes the context that a
> block is evaluated in).
>
> When you use instance_eval you can't call local methods anymore
> (because, the scope has changed, and those methods aren't local
> anymore).
>
> But how come I can still use local variables?
>
>     myStr = "myString"
>     o = Object.new
>     o.instance_eval do
>       @str = myStr    # <- why does this still work?
>       def method
>         puts @str
>       end
>     end
>
> So are local variables sort of an exception to instance_eval? And in the
> above example, how come it won't work anymore if I change myStr into an
> instance variable @myStr?

instance_eval doesn't change scope. Its only effect is to change the value of 
the special local variable self. This explains why you can't access instance 
variables anymore: with self changed, the instance variables are instance 
variables of the new self (that is, the receiver of instance_eval). The same 
is true for instance methods. Local variables, on the other hand, are 
unaffected by the change of self and so you can still use them.

I hope this helps

Stefano