From: Joel VanderWerf <vjoel@...>
Date: 2002-09-12T08:45:07+09:00
Subject: Re: instance_eval with a block


See embedded comments:

Brad Hilton wrote:
> Hello,
> 
> Can someone please explain why the following code doesn't work?
> 
> P.S.  If I move the methods out of class 'TestEval', things work as
> expected.  I'm guessing it has to do with Binding or some such trickery,
> but I'm stumped.
> 
> Thanks,
> -Brad
> 
> ---------------
> class TestEval
>   def install_attrs(attrs)
>     attrs.each { |k, v|
>       type.send(:attr_accessor, k)
>       send("#{k}=", v)
>     }
>   end
> 
>   def output(params, block)
>     install_attrs(params)
>     instance_eval { block.call }

       instance_eval &block  # Fixes it. This changes the "self"
                             # binding for block.

>   end
> end # class TestEval
> 

@name = "FOO"    # This is what the @name below was getting bound
                  # to, and "block.call" preserved that binding.

> block = proc { @name }
> 
> test = TestEval.new
> 
> params = {
>   'name' => 'Joe',
> }
> 
> puts test.output(params, block)
> 
> params['name'] = 'Marty'
> puts test.output(params, block)
> 
> -----------------------------------



-- 
Joel VanderWerf                          California PATH, UC Berkeley
mailto:vjoel@path.berkeley.edu                     Ph. (510) 231-9446
http://www.path.berkeley.edu                       FAX (510) 231-9512