From: "David A. Black" <dblack@...>
Date: 2004-08-26T06:56:13+09:00
Subject: Re: *with* block?

Hi --

On Wed, 25 Aug 2004, Jim Weirich wrote:

> The above works, but some very similar code is broken ... For example:
>   class Test
>     def test
>       puts "i'm testing"
>     end
>   end
> 
>   class Dog
>     def talk
>       puts "i'm talking"
>     end
> 
>     def testing_dog
>       talk             # Class Dog#talk
>       t = Test.new
>       with t do
>         test           # Calls Test#test
>         talk           # ERROR: undefined local variable or method `talk'
>       end
>     end
>   end
> 
> This is because the second call to talk is not evaluated in the lexical
> scope of the calling object Dog, but within the scope of the Test object. 
> Dog#talk exists, but Test#talk does not.  (Your example missed the problem
> by putting +talk+ into the global scope.
> 
> There is an awkward work around.  First write +with+ like this:
> 
>   module Kernel
>     def with(obj, &block)
>       obj.instance_eval { @self = eval("self", block) }
>       obj.instance_eval &block
>     end
>   end
> 
> Then reference Dog#talk like this ...
> 
>   class Dog
>     def talk
>       puts "i'm talking"
>     end
> 
>     def testing_dog
>       talk             # Class Dog#talk
>       t = Test.new
>       with t do
>         test           # Calls Test#test
>         @self.talk     # Calls Dog#talk
>       end
>     end
>   end

You could also do:

  def testing_dog
    s = self
    t = Test.new
    talk
    with t do
      test
      s.talk
    end
  end

(i.e., put the burden of providing a reference to self-the-Dog to on
the method, rather than on 'with')


David

-- 
David A. Black
dblack@wobblini.net