From: Pit Capitain <pit@...>
Date: 2003-11-29T07:49:46+09:00
Subject: Re: Controlled block variables

Hi T.

> (...) the core problem (...)
>   
>   a = [1,2,3]
>   class_eval {
>     def ameth
>       p a
>     end
>   }
> 
> (...) So how does your solution deal with this problem? 

I thought you could tell from my examples, but let's try it with your 
exact problem...

You know you can pass objects to eval:

   a = [1,2,3]
   eval "p a"

results in

   [1, 2, 3]

You can even use those objects in blocks:

   a = [1,2,3]
   eval( "proc{ p a }" ).call

also results in

   [1, 2, 3]

But you can't do so in a method definition:

   a = [1,2,3]
   self.class.class_eval "def ameth; p a; end"
   ameth

results in

   (eval):1:in `ameth': undefined local variable or method `a' for
    main:Object (NameError)

That's because "a" is interpreted in the scope of the method definition, 
where it must be a local variable or a method, which it isn't.

So the trick is to use a proc as shown above and use it as a method, 
which can be done with define_method:

   a = [1,2,3]
   self.class.class_eval "define_method :ameth, proc{ p a }"
   ameth

results in

   [1, 2, 3]

If I understood you right, this is what you wanted.

Regards,
Pit