From: Ronny <ro.naldfi.scher@...>
Date: 2008-01-10T03:14:59+09:00
Subject: module_eval and scope question

When trying to understand a piece of code from the Ruby-On-Rails
framework,
I found that I'm still lacking knowledge about scope issues in Ruby.
Here is a
stripped down version of the code I'm trying to understand (in case
you
happen to use Rails: This is from file scaffolding.rb):

module ActionController
  module Scaffolding
    ..
    module ClassMethods
      def scaffold(model_id, options = {})
         ...
         unless options[:suffix]
          module_eval <<-"end_eval", __FILE__, __LINE__
            def index
              list
            end
          end_eval
        end
        ...
     end
  end
end

The purpose of this code is to create at run-time the function
'index'.
My question is: Why do I need 'module_eval' here? I understand that
I can use module_eval to evaluate an expression in the context of
a different class/module, for example

  mymodule.module_eval <<"END"
      def foo
      end
  END

would define foo in the context of mymodule. In the example code
above,
however, there is no SOMETHING.module_eval defined, so the scope
would be ClassMethods anyway and, so I had thought, the author could
have written simply:

module ActionController
  module Scaffolding
    ..
    module ClassMethods
      def scaffold(model_id, options = {})
         ...
         unless options[:suffix]
            def index
              list
            end
        end
        ...
     end
  end
end

Of course I'm pretty sure that there was some purpose in using
module_eval
here, though. Could anybody enlighten me why it is necessary, and why
my
simpler solution would not work?

Ronald