From: Paolo Perrotta <paolo.nusco.perrotta@...>
Date: 2010-07-26T07:45:07+09:00
Subject: Re: Module inclusion and class macro problem

Hello, Rolf. Let's wrap up what happened and why. The issue comes from
these lines:

> module Interface
>   module ILockable
>     add_interface_methods :lock, :unlock
>   end
>
>   # ...
> end

Coming from C#, you're probably tempted to assume that the code inside
ILockable (and IDrivable) is executed only when a class/module is
"loaded". Actually, that code is executed immediately as Ruby goes
through the definition.

So, at this point (and independently of all the other code in your
example), Ruby tries to execute add_interface_methods() in the scope
of Interface::ILockable. Inside that scope, the role of self is taken
by Interface::ILockable itself - so that's the implicit receiver of
any method call. So Ruby tries to call
Interface::ILockable.add_interface_methods(), doesn't find that
method, and fails.

In the real world, I'd go for something simpler (and I'd be a real
prick in questioning the notion of strongly typed interfaces in
Ruby ;) ). However, because this is a great learning exercise, here's
something along your own lines that probably works as you expect:

---
module Interface
  module Base
    def self.included(base)
      base.extend ClassMethods
    end
    module ClassMethods
      def implement_interfaces(*interfaces)
        interfaces.each do |interface|
          @interface_methods = (@interface_methods || []) +
interface.instance_methods
          puts "self: " + self.inspect.to_s
        end
      end
      def enforce_interfaces
        if not (not_implemented_interface_methods = @interface_methods
- self.instance_methods).empty?
          raise RuntimeError, "The following interface methods are not
implemented in class #{self.to_s}:\n "
+not_implemented_interface_methods.inspect
        end
      end
    end
  end
end

module Interface
  module ILockable
    def lock; end
    def unlock; end
  end

  module IDrivable
    def drive; end
    def end; end
  end
end

class MyClass
  include Interface::Base
  implement_interfaces Interface::ILockable, Interface::IDrivable
  # methods and other stuff
  enforce_interfaces
end


---
  Paolo "Nusco" Perrotta
  Metaprogramming Ruby (http://www.pragprog.com/titles/ppmetr)