From: Freeman Pascal <pascal@...>
Date: 2001-03-13T17:09:52+09:00
Subject: [ruby-talk:12561] RE:  Accessing caller of include or extend...

My apologies, I seem to have slaughtered the case..when...end
structure below in my code please do the appropriate regex
in your head as you read it =)

-Freeman

-----Original Message-----
From: Freeman Pascal [mailto:pascal@pascal.org]
Sent: Monday, March 12, 2001 11:35 PM
To: ruby-talk ML
Cc: Freeman P Pascal IV
Subject: [ruby-talk:12557] Accessing caller of include or extend...


Hi,

I'm fairly new to Ruby and OO in general, but have read the pickaxe
book through twice already and have been browsing the mailing list
archives.  I have a couple of questions for the more seasoned Ruby
users.  

First, when creating a module for inclusion by a class, is there
a way to initialize module instance variables being added to the 
including class?

The following does not work (though runs without error) and I now 
understand it's because I'm adding the instance variables to the 
module class not the module I'm creating:

  module ThreeDSpace
    @position = Vector.new ()		# current position
    @vector   = Vector.new ()		# speed and heading

    attr_accessor :position
    attr_accessor :vector
  end

  class MyEntity
    include ThreeDSpace
  end

  ball = MyEntity.new Sphere

  p ball.position				# -> nil
  p ball.vector				# -> nil

  ball.position = Vector.new (0,0,0)
  p ball.position				# -> [Vector: @x=0, @y=0, @z=0]

This seems counter intuitive and surprised me.  I was expecting to
have my instance variables initialized and available to the accessors
much the same way that method variables (@@symbol) are.

I naively tried even adding an "initialize" method to the module
which  didn't work.  How does one initialize module instance 
variables without resorting to defining a method to do so and 
calling it within the class after including the module?

Second, is there a way to determine within the module definition at 
run-time who the including class is (i.e. who called "include")?

I'd like to use modules to implement feature sets that extend
my classes, but don't want to resort to uniquely identifying and
including modules that are tightly coupled to the including class.

  module ThreeDSpace
    switch (self.including_class)		# NO SUCH METHOD!
      with VEEnvironment
	  # Environment extensions here	
      with Entity
        attr_accessor :position		# Vector
	  attr_accessor :velocity		# Vector
    end
  end

  module Simple Gravity
    switch (self.including_class)		# NO SUCH METHOD!!!
      with VEEnvironment
  	def simple gravity (entity)
	    # implementation of simple gravity
	  end

	  VEEnvironment.register_law (simple_gravity)

      with VEEntity
	  attr_accessor :simple_gravity_exempt
    end
  end

  class VEEnvironment
    # Base class for simulated environment

    def register_law (&proc)
      # ...
    end

    def add_entity (entity)
      # ...
    end
  end

  class VEEntity
    # base class for simulated entities
  end

  class MyEnvironment << VEEnvironment
    include ThreeDSpace
    include SimpleGravity
  end

  class MyEntity << VEEntity
    include ThreeDSpace
    include SimpleGravity
  end

  env  = MyEnvironment.new
  ball = MyEntity.new

  env.add_entity ball

The closest I've been able to come to doing this is providing
modules specific to the class doing the inclusion and specifically
including that module within the class.

Ideally, I did like to see something like:

  method SimpleGravity
    extend VEEnvironment
	def simple_gravity (entity)
        # ...
      end
    extend VEEntity
      attr_accessor :simple_gravity_exempt
  end

-Freeman