From: ahoward <ahoward@...>
Date: 2003-01-22T05:38:05+09:00
Subject: Re: accessors for module instance vars

On 21 Jan 2003, Phil Tomson wrote:

> I've got a module that that has several classes defined within it.  These
> classes need to access some instance variables defined at the module
> level.
>
> For example:
>
> module Evolvable
>   PC = 0.2
>   PM = 0.01
<snip>
> end
>
> Now this works ok, but I'm wondering if there's a way to create the
> accessor methods for the Evolvable module automatically like with
> attr_accessor and attr_reader.  The problem with attr_accessor is that it
> creates an instance method, not a module method - is there a way to
> automatically create a module method?

i'm unsure of the semantics of instance variables in a module (something you
cannot declare an instance of) but assuming you need both the module defaults
AND 'instance variable' in your module (perhaps because you will want to tweak
these 'instance variables' later and yet have sensible defaults) why not
simply :


  class Evolvable
    require 'singleton'
    include Singleton

  private

    PC = 0.2

    def initialize
      @pc = PC
    end

    attr_writer :pc

  public

    attr_reader :pc
  end


  e = Evolvable.instance
  puts e.pc 		# >> 0.2

perhaps PC should be @@pc or something, but you get the picture :

by putting the vars in a class you can use attr_*, but making the class a Singleton you ensure only one copy of @pc will ever exist.

-a



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