From: avi@...4.com (Avi Bryant)
Date: 2002-02-26T04:04:22+09:00
Subject: Re: Style Question

Ron Jeffries <ronjeffries@REMOVEacm.org> wrote in message news:<12B6609B12CD5F7D.4DEF63A6DE4AC5F0.77925861F11FD441@lp.airnews.net>...

> So what I've got now is that Sets are created by factory methods such as
> 
>   Set.create(anArray)
> 
> which returns an EnumeratedSet whose contents are the elements of the
> Array, wrapped as they need to be for the set's purposes.
> 
> Of course (he said, after finally figuring it out) this means that no
> one can do Set.new(anything), because that will implicitly call
> initialize and MUST return an instance of Set, which is illegal because
> Set is abstract. I guess that's OK with me.

What about using double dispatch?  For example,

class Set
  class << self
    alias :new :basicNew

    def new(anObject)
      anObject.asSet
    end
   end
end

class Array
  def asSet
    EnumerableSet.basicNew(self)
  end
end

class Fibble
  def asSet
     FibbleSet.basicNew(self)
  end
end

Set.new(anArray)  #=> an ArraySet
Set.new(aFibble)  #=> a FibbleSet