From: Daniel Berger <djberg96@...>
Date: 2010-11-02T06:32:54+09:00
Subject: Re: the dark side of inherited methods



On Oct 31, 5:49 pm, James Edward Gray II <ja...@graysoftinc.com>
wrote:
> On Oct 31, 2010, at 5:30 PM, timr wrote:
>
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> > Let's say I want to make a new class, Vector (that will function,
> > eventually, like R vectors for mathematical operations), and I write
> > this:
>
> > class Vector < Array
> >  def initialize(*arr)
> >    super(arr.flatten)
> >  end
> > end
>
> > v = Vector.new(1,2,3) # => [1, 2, 3]
> > v.class # => Vector
> > v.collect{|item| item * 3}.class # => Array
> > v.collect!{|item| item * 3}.class # => Vector
>
> > I noticed that I inherited the Array#collect and Array#collect!
> > methods. Yeah for inheritance! But then look at the returned classes.
> > Array#collect returns and array. Array.collect! returns a Vector.
> > Is there a simple way to fix this for all similar methods? Or do I just have to live with the dark side of inherited methods?
>
> It is a problem with inheritance and another reason why inheritance is almost never what we want.  Luckily, this is Ruby where anything is possible:
>
> class Vector < BasicObject
>   def initialize(*array)
>     @array = array.flatten
>   end
>
>   def class
>     ::Vector
>   end
>
>   def method_missing(meth, *args, &blk)
>     result = @array.send(meth, *args, &blk)
>     if result.object_id == @array.object_id
>       self
>     elsif result.class == ::Array
>       self.class.new(result)
>     else
>       result
>     end
>   end
> end
> v = Vector.new(1,2,3)                # => [1, 2, 3]
> v.class                              # => Vector
> v.collect { |item| item * 3 }.class  # => Array
> v.collect!{ |item| item * 3 }.class  # => Vector
>
> That's a solution for Ruby 1.9, but a similar trick is possible with 1.8 using a touch more code.  It's probably not perfect yet, but you get the idea.

Eeep, method_missing. I was thinking this might be accomplished via
some sort of AOP. I looked at AspectR and Aquarium but couldn't make
it work. Maybe that's the wrong approach, though.

Regards,

Dan