From: "David A. Black" <dblack@...>
Date: 2004-12-12T12:31:22+09:00
Subject: Re: Avoiding if requires type mutation -- evil.rb?

Hi --

On Sun, 12 Dec 2004, Mikael Brockman wrote:

> I am writing a tree editor.  The algorithm for displaying a node is
> recursive.  On leaf nodes, it's trivial.  Branches need to display all
> their kids.
>
> I would like to implement this with three classes:  the Leaf, the
> Branch, and the Node.  Leaf & Branch are subclasses of Node.
>
> The problem:  nodes are mutable.  You can add a kid to a node.  If that
> node's a Leaf, it needs to become a Branch.
>
> I can think of three solutions.
>
> - Add a wrapper object around every Node, and give it a #replace
>   method.
>
> - Use Object#become from evil.rb.
>
> - Represent nodes and leaves as instances of the same class, and use if
>   statements.
>
> I'd like #2 the best if evil.pb wasn't likely to win awards for being
> the nastiest hack of all time.  (Props to the authors!)  I don't like #1
> or #3, but if I had to choose, I'd go with #3.
>
> Making nodes non-mutable would be annoying and unintuitive, I think.
>
> What would you do?

One possibility would be have a  Branchlike module and extend leaf
objects with it when they became branches:

   module Branchlike
     def show_kids
      ...
     end
     # etc.
   end

   class Branch
     include Branchlike
     # etc.
   end

   class Leaf
     def add_node
       extend(Branchlike)
     end
     # etc.

or something along those lines, maybe even more modularized.


David

-- 
David A. Black
dblack@wobblini.net