From: Joel VanderWerf <vjoel@...>
Date: 2003-10-02T05:12:53+09:00
Subject: Re: Making == symmetric?

Zach Dennis wrote:
> But on the Polar Points and the Cartesian Points, wouldn't
> you want them to inherit from the Point class and allow the
> Point class to handle conversions, translations of it's
> instantiated objects? Why would you want == to handle the
> translation/conversion from Polar to Cartesian. Shouldn't the
> class Point handle that itself outside of equality comparion.
> 
> Why have 2 Point classes? Why not just have 1 that has 2 methods:
> 
> convertToCartesian
> convertToPolar
> 
> and then check for equality.

You're probably right. One class with implementation hidden from the 
user is better. Even if you want to specify to #initialize that, 
internally, a certain coordinate system would be optimal for the kinds 
of operations you are about to do with this point yadda yadda, that 
doesn't really justify having two classes.

I was trying to come up with an analogue of 1 == 1.0 for some other 
kinds of entities besides numbers. Maybe if you subclass String in a 
semantically uninteresting way:

   class MyString < String
     def <<(s)
       raise if s == "bad data"
       super
     end
   end

   my_string = MyString.new

   my_string << "x" << "y"

   if my_string == "xy"
     puts "found xy"
   end

In this case, == already does the right thing, though.