From: Brian Candler <b.candler@...>
Date: 2011-04-08T01:06:39+09:00
Subject: Re: Hash Surprises with Fixnum, #hash, and #eql?

Phillip Gawlowski wrote in post #991471:
>> If you see inheritance as "is a" relationship then inheritance would
>> be Rational < Real < Complex but not the other way round. Otherwise
>> you cannot use a subclass instance everywhere you were using a
>> superclass instance.
>
> Though, does the "is a" relationship hold up? I think it's more of a
> "kind of" relationship, where subsequent classes are defined in ever
> more detail (so, you'd inherit Floats from Integers, and Complex from
> Float).

Isn't this the old "ellipse is_a circle, or vice versa" debate?

If you make Circle the top class, then Ellipse reimplements pretty much 
everything (draw, area, etc); there's no useful code sharing. If you 
make Ellipse the top class, then Circle is just a special constrained 
case of Ellipse.

Translating to the current discussion, substitute Float for Circle and 
Ellipse for Complex.

Ruby's answer is: neither is a subclass of the other. Both inherit from 
Numeric. That is, Circle and Ellipse are both a Shape. Or in other 
words, "who cares"?

Eventually you come to realise that a lot of what is taught in object 
oriented classes and textbooks is tosh :-)

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