From: Peter Suk <peter.kwangjun.suk@...>
Date: 2005-04-24T06:53:02+09:00
Subject: Re: Abstract Class in ruby

On Apr 23, 2005, at 2:14 PM, Paulo S�rgio wrote:

> Hi all,
>
> My question is very "simple" ... how to define an abstract class (or 
> method)  in ruby?

Paulo brings up a key issue.  We should have a standard 
"subclass_responsibility" method to denote this for methods.  If there 
is s standard idiom for Ruby, then code tools will be able to identify 
purely abstract classes.  (Classes that have no concrete methods at 
all.)

But to elaborate, nothing prevents someone from putting concrete 
methods into an abstract class.  Ruby is not about enforcement.  If you 
want to communicate this, there is an idiom in Smalltalk where people 
define a class-side method isAbstract like:

---- code follows ----
class Node
	def Node.abstract?
		return self == Node
	end
end
---- end code ----

Now you can make subclasses of Node and they will automatically return 
false when asked "abstract?"  If one of your subclasses also turns out 
to be abstract, just write a similar method.  Now if you want to know 
if a class is abstract, you can just ask.

--Peter


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