From: kwatch <kwa@...>
Date: 2006-03-12T07:23:44+09:00
Subject: Re: [RCR] abstract method in Ruby

Thanks Austin,

Austin Ziegler wrote:
>
> In the last four years of using Ruby, I thought I needed abstract
> methods in the first few months. Then I learned what value Modules as
> mixins gave me.
>
> I do not believe that this is a common enough need that it needs to be
> in the core of Ruby.

Well, I think that abstract method is more basic than 'singleton.rb'
or 'delegate.rb' which are bundled in Ruby.


> I encourage you to do what others have done with this sort of
> mismatched feature and make a library that implements it and make it
> available for use. Your pure-Ruby implementation looks more than
> sufficient.

I'll make it library and register it to RubyForge.


> More than that, though, I encourage you to rethink why you need
> abstract methods. Most of the time this is because you're thinking in
> terms of C++ or Java inheritance, when Ruby's mixins are both more
> powerful and applicable in most cases where you would define a
> hierarchy that has abstract methods.

In my opinion, 'mixin' is one thing and 'abstract method' is another.
Mixin doesn't cover abstract method.

The following is an example of visitor pattern.
It shows that mixin doesn't cover abstract method.

----------
module Visitor
  def visit_foo(acceptor)  # abstract method
    mesg = "#{self.class.name}#visit_foo() is not implemented."
    raise NotImplementedError.new(mesg)
  end

  def visit_bar(acceptor)  # abstract method
    mesg = "#{self.class.name}#visit_foo() is not implemented."
    raise NotImplementedError.new(mesg)
  end
end

class MyVisitor
  include Visitor     # mix-in

  def visit_foo(acceptor)
    puts "visit_foo() called."
  end

  def visit_bar(acceptor)
    puts "visit_bar() called."
  end
end

class Foo
  def accept(visitor)
    visitor.visit_foo(self)
  end
end

class Bar
  def accept(visitor)
    visitor.visit_bar(self)
  end
end
----------

Ruby's mix-in is more sophisticated solution than interface of Java
or multiple inheritance of C++, I think.
But mix-in and abstract method are different thing.

--
regards,
kwatch