From: Steve Klabnik <steve@...>
Date: 2012-04-06T06:26:10+09:00
Subject: Re: Design pattern question

> Abstract classes are sometimes useful (although rarely). For example,
>  how else could you do the kind of database adapter handling Sequel[1]
>  does?

Use duck typing.

This:

class Abstract
  def foo
    throw NotImplementedException
  end
end

class Foo < Abstract
  def foo
    puts "in foo!"
  end
end

class Bar < Abstract
  def foo
    puts "in bar's foo"
  end
end

Is the exact same thing as

class Foo
  def foo
    puts "in foo!"
  end
end

class Bar
  def foo
    puts "in bar's foo"
  end
end

in fact...

class Baz
end

Baz.new.foo # => NoMethodError

class Baz < Abstract
end

Baz.new.foo # => NotImplementedError

Almost the exact same thing. Just a different exception.