Abstract Method in Ruby

Abstract methods are supposed to be less useful in Ruby because it's not strongly statically typed.

However, this is what I do:

class AbstractThing
  MESS = "SYSTEM ERROR: method missing"

  def method_one; raise MESS; end
  def method_two; raise MESS; end
end

class ConcreteThing < AbstractThing
  def method_one
     puts "hi"
  end
end

a = ConcreteThing.new
a.method_two # -> raises error.

It rarely seems to be necessary, however.

I like the answer by pvandenberk, but I would improve it as follows:

module Canine      # in Ruby, abstract classes are known as modules
  def bark
    fail NotImplementedError, "A canine class must be able to #bark!"
  end
end

Now if you make a class belonging to Canine "abstract class" (ie. a class that has Canine module in its ancestors), it will complain if it is found that #bark method is not implemented:

class Dog
  include Canine   # make dog belong to Canine "abstract class"
end

Dog.new.bark       # complains about #bark not being implemented

class Dog
  def bark; "Bow wow!" end
end

# Now it's OK:
Dog.new.bark #=> "Bow wow!"

Note that since Ruby classes are not static, but always open to changes, Dog class itself cannot enforce existence of #bark methods, since it doesn't know when is it supposed to be finished. If you as a programmer do, it is up to you to test it at such time.