judywu29 / meta_programming

Extend and define the attribute method

To define a single method 'attribute' which behaves much like the built-in 'attr', but whose properties require delving deep into the depths of meta-ruby. Usage of the 'attribute' method follows the general form of

       class C
         attribute 'a'
       end
       
       o = C::new
       o.a = 42  # setter - sets @a
       o.a       # getter - gets @a 
       o.a?      # query  - true if @a

but reaches much farther than the standard 'attr' method. 'attribute' must provide getter, setter, and query to instances

      def koan_1
        c = Class::new {
          attribute 'a'
        }

        o = c::new

        assert{ not o.a? }
        assert{ o.a = 42 }
        assert{ o.a == 42 }
        assert{ o.a? }
      end

'attribute' must provide a method for providing a default value as hash

      def koan_6
        c = Class::new {
          attribute 'a' => 42
        }

        o = c::new

        assert{ o.a == 42 }
        assert{ o.a? }
        assert{ (o.a = nil) == nil }
        assert{ not o.a? }
      end

'attribute' must provide a method for providing a default value as block which is evaluated at instance level

      def koan_7
        c = Class::new {
          attribute('a'){ fortytwo }
          def fortytwo
            42
          end
        }

        o = c::new

        assert{ o.a == 42 }
        assert{ o.a? }
        assert{ (o.a = nil) == nil }
        assert{ not o.a? }
      end

Solution

We can extend Object with a method that relies on methods definded in module and the method is available in places where it really didn't ought to be. and then we can use class_eval(), which just executes the code in the context of the class where we can define our setter, getter and querier there.