defnil.rb

module DefNil
  def defnil(meth)
    self.class.class_eval do
      alias_method "#{meth}_before_defnil", meth
      define_method meth do |first_arg, *args, &block|
        if first_arg
          send("#{meth}_before_defnil", first_arg, *args, &block)
        else
          nil
        end
      end
    end
  end
end

extend DefNil

def f(x,y)
  x + y
end
defnil :f
 
f(1,2) # => 3
f(nil,2) # => nil