DEV Community

Discussion on: Explain the 'super' keyword in ruby like I'm five

View post
Collapse
 
cathodion profile image
Dustin King • • Edited

When you inherit from a parent class, sometimes you want the child's method to behave slightly differently but still use the parent method's behavior.

Let's say you have a Greeter, and their job is to say hi to everyone they meet. If they have the person's name, they'll greet them by name.

class Greeter
  def greet(name=nil)
    if name
      puts "Hi, #{name}, nice to meet you!"
    else
      puts "Hi, nice to meet you!"
    end
  end
end

Greeter.new.greet
#=> Hi, nice to meet you!

Greeter.new.greet 'Jess'
#=> Hi, Jess, nice to meet you!
Enter fullscreen mode Exit fullscreen mode

Now suppose we hire a new greeter from the planet Marklar, who has a religious belief that everyone's name is Marklar. When their greet method calls super, it calls the greet method from their parent class, Greeter:

class MarklarGreeter < Greeter
  def greet
    super 'Marklar'
  end
end

MarklarGreeter.new.greet
#=> Hi, Marklar, nice to meet you

MarklarGreeter.new.greet 'Jess'
#=> ArgumentError: wrong number of arguments (given 1, expected 0)
Enter fullscreen mode Exit fullscreen mode

Okay, we still need greet to take a name parameter, so that we can use a MarklarGreeter anywhere we use a Greeter. This is called the Liskov Substitution Principle. I can't find a link that explains that like you're five, but it basically says an instance of a child class should be usable anywhere you can use an instance of the parent class.

class MarklarGreeter < Greeter
  def greet(name=nil)
    # We accept a name parameter here, but we ignore it
    # because everyone's name is Marklar
    super 'Marklar'
  end
end

MarklarGreeter.new.greet
#=> Hi, Marklar, nice to meet you!

MarklarGreeter.new.greet 'Jess'
#=> Hi, Marklar, nice to meet you!
Enter fullscreen mode Exit fullscreen mode

More info about Ruby's super here.