What does 'extend self' mean in Ruby?

I came across the following in Pagy’s code base:

class Pagy  
  module I18n
    extend self # what does this mean?
	end
end

What does extend self mean?

You can think about extend as opening up the eigenclass (or singleton class) of a particular object.

In this case, we are opening the singleton class of the I18n module and adding the methods in I18n to itself. If that is confusing, consider the following example:

module Car
    extend self

    def drive
        puts "Don't stop me now! I'm having such a good time, havin' a ball!"
    end

end

class Toyota
    include Car
end

t = Toyota.new
t.drive
# => Don't stop me (driving)! I'm having such a good time, havin' a ball!

Car.drive 
# => Don't stop me (driving)! I'm having such a good time, havin' a ball!
# extend self makes the above possible.

Quiz: Will this work?

Let’s test your knowledge:

# Let's go one step further:
Toyota.drive # will this work? What do you think?

If that won’t work, what will you need to do in order to make it work?

Hint: extend yourself - in finding the answer.

class Toyota
    extend Car
end

Toyota.drive # now will it work?

Written on January 25, 2024