Question

I’m fairly new to Ruby but I came across this weird gist where a function def code is written inside a do-block code. What is the purpose of this:

 module Hi
   def self.included(base)
     base.class_eval do
       def hello; puts 'Hello' end
     end
    end
 end

I’m sure I should first get my Mixin concepts straight before moving ahead (and I’m in the process of diving deeper into mixin’s and whatnot) but the way def :hello is used inside the do-block is confusing me

Answer 1

What this particular bit of code does is a define a method (hello) in any class that includes this module. Given this, the following two bits of code are (roughly) equivalent:

class Foo
  include Hi
end

# ...is equivalent to...

class Foo
  def hello
    puts 'Hello'
  end
end

You’ll find the documentation for Module#class_eval and Module.included informative.