about_extend_and_include.rb

module B                                                                             
  def greeting                                                                       
    puts "Hi there"                                                                  
  end                                                                                
end                                                                                  
                                                                                     
module C                                                                             
  def yell                                                                           
    puts "AAaaaaaaaaa!!"                                                             
  end                                                                                
end                                                                                  
                                                                                     
class A                                                                              
  extend B                                                                           
  include C                                                                          
end                                                                                  
                                                                                     
                                                                                     
A.greeting                                                                           
A.new.yell                                                                           
                                                                                     
class D; end                                                                         
                                                                                     
D.extend B                                                                           
D.send :include, C                                                                   
                                                                                     
D.greeting                                                                           
D.new.yell                                                                           

about_inheritance.rb

class B                                                                              
  def greeting                                                                       
    puts "holaaaaa"                                                                  
  end                                                                                
end                                                                                  
                                                                                     
class A < B                                                                          
                                                                                     
end                                                                                  
                                                                                     
A.new.greeting                                                                       
                                                                                     
# Re open A class                                                                    
class A                                                                              
  def greeting                                                                       
    puts "The new hoolaaaaaa"                                                        
  end                                                                                
end                                                                                  
                                                                                     
A.new.greeting

about_procs_and_lambdas.rb

proc1 = lambda { puts "Created as a lambda" }                                        
proc2 = Proc.new do                                                                  
  puts "Created as a proc"                                                           
end                                                                                  
                                                                                     
proc1.call                                                                           
proc2.call                                                                           
                                                                                     
def greeting a_proc                                                                  
  a_proc.call                                                                        
end                                                                                  
                                                                                     
greeting proc1                                                                       
greeting proc2                                                                       
                                                                                     
proc1 = lambda { |name| puts "Hi #{name}" }                                          
proc2 = Proc.new do |name|                                                           
  puts "Hi #{name}"                                                                  
end                                                                                  
                                                                                     
proc1.call "Juan"                                                                    
proc2.call "Pepe"                                                                    
                                                                                     
def greeting a_proc, name                                                            
  a_proc.call name                                                                   
end                                                                                  
                                                                                     
greeting proc1, "Peca"                                                               
                                                                                     
                                                                                     
def greeting name                                                                    
  puts "Inside the greeting before yielding"                                         
  yield name                                                                         
  puts "Inside the greeting after yielding"                                          
end                                                                                  
                                                                                     
greeting "Maria" do |name|                                                           
  puts "Greeting #{name}"                                                            
end

test.rb

class Configuration

  attr_accessor :username, :password

  def initialize
    yield self
  end
end

c = Configuration.new do |config|
  config.username = 'juan'
  config.password = '123'
end

puts c.username
puts c.password