modulemethods.rb

module Foo1
  def self.bar
    "Yolo"
  end
end
"Foo1.bar => #{Foo1.bar}" # => "Foo1.bar => Yolo"

module Foo2
  def bar # !> previous definition of bar was here
    "Yolo"
  end
  extend self
end

# extend self
"Foo2.bar => #{Foo2.bar}" # => "Foo2.bar => Yolo"
object = Object.new
object.extend(Foo2)
"object extended bar: #{object.bar}" # => "object extended bar: Yolo"

module Foo2
  def bar # !> method redefined; discarding old bar
    "altered Yolo"
  end
end

# altering def bar
"Foo2.bar => #{Foo2.bar}" # => "Foo2.bar => altered Yolo"
object = Object.new
object.extend(Foo2)
"object extended bar: #{object.bar}" # => "object extended bar: altered Yolo"

module Foo3
  def bar
    "Yolo"
  end
  module_function :bar
end

# module_function :bar
"Foo3.bar => #{Foo3.bar}" # => "Foo3.bar => Yolo"
object = Object.new
object.extend(Foo3)
# "object extended bar: #{object.bar}" # => private method `bar' called for #<Object:0x007fab199590e0> (NoMethodError)

class Calling
  include Foo3
  
  def call_bar
    bar
  end
end

# include with module_function
"Foo3.bar => #{Foo3.bar}" # => "Foo3.bar => Yolo"
object = Calling.new
"object extended call_bar: #{object.call_bar}" # => "object extended call_bar: Yolo"

module Foo3
  def bar
    "altered instance Yolo"
  end
end

# altering def bar which is a module_function
"Foo3.bar => #{Foo3.bar} (remains the same)" # => "Foo3.bar => Yolo (remains the same)"
object = Object.new
object.extend(Foo3)
"object extended bar: #{object.bar}" # => "object extended bar: altered instance Yolo"