Ruby `module_function` example

module_function.rb

module B
  module_function

  def b
    puts 'b'
  end
end

B.b # => 'b'

class IncludeB
  include B
end

IncludeB.b     # => undefined method `b' for IncludeB:Class (NoMethodError)
IncludeB.new.b # => private method `b' called for #<IncludeB:0x007fbf9a07d828> (NoMethodError)

class ExtendB
  extend B
end

ExtendB.b     # => private method `b' called for ExtendB:Class (NoMethodError)
ExtendB.new.b # => undefined method `b' for #<ExtendB:0x007fd4b48cd530> (NoMethodError)