如何在ruby中声明模块中的函数

Question

In the below code, what is the difference between declaring two methods differently. Second method is declared using Week but first method is declared without using Week . And we are also not able to access second method by the class object d1 . It gives the error

undefined method `weeks_in_year' for #<Decade:0x2c08a28> (NoMethodError)

then what is the use of declaring methods using Week prefix in second method when it is of no use.

module Week
   def weeks_in_month
      puts "You have four weeks in a month"
   end
   def Week.weeks_in_year
      puts "You have 52 weeks in a year"
   end
end

class Decade
   include Week
end

d1=Decade.new
d1.weeks_in_month
d1.weeks_in_year

Answer 1

The way you have defined the method weeks_in_year is a class method of the Week class, not an instance method. That's why it didn't get inherited and you got the error as you posted.

You can use module_function to use the same method as a class method or instance method.

module Week
  def weeks_in_month
    puts "You have four weeks in a month"
  end

  def weeks_in_year
    puts "You have 52 weeks in a year"
  end

  module_function :weeks_in_year
end

class Decade
  include Week

  def wrapper_of_weeks_in_year
    weeks_in_year
  end
end

d1 = Decade.new

d1.weeks_in_month
# You have four weeks in a month
d1.wrapper_of_weeks_in_year
# You have 52 weeks in a year
Week.weeks_in_year
# You have 52 weeks in a year

While you will be using module_function , The instance-method versions are made private . That's why you need to use a wrapper method to call it as direct invocation is not possible.