Ruby: module, require and include

Question

I'm trying to use Ruby modules (mixins).

I have test.rb:

#!/usr/bin/env ruby require_relative 'lib/mymodule'  class MyApp   include MyModule   self.hallo end 

and lib/mymodule.rb:

module MyModule   def hallo     puts "hallo"   end end 

Quite simple setup. But it does not work :( :

ruby test.rb test.rb:8:in `<class:MyApp>': undefined method `hallo' for MyApp:Class (NoMethodError)         from test.rb:6:in `<main>' 

Where is my error?

Answer 1

In short: you need to extend instead of include the module.

class MyApp   extend MyModule   self.hallo end 

include provides instance methods for the class that mixes it in.

extend provides class methods for the class that mixes it in.

Give this a read.

Answer 2

The issue is that you are calling hallo in the class definition, while you add it as an instance method (include).

So you could either use extend (hallo would become a class method):

module MyModule   def hallo     puts "hallo"   end end  class MyApp   extend MyModule   self.hallo end 

Or either call hallo in an instance of MyApp:

module MyModule   def hallo     puts "hallo"   end end  class MyApp   include MyModule end  an_instance = MyApp.new an_instance.hallo