How can I call a static method in a Ruby module from a class that includes the module?

Question

Is it possible to declare static methods in a module in ruby?

module Software   def self.exit     puts "exited"   end end  class Windows   include Software    def self.start     puts "started"     self.exit   end end  Windows.start 

The example above will not print out "exited".

Is it only possible to have instance methods in a module?

Answer 1

Define your module like this (i.e. make exit an instance method in the module):

module Software   def exit     puts "exited"   end end 

and then use extend rather than include

class Windows   extend Software   # your self.start method as in the question end 

In use:

irb(main):016:0> Windows.start started exited => nil 

Explanation

obj.extend(module, ...) adds to obj the instance methods from each module given as a parameter

...so when used within the context of a class definition (with the class itself as the receiver) the methods become class methods.

Answer 2

Put your class methods in a nested module, and then override the "included" hook. This hook is called anytime your module is included. Inside the hook, add the class methods to whomever did the include:

module Foo    def self.included(o)     o.extend(ClassMethods)   end    module ClassMethods      def foo       'foo'     end    end  end 

Now any class including Foo gets a class method named foo:

class MyClass   include Foo end  p MyClass.foo    # "foo" 

Any non-class methods may be defined in Foo as usual.