Ruby：如何从模块中调用类的实例方法？

Question

I have an instance method defined on a class:

class Foo
  def is_linux
    # some code...
    # returns true or false
  end
end

And i would like to call this method in a Module:

class Foo
  module Dog
    is_linux? ? puts "Is Linux" : puts "Is Windows"
  end
end

This gives me the following error:

NoMethodError: undefined method `is_linux?' for Foo::Foo:Module

I know i can write

class Foo
  module Dog
    Foo.new.is_linux? ? puts "Is Linux" : puts "Is Windows"
  end
end

But i was wondering if there is a way for the module to access the current instance?

Answer 1

There is no such option because subtypes (like module) are not connected with their parents instances. If you need such construct you have can only make is_linux? class method. If there is no such possibility then propably you have wrong design.

Answer 2

Basically you'll need somehow to "share" your utility methods.

I would suggest to put them into a mixin:

module Mixin
  def is_linux?
    true
  end
end

class Foo
  include Mixin # adding methods at instance level
end

class Foo
  module Dog

    extend Mixin # adding methods at class level

    puts is_linux? ? "Is Linux" : "Is Windows"

  end
end

#=> Is Linux