Can I invoke an instance method on a Ruby module without including it?

Question

Background:

I have a module which declares a number of instance methods

module UsefulThings
  def get_file; ...
  def delete_file; ...

  def forma         


        

Answer 1

As I understand the question, you want to mix some of a module's instance methods into a class.

Let's begin by considering how Module#include works. Suppose we have a module UsefulThings that contains two instance methods:

module UsefulThings
  def add1
    self + 1
  end
  def add3
    self + 3
  end
end

UsefulThings.instance_methods
  #=> [:add1, :add3]

and Fixnum includes that module:

class Fixnum
  def add2
    puts "cat"
  end
  def add3
    puts "dog"
  end
  include UsefulThings
end

We see that:

Fixnum.instance_methods.select { |m| m.to_s.start_with? "add" }
  #=> [:add2, :add3, :add1] 
1.add1
2 
1.add2
cat
1.add3
dog

Were you expecting UsefulThings#add3 to override Fixnum#add3, so that 1.add3 would return 4? Consider this:

Fixnum.ancestors
  #=> [Fixnum, UsefulThings, Integer, Numeric, Comparable,
  #    Object, Kernel, BasicObject] 

When the class includes the module, the module becomes the class' superclass. So, because of how inheritance works, sending add3 to an instance of Fixnum will cause Fixnum#add3 to be invoked, returning dog.

Now let's add a method :add2 to UsefulThings:

module UsefulThings
  def add1
    self + 1
  end
  def add2
    self + 2
  end
  def add3
    self + 3
  end
end

We now wish Fixnum to include only the methods add1 and add3. Is so doing, we expect to get the same results as above.

Suppose, as above, we execute:

class Fixnum
  def add2
    puts "cat"
  end
  def add3
    puts "dog"
  end
  include UsefulThings
end

What is the result? The unwanted method :add2 is added to Fixnum, :add1 is added and, for reasons I explained above, :add3 is not added. So all we have to do is undef :add2. We can do that with a simple helper method:

module Helpers
  def self.include_some(mod, klass, *args)
    klass.send(:include, mod)
    (mod.instance_methods - args - klass.instance_methods).each do |m|
      klass.send(:undef_method, m)
    end
  end
end

which we invoke like this:

class Fixnum
  def add2
    puts "cat"
  end
  def add3
    puts "dog"
  end
  Helpers.include_some(UsefulThings, self, :add1, :add3)
end

Then:

Fixnum.instance_methods.select { |m| m.to_s.start_with? "add" }
  #=> [:add2, :add3, :add1] 
1.add1
2 
1.add2
cat
1.add3
dog

which is the result we want.