Question

You can call include to mixin a module with a class in ruby, but it must be done at the beginning of the class definition. Why can’t it be done inside a class function? Is there an alternate syntax?

EX:

module UsefulThings
  def a() puts "a" end
end

class IncludeTester
  include UsefulThings
    def initialize
  end
end

n = IncludeTester.new
n.a()

^^ This works, but if I change IncludeTester to the following, I get the error “undefined method `include'”

class IncludeTester
  def initialize
    include UsefulThings
  end
end

Answer 1

It can be done in a class method.

This works:

module UsefulThings
  def a
    puts "a" 
  end
end

class IncludeTester
  def self.mix_in_useful_things
    include UsefulThings
  end 
end

x = IncludeTester.new

IncludeTester.mix_in_useful_things

x.a # => a

But “initialize” is not a class method, it’s an instance method.

“new” is a class method. You can think of new as allocating a new object and then calling initialize on it, passing initialize whatever arguments were passed to new.

You can’t call include directly in initialize because include is a private method of Class (inherited from Module), not of the newly created IncludeTester instance.

If you want to include a module into a class from an instance method, you have to do something like this:

class IncludeTester
  def initialize
    self.class.send(:include, UsefulThings)
  end
end

It’s necessary to use “send” here because include is private method, which means it can only be directly invoked with an implicit receiver (of self).

When you call initialize normally in a class definition, you’re actually calling it with an implicit receiver of “self”, referring to the class being defined.

This is what is actually happening when you do this:

class IncludeTester
  include UsefulThings
end