Question

I need to define the constant in the module that use the method from the class that includes this module:

module B 
  def self.included(base)
    class << base
  CONST = self.find
    end
  end
end 

class A
  def self.find
    "AAA"
  end
  include B 
end

puts A::CONST

But the compiler gives the error on the 4th line.

Is there any other way to define the constant?

Answer 1

The more idiomatic way to achieve this in Ruby is:

module B 
  def self.included(klass)
    klass.class_eval <<-ruby_eval
      CONST = find
    ruby_eval

    # note that the block form of class_eval won't work
    # because you can't assign a constant inside a method
  end
end

class A
  def self.find
    "AAA"
  end
  include B 
end

puts A::CONST

What you were doing (class << base) actually puts you into the context of A’s metaclass, not A itself. The find method is on A itself, not its metaclass. The thing to keep in mind is that classes are themselves objects, and so have their own metaclasses.

To try to make it clearer:

class Human
  def parent
    # this method is on the Human class and available
    # to all instances of Human.
  end

  class << self
    def build
      # this method is on the Human metaclass, and
      # available to its instance, Human itself.
    end

    # the "self" here is Human's metaclass, so build
    # cannot be called.
  end

  def self.build
    # exactly the same as the above
  end

  build # the "self" here is Human itself, so build can
        # be called
end

Not sure if that helps, but if you don’t understand it, you can still use the class_eval idiom above.