wtf.rb

# -*- coding: utf-8 -*-
class Charge
  def visit(obj, method, *args)
    obj.public_send(:"#{method}_for_charge", args)
  end
end

class Refund
  def visit(obj, method, *args)
    obj.public_send(:"#{method}_for_refund", args)
  end
end


module FeeCalculator
  def self.fees_for(obj)
    obj.visit("fees", self)
  end

  def self.fees_for_charge(ch)
    # …
  end

  def self.fees_for_refund(re)
    # …
  end
end