example.rb

class User
  ROLES = %w( doctor patient )
  attr_accessor :role

  def initialize( role )
    raise ArgumentError.new("#{role} isn't a valid role") unless ROLES.include?(role.to_s)
    @role = role.to_s
  end

  ROLES.each do |role|
    define_method("#{role}?".to_sym) do
      self.role == role
    end
  end
end