How can I memoize a method that may return true, false, or nil in Ruby?

Question

Obviously ||= won\'t work

def x?
  @x_query ||= expensive_way_to_calculate_x
end

because if it turns out to be false

Answer 1

To account for nil, use defined? to see if the variable has been defined:

def x?
  return @x_query if defined? @x_query
  @x_query = expensive_way_to_calculate_x
end

defined? will return nil if the variable hasn't been defined, or the string "instance_variable" otherwise:

irb(main):001:0> defined? @x
=> nil
irb(main):002:0> @x = 3
=> 3
irb(main):003:0> defined? @x
=> "instance-variable"