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 or nil, then expensive_way_to_calculate_x will get run over and over.

Currently the best way I know is to put the value into an Array:

def x?
  return @x_query.first if @x_query.is_a?(Array)
  @x_query = [expensive_way_to_calculate_x]
  @x_query.first
end

Is there a more conventional or efficient way of doing this?

UPDATE I realized that I wanted to memoize nil in addition to false - this goes all the way back to https://rails.lighthouseapp.com/projects/8994/tickets/1830-railscachefetch-does-not-work-with-false-boolean-as-cached-value - my apologies to Andrew Marshall who gave an otherwise completely correct answer.

Answer 1

Explicitly check if the value of @x_query is nil instead:

def x?
  @x_query = expensive_way_to_calculate_x if @x_query.nil?
  @x_query
end

Note that if this wasn't an instance variable, you would have to check if it was defined also/instead, since all instance variables default to nil.

Given your update that @x_query's memoized value can be nil, you can use defined? instead to get around the fact that all instance variables default to nil:

def x?
  defined?(@x_query) or @x_query = expensive_way_to_calculate_x
  @x_query
end

Note that doing something like a = 42 unless defined?(a) won't work as expected since once the parser hits a =, a is defined before it reaches the conditional. However, this isn't true with instance variables since they default to nil the parser doesn't define them when it hits =. Regardless, I think it's a good idiom to use or or unless's long block form instead of a one-line unless with defined? to keep it consistent.

Answer 2

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"