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"

Answer 2

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.