These two are similar, but you call them different

Ruby’s defined? method is useful, but the syntax is a little surprising at first glance!

# These two are similar, but you call them different
defined? @whatever # call with the actual identifier
instance_variable_defined? :@whatever # call with a symbol of the identifier

defined? returns a string of what identifier is. In this case, nil or "instance-variable".

instance_variable_defined? returns a bool.

When nil is an acceptable memoized value, I often reach for a pattern of.

def whatever
  return @whatever if instance_variable_defined? :@whatever
  @whatever = some_expensive_computation
end

For tersness that communicates just as much, this could become

def whatever
  return @whatever if defined? @whatever
  @whatever = some_expensive_computation
end