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