Class variables in Ruby are not the same as instance variables of a
class. They're particularly tricky when it comes to inheritance and
modules.
In the case of the Measurable module, for example, using a class
variable will make all classes including the Measurable module share
the same value. However, that's not what we want; we want the variable
to be different for every class. And that's accomplished using a class
instance variable.
Although in this case it would probably be better to remove the caching
variable. I don't think these methods are called more than once during a
request, and even if they did it's highly unlikely the would become a
bottleneck.
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