It looks possible to call the value-to-be-cached twice

[vemv]

Related: #45

Consider the plain ruby memoization pattern:

      def foo
        @foo ||= (...)
      end

This looks thread-unsafe, because two concurrent threads which call foo for the first time at the same time could trigger the code in (...). Only one would 'win', and everything would normally from then on, but (...) has been called twice.

If that was an expensive operation, it would have been called twice (suboptimal). If it was an operation with side-effects, we might encounter actual problems.

From my understanding of memoist.rb, this doesn't seem prevented at all.

What do you think - is there room for improvement here?

Cheers - Victor