If the author is reading, I believe the problem noted at the end is actually that the code isn’t going through the thunk API and thus no memoization occurs. (Consider that there is no call to Set in the evaluation tree!)

  fib = func(n int) int {
    if cache[n].o.IsSome() {
      return *cache[n].o.val
    }
    return fib(n-1) + fib(n-2)
  }

Should be:

  fib = func(n int) int {
    return cache[n-1].Force() + cache[n-2].Force()
  }