That's what I mean by hard to optimize. Memoization is hard to get right, because if you compare only by value, it can be slow, but if you compare by identity,

That's what I mean by hard to optimize. Memoization is hard to get right, because if you compare only by value, it can be slow, but if you compare by identity, the cache can take a lot of memory. And of course, forcing a purely immutable model comes with its own unique challenges.