Comment by xeubie
2 hours ago
If each "branch" is read only, it's not a branch at all. The entire idea of branching implies that you can make changes on one branch, then switch to another branch and make changes to it. They start from the same point and grow in different directions, as the metaphor of branches on a tree depicts.
I don't disagree with anything you've said here, I just don't see how it applies to the situation.
With `datomic.api/with`, you can apply new datoms and get back a new DB value. Repeat this process as many times as you want, in as many directions as you want, switching as you choose. You're building a tree of immutable DB values—seems clearly like branching to me.
If by "read only" you mean that they're not persisted to disk, then that's an important point, but it surely doesn't obviate the utility of the functionality. It's useful in a number of cases, and especially testing scenarios like the Xata article describes.
If you built an immutable database that persists the branches, that is very cool and sounds useful—kudos! That said, I also don't want to downplay the utility of what Datomic does; it's a major help to me.
Yes the article I linked mentioned d/with (speculative writes), and you are right that it is useful for testing -- but not much else, since it is purely in-memory. If you want to call that in-memory branching that's fine, I'll concede that.
My database supports persisted branching, but not just at the database level. You can "branch" (i.e., make a fast clone) data at any level, such as data for a specific user. Many production uses for this, not just testing, yet almost no database supports this. It uses the same HAMT algorithm that Clojure uses.