I’ve not really spent much time with Datomic in anger because it’s super weird, but is any of this surprising? Datomic transactions are basically just batches and I always thought it was single threaded so obviously it doesn’t have a lot of race conditions. It’s slow and safe by design.
In Clojure the answer is [2 2]. A beginner might guess [2 2] or [2 3]. Both are reasonable guesses, so a beginner needs to be quite careful!
But that isn't particularly interesting, because beginners always have to be quite careful. When you are learning any technology, you are a beginner once and experienced ever after. Tool design should optimize for the experienced practitioner. Immutability removes an enormous source of complexity from programs, so where it is feasible it is often desirable.
this is a lot less of a convincing argument when you consider the fact that we’re talking about a database interaction. If we want to do the FP weenie thing, I would assume a transaction context is monadic!
Another ding on your argument is that in the metaphor you _would_ get [2 3] if you aren’t in a transaction! Of course the reasons for this are not incorrect, but I think generally “the end result of this list of actions is different if you are in a transition for a part of it, even if you are the only client operating” is a bit surprising compared to default transaction semantics in other systems.
Of course, people should learn about their tools and not assume everything works exactly the same. I think it’s incorrect to assume that the choice made here was the only one that would make sense for Datomic.
