Datomic is immutable in the sense that what you had for lunch today doesn't change what you had for lunch yesterday, where "lunch" is any arbitrary fact stored in the database.
I.e., you can ask to look at the entire database as it was yesterday, and run arbitrary queries against it.
You can also do speculative updates to it, in the sense of "show me the entire database as it would be if I were to have pizza for lunch".
It models this as a strictly linear succession of assertions and retractions of facts. Yesterday, `A` was true, today `A` is no longer true. While this new fact is recorded, it doesn't change the fact that yesterday, `A` was true.
I.e., you can ask to look at the entire database as it was yesterday, and run arbitrary queries against it.
You can also do speculative updates to it, in the sense of "show me the entire database as it would be if I were to have pizza for lunch".
It models this as a strictly linear succession of assertions and retractions of facts. Yesterday, `A` was true, today `A` is no longer true. While this new fact is recorded, it doesn't change the fact that yesterday, `A` was true.