There is nothing more annoying than databases. Absolutely all DBs nowadays are based on some kind of pre-determined data structure (tables, documents, key/val stores, whatever) plus some methods to mutate data on them. They're the functional programmer's worst nightmare and one of the few "imperative" things that still impregnate Haskell programs. I wonder if there isn't, on this human world, a single functional-oriented DB.
I'm thinking of an app-centric, append-only-log database. That is, rather than having tables or documents with operations that mutate the database state - like all DBs nowadays do, and which is completely non-functional - it would merely store an immutable history of transactions. You would then derive the app state from a reducer. Let me explain with an example. Suppose we're programming a collective TODO-list application. In order to create a DB, all you need is the specification of your app and a data path:
import MyDreamDB