Haskey is a transactional, ACID compliant, embeddable, scalable key-value store written entirely in Haskell. It was developed as part of the Summer of Haskell 2017 project.
A full tutorial can be found in the haskey-mtl library, along with a full code example.
Some blog posts have been written on Haskey's design an internals. These give an insight in the inner workings of Haskey, but the used APIs might be a bit outdated.