Awesome
haskey
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.
Tutorial
A full tutorial can be found in the haskey-mtl library, along with a full code example.
Historical blog posts
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 These give an insight in the inner workings of Haskey, but the used APIs might be a bit outdated.