A library that makes it nicer to work with nested functional data structures

An accessor is a value that understands how to reach data within a larger data structure, independently from what you intend to do with it. Accessors allow you to read and write data and perform possibly-monadic traversals. By composing accessors, you can work with increasingly complex data structures.

In case you have ever heard of "lenses", this is an OCaml implementation of that idea.