https://github.com/PostgREST/postgrest/blob/master/test/Main...
and
https://github.com/PostgREST/postgrest/blob/master/test/Quer...
I don't know, maybe it lacks comments ? The code is really not easy to follow if you are not using Haskell 100% of your coding time.
While the library may work well in practice, it's a maintainability red flag and, by using this library, you rely on rare Haskell programmers for the future.