I really wish that Aeson's lookups and fromJSON returned Eithers, and not Maybes. If you try to parse a deeply nested structure represented in JSON, and just get a Nothing back, it really doesn't help you very much. Having an Either is much more helpful.

(If anyone is aware of a canonical library for this, let me know. I wrote my own JSON library which has this functionality but it's not particularly robust or performant).