Lenses, and going from lenses to optics more generally. You start with what looks like a bad reinvention of "dot notation" to deal with updates on nested records, but then as you generalise, it turns into a deep well to draw from. You get an incredibly powerful toolkit to query/summarise/transform/rewrite data structures, and it's the only library I know where compositionality is _enhanced_ by _exposing_ its inner workings:

Here is SPJ giving a talk on them: https://www.youtube.com/watch?v=k-QwBL9Dia0