foldl traverses with State, foldr traverses with anything

Avi Press gave an excellent talk at Scale By the Bay 2023 about difficulties using Haskell at a startup. He mentions that even experienced Haskellers don’t always know how to use fundamental parts of the language. In particular,

even experienced Haskell engineers aren’t always going to know whether to foldl or foldr.

In this article I’ll deduce a firm rule that allows you to make the correct choice...