(Boy is there a lot of Haskell here now. What’s up with that?) This one comes by way of Shachaf Ben-Kiki, who found it originally and mentioned it to me. Data.Traversable and Data.Foldable are two Haskell typeclasses that express related notions of a data structure being “list-like” in the sense that it contains zero or more elements, and an externally-provided function can be mapped over those elements in some way. GHC provides support (as a language extension) for automatically “der...
