Lenses, Folds, and Traversals in Haskell

HN calls it dense but mind-blowing, the go-to way to understand lenses through algebraic derivation.

Edward Kmett's talk to the NY Haskell User Group on the lens library, building from fmap and function composition up to lenses, folds and traversals. It shows how these composable combinators access and modify parts of data structures.