In denotational semantics and functional programming, the terms monad morphism, monad layering, monad constructor, and monad transformer have by now accumulated 20 years of twisted history. The exchange between Eric Kidd and sigfpe about the probability monad prompted me to investigate this history
"Generalized Algebraic Data Structures" have become a a hot new topic. They have recently been added to the GHC compiler. They support the construction, maintenance, and propagation of semantic properties of programs using powerful old ideas about types (the Curry-Howard Isomorphism) in surprisingly easy to understand new ways. The language Omega was designed and implemented to demonstrate their utility. Here a a few talks I gave that explains how they work. Also class lectures