Data Types a la Carte. Wouter Swierstra.
This paper describes a technique for assembling both data types and functions from isolated individual components. We also explore how the same technology can be used to combine free monads and, as a result, structure Haskell’s monolithic IO monad.
This new Functional Pearl has been mentioned twice in comments (1, 2), and has now also appeared with comments on Phil Wadler's blog. Obviously it's time to put it on the front page.