Friends I'm giving a series of five lectures at the Laser Summer Schoolhttp://laser.inf.ethz.ch/2012/ (2-8 Sept), on "Adventures with types in Haskell". My plan is: 1. Type classes 2. Type families [examples including Repa type tags] 3. GADTs 4. Kind polymorphism 5. System FC and deferred type errors This message is to invite you to send me your favourite example of using a GADT to get the job done. Ideally I'd like to use examples that are (a) realistic, drawn from practice (b) compelling and (c) easy to present without a lot of background. Most academic papers only have rather limited examples, usually eval :: Term t -> t, but I know that GADTs are widely used in practice. Any ideas from your experience, satisfying (a-c)? If so, and you can spare the time, do send me a short write-up. Copy the list, so that we can all benefit. Many thanks Simon