GADTs in the wild

[Simon Peyton-Jones]

Friends

I'm giving a series of five lectures at the Laser Summer School<http://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
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