We use a form of stream transducer here at Capital IQ that uses GADTs, kind polymorphism and data kinds: data SF k a b = Emit b (SF k a b) | Receive (k a (SF k a b)) data Fork :: (*,*) -> * -> * where L :: (a -> c) -> Fork '(a, b) c R :: (b -> c) -> Fork '(a, b) c type Pipe = SF (->) type Tee a b = SF Fork '(a, b) instance Category (SF (->)) where id = arr id Emit a as . sf = Emit a (as . sf) Receive f . Emit b bs = f b . bs sf . Receive g = Receive (\a -> sf . g a) arr :: (a -> b) -> Pipe a b arr f = z where loop a = Emit (f a) z z = Receive loop repeat :: b -> SF k a b repeat b = Emit b z where z = repeat b filter :: (a -> Bool) -> Pipe a a filter p = z where loop a | p a = Emit a z | otherwise = z z = Receive loop You can extend the model to support non-deterministic read from whichever input is available with data NonDetFork :: (*,*) -> * -> * where NDL :: (a -> c) -> NonDetFork '(a, b) c NDR :: (b -> c) -> NonDetFork '(a, b) c NDB :: (a -> b) -> (b -> c) -> NonDetFork '(a, b) c These could admittedly be implemented using a more traditional GADT without poly/data kinds, by just using (a,b) instead of '(a,b), though. -Edward Kmett On Tue, Aug 14, 2012 at 7:32 AM, Simon Peyton-Jones <simonpj@microsoft.com>wrote:
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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