On Sun, Apr 11, 2010 at 10:54 PM, Jason Dagit
Or, you could use witness types: data Vehicle classification = Vehicle { ... } mkCar :: Vehicle Car mkTruck :: Vehicle Truck Then you would export the smart constructors, (mkCar/mkTruck) without exporting the Vehicle constructor. moveVehicle :: Vehicle c -> Simulation () carsOnly :: Vehicle Car -> ... In the witness type version you may find that defining Vehicle as a GADT is even better: data Vehicle classification where mkCar :: ... -> Vehicle Car mkTruck :: ... -> Vehicle Truck
A minor point of syntax: GADT constructors should begin with a capital letter. A more important point: GADT constructors are useful for some things but aren't appropriate for this case because they aren't "smart" - they can't take an Int parameter and then make either a Car or a Truck depending on its value. A smart constructor will need to account for the possibility that the parameter isn't valid: mkCar :: Integer -> Maybe Vehicle mkCar weight | weight > carWeight = Nothing | otherwise = Car { weight = weight } -- or mkVehicle :: Integer -> Vehicle mkVehicle weight | weight > carWeight = Truck weight | otherwise = Car weight Even with GADTs, you can't pattern match on the constructors unless you export them, in which case you have to allow for the possibility that someone might construct an invalid value of the type, so you might then also need "smart" field labels that retrieve values from the Vehicle without allowing you to set them as well (as you would be able to if you exported the field label itself). Personally I think this approach is all rather OO. The way that seems most natural to me is: moveVehicleAcrossBridge :: Bridge -> Vehicle -> Maybe Move moveVehicleAcrossBridge bridge { maxWeight = max } vehicle { weight = w } | w > max = Nothing | otherwise = {- ... moving stuff ... -} so you just test the properties directly as and when they are interesting to you.