Let's look at it from the operational perspective. In the GADT case, the set of possibilities is fixed in advance (closed).
Every GADT constructor has a corresponding tag (a small integer) which, when pattern-matching, tells us which branch to take. In the data family case, the set of possibilities is open. It is harder to do robust tagging over all the instances, given that new instances can be added after the module is compiled. The right way to do what you want is to use a type class and associate your data family with that class: class C a where data D a a :: D a -> a instance C Int where data D Int = DInt Int a (DInt x) = x instance C Bool where data D Bool = DBool Bool a (DBool x) = x Roman * Alexey Egorov <elect...@list.ru> [2013-04-25 20:29:16+0400] > > Hi, > suppose that there is following data family: > > data family D a > > data instance D Int = DInt Int > > data instance D Bool = DBool Bool > it is not possible to match on constructors from different instances: > > -- type error > > a :: D a -> a > > a (DInt x) = x > > a (DBool x) = x > however, following works: > > data G :: * -> * where > > GInt :: G Int > > GBool :: G Bool > > > > b :: G a -> D a -> a > > b GInt (DInt x) = x > > b GBool (DBool x) = x > The reason why second example works is equality constraints (a ~ Int) and (a > ~ Bool) introduced by GADT's constructors, I suppose. > I'm curious - why data families constructors (such as DInt and DBool) doesn't > imply such constraints while typechecking pattern matching? > Thanks. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe