At Thu, 25 Apr 2013 20:29:16 +0400, Alexey Egorov wrote: > I'm curious - why data families constructors (such as DInt and DBool) doesn't > imply such constraints while typechecking pattern matching?
I think you are misunderstanding what data families do. ‘DInt :: DInt -> D Int’ and ‘DBool :: DBool -> D Bool’ are two data constructors for *different* data types (namely, ‘D Int’ and ‘D Bool’). The type family ‘D :: * -> *’ relates types to said distinct data types. On the other hand the type constructor ‘D :: * -> *’ parametrises a *single* data type over another type—the fact that the parameter can be constrained depending on the data constructor doesn’t really matter here. Would you expect this to work? > newtype DInt a = DInt a > newtype DBool a = DBool a > > type family D a > type instance D Int = DInt Int > type instance D Bool = DBool Bool > > a :: D a -> a > a (DInt x) = x > a (DBool x) = x Francesco _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
