Norman Ramsey <nr@eecs.harvard.edu> writes
[..] class Formula f where eval :: f a -> Environment a -> Bool [..] type Environment a = Var -> a type Var = String [..] data Formula f => Row a = Row (f a, Weight) type Weight = Float
Hugs rejects this program: [ .. ]
Maybe, the contexts are not needed in the data declarations. I would write in your case something like data Row f a = Row (f a, Weight) and then, define the same instances as the initially intended to define for Row: instance (Context1 f a) => Foo1 (Row f a) where ... instance (Context2 f a) => Foo2 (Row f a) where ... ... But there arises a question: as the instances define what is allowed to do with the data type under various conditions on its parameters, then why at all the language allows the `data' contexts? ----------------- Serge Mechveliani mechvel@botik.ru