type error. reply
Thu, 05 Apr 2001 10:04:50 +0400
Norman Ramsey <email@example.com> 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?