I'm learning Haskell, and I'm having trouble understanding the meaning of a `context' as applied to the declaration of an algebraic datatype. I would like to say the following: A formula is a predicate over environments.
class Formula f where eval :: f a -> Environment a -> Bool -- evaluate the formula eq :: Var -> a -> f a -- basic equality (&) :: f a -> f a -> f a -- conjunction
type Environment a = Var -> a type Var = String
A row has a formula and a weight.
data Formula f => Row a = Row (f a, Weight) type Weight = Float
Hugs rejects this program: ERROR "hard.lhs" (line 14): Undefined type variable "f" Line 14 is the definition of Row. Can anyone explain the proper use of a context in a data definition? Thanks, Norman