DavidA wrote:
Hi,
I am trying to implement quadratic fields Q(sqrt d). These are numbers of the form a + b sqrt d, where a and b are rationals, and d is an integer.
...
class IntegerType a where value :: Integer
The problem is, this doesn't work. GHC complains: The class method `value' mentions none of the type variables of the class IntegerType a When checking the class method: value :: Integer In the class declaration for `IntegerType'
Is what I'm trying to do reasonable? If no, what should I be doing instead? If yes, why doesn't GHC like it?
You are on the right track. The problem with the class method is that it doesn't use type 'a' anywhere, consider
f :: Integer f = value What class instance should be used here?
The solution is to use a dummy parameter:
class IntegerType a where value :: a -> Integer And call it like: f = value (undefined :: Two)
So for instance:
instance IntegerType d => Show (QF d) where show (QF a b) = show a ++ " + " ++ show b ++ " sqrt " ++ show (value (undefined::d))
The problem is that this doesn't work, because d is not in scope, you need the scoped type variables extension:
valueOfQF :: forall a. IntegerType a => QF a -> Integer valueOfQF qf = value (undefined :: a)
or maybe better, change the class:
class IntegerType a where value :: QF a -> Integer
Now you can simply use
instance IntegerType d => Show (QF d) where show qf@(QF a b) = show a ++ " + " ++ show b ++ " sqrt " ++ show (value qf)
Twan