I have a class:
import Linear
class Coordinate c where
rotate :: Num a => Quaternion a -> c a -> c a
translate :: Num a => V3 a -> c a -> c a
, for which I have defined the instances
instance Coordinate V3 where
rotate _ = id
translate p = (p+)
instance Coordinate Quaternion where
rotate o = (o*)
translate _ = id
Now I want to define an instance for a pair of members of the class.
instance (Coordinate a, Coordinate b) => Coordinate (a, b) where
rotate o (a, b) = (rotate o a, rotate o b)
translate p (a, b) = (translate p a, translate p b)
The problem is that this does not work, since the compiler expects an argument for a
and b
. However adding a type-constraint like
instance (Coordinate a, Coordinate b, Num c) => Coordinate (a c, b c) where
move p (a, b) = (move p a, move p b)
translate p (a, b) = (translate p a, translate p b)
It does not work either, since this results in an expression with the kind *
rather than * -> *
. I can see how both of the above are incorrect, but I am unsure of how to solve this. I suppose there should be some form of constraint that keeps the Num
types for both a
and b
the same, but I don't know what that would look like syntactically.