On Sun, 10 Jul 2011 11:49:44 +0100, Patrick Browne
My main question is in understanding the relationship between the arguments of the functions getX and getY in the class and in the instance. It seems to me that the constructor Pt 1 2 produces one element of type Point which has two components. How does this square with the class definition of getX which has two arguments? Is there a difference between: getX :: p a -> a
This applies the type constructor p, in this case Point to the type variable a. It is not a function that takes two arguments. A minor clarification: Pt 1 2 produces a value of type "(Num a) => Point a", so a type where the type constructor Point is already applied to something. Just "Point" is not a valid type a value can have, but something that you have to apply to another type, called a type constructor.
and getX :: p -> a -> a
This is a function that takes two arguments. Cheers, Daniel