How do I define a data type that only accepts numbers?

Question

I am attempting to create a data type, Point, that takes three numbers for its constructor. Initially, I had written

data Point = Point Double D         


        

Answer 1

You can use GADT to specify the constraint:

{-# Language GADTs #-}

data Point a where
  Point :: (Num a) => a -> a ->  a -> Point a