How do I complete the function below so that it takes a list of all products that have been ordered, with the total quantity of each.

 

productQuantities :: [Order] -> [(Product, Double)] 

Type definitions:

type Customer = String 

type Product = String 

An order of some quantity of a product by a customer is described by the type

data Order = Order Customer Product Double deriving (Show) 

A delivery to the supplier of some quantity of a product is described by the type 

data Delivery = Delivery Product Double deriving (Show)