Hi everyone, I'm trying to set up some type safe functions for doing weighted averages and sum of products. The example I give below is to try and calculate the average miles per gallon for a collections of vehicles. Unfortunately, I am unable to get my weightedAverage function to work, seemingly due to an ambiguity for which instance to use. I think that the issue is that my "class Multiplicable" should only have two parameters, as opposed to the three it currently has. However, I can't figure out how to get that to work. Any help is greatly appreciated. Thank you, Michael ---------------------------------- {-# LANGUAGE MultiParamTypeClasses #-} import Prelude hiding (sum, product) class Addable a where add :: a -> a -> a zero :: a sum :: [a] -> a sum = foldr add zero class Multiplicable a b c where mult :: a -> b -> c product :: [a] -> [b] -> [c] product x y = map (\(x1, y1) -> x1 `mult` y1) $ zip x y sumProduct :: (Addable c, Multiplicable a b c) => [a] -> [b] -> c sumProduct x y = sum $ product x y weightedAverage x y = (sumProduct y x) `divide` (sum y) class Dividable a b c where divide :: c -> a -> b newtype MilesPerGallon = MilesPerGallon Double deriving Show newtype Gallon = Gallon Double deriving Show newtype Mile = Mile Double deriving Show instance Addable Gallon where add (Gallon x) (Gallon y) = Gallon $ x + y zero = Gallon 0 instance Addable Mile where add (Mile x) (Mile y) = Mile $ x + y zero = Mile 0 instance Multiplicable Gallon MilesPerGallon Mile where mult (Gallon x) (MilesPerGallon y) = Mile $ x * y instance Dividable Gallon MilesPerGallon Mile where divide (Mile x) (Gallon y) = MilesPerGallon $ x / y milesPerGallon :: [MilesPerGallon] milesPerGallon = map MilesPerGallon [35, 25, 29, 20, 52] gallons :: [Gallon] gallons = map Gallon [500, 190, 240, 100, 600] totalGallons :: Gallon totalGallons = sum gallons totalMiles :: Mile totalMiles = sumProduct gallons milesPerGallon totalMilesPerGallon :: MilesPerGallon totalMilesPerGallon = totalMiles `divide` totalGallons -- I would like some way to get the following line to replace the previous --totalMilesPerGallon = weightedAverage milesPerGallon gallons main = do putStrLn $ "Total gallons of gas used: " ++ show totalGallons putStrLn $ "Total miles traveled: " ++ show totalMiles putStrLn $ "Average miles per gallon: " ++ show totalMilesPerGallon