Orange, Apple etc are values of type FruitType, not types themselves. module Main where data FruitType = Orange | Apple | Banana | Watermelon deriving (Eq, Show) data Fruit = Fruit {fruitRadius::Double, fruitLength::Double, fType::FruitType} data VegType = Tomato | StringBean deriving (Eq, Show) data Veg = Veg {vegRadius::Double, vegLength::Double, vType::VegType} data GeoType = Sphere | Elipsoid deriving (Eq, Show) data Geometric = Geo {radius1::Double, radius2::Double, radius3::Double, gType::GeoType} class ObjectCalc a where volume :: a -> Double surfaceArea :: a -> Double instance ObjectCalc Geometric where volume g | gType g == Sphere = (4.0/3.0) * pi * ( (radius1 g) ** 3.0) | gType g == Elipsoid = (4.0/3.0) * pi * (radius1 g) * (radius2 g) * (radius3 g) | otherwise = undefined surfaceArea g = undefined instance ObjectCalc Fruit where volume f | fType f == Orange = volume (Geo (fruitRadius f) undefined undefined Sphere) | fType f == Apple = volume (Geo (fruitRadius f) undefined undefined Sphere) | fType f == Banana = volume (Geo (fruitRadius f) (fruitRadius f) (fruitLength f) Elipsoid) | fType f == Watermelon = volume (Geo ((fruitRadius f)*2.0) ((fruitLength f)*0.5) (fruitLength f) Elipsoid) | otherwise = undefined surfaceArea f = undefined instance ObjectCalc Veg where volume v = undefined surfaceArea v = undefined initFruit :: [Fruit] initFruit = [ (Fruit 3.0 0.0 Orange), (Fruit 3.0 0.0 Apple), (Fruit 3.0 2.0 Banana), (Fruit 40.0 20.0 Watermelon) ] main = let f = initFruit v = map volume f ft = map fType f s = zipWith (\a b -> putStrLn ("Volume -> " ++ (show a) ++ " = " ++ (show b))) ft v in sequence_ s