[Haskell-beginners] Type Class Woes ..

[Tom Poliquin]

I've been writing Haskell programs (even useful ones :-) )
for awhile and I thought it was time to experiment with my own 
type classes. I chose the (contrived toy) problem of computing the volume
of various fruits.

First I wrote the code using algebraic data types ..
(shown below); then I 'converted' it to use type classes
(also shown below)
The type class version gives me the errors ..

Main.hs:12:16: Not in scope: type constructor or class `Banana'
Main.hs:15:16: Not in scope: type constructor or class `Watermelon'

.. which makes sense since Banana and Watermelon are
'data constructors' and not 'type constructors'. The problem is I'm
not sure how to get around it .. If I try putting

data Orange = Orange 
data Banana = Banana ..

then what does 'initFruit' return .. ?
it can't return [Fruit FruitType] anymore ..

It seems like this should be a simple problem.
I'm clearly not 'getting it'.

Any help greatly appreciated,

Tom


----------
Algegraic Data Type Version (works)
----------

module Main where

data Fruit a = F {radius::Double, len::Double, fType::a}

data FruitType = Orange | Apple | Banana | Watermelon
  deriving Show


initFruit :: [Fruit FruitType]
initFruit = [
              (F  3.0  0.0 Orange),
              (F  3.0  0.0 Apple),
              (F  3.0  2.0 Banana),
              (F 40.0 20.0 Watermelon)
            ]

volume :: Fruit FruitType -> Double
volume F{radius=r,len=l,fType=Orange} = (4.0/3.0) * pi * r * r * r
volume F{radius=r,len=l,fType=Apple}  = (4.0/3.0) * pi * r * r * r
volume F{radius=r,len=l,fType=Banana} = pi * (r * r) * l 
volume F{radius=r,len=l,fType=Watermelon} = (4.0/3.0) * pi * (2.0 * r) 
                                                         * l * (0.5 * l) 

----------
-- Main --
----------

main = do

   fruit <- return $ initFruit

   mapM (\f@(F{fType=t}) -> putStrLn ("Volume -> " ++ show t ++ 
                                    " = " ++ show (volume f))) fruit

Volume -> Orange = 113.09733552923255
Volume -> Apple = 113.09733552923255
Volume -> Banana = 56.548667764616276
Volume -> Watermelon = 67020.64327658225



----------
Type Class Version (fails)
----------

module Main where

data Fruit a = F {radius::Double, len::Double, fType::a}

data FruitType = Orange | Apple | Banana | Watermelon

class Volume a where
  volume :: (FruitType a) =>  Fruit a -> Double
  -- default spherical fruit ..
  volume F{radius=r,len=l} = (4.0/3.0) * pi * r * r * r
 
instance Volume Banana where
  volume F{radius=r,len=l} = pi * (r * r) * l 

instance Volume Watermelon where
  volume F{radius=r,len=l} = (4.0/3.0) * pi * (2.0 * r) * l * (0.5 * l) 


initFruit :: [Fruit FruitType]
initFruit = [
              (F  3.0  0.0 Orange),
              (F  3.0  0.0 Apple),
              (F  3.0  2.0 Banana),
              (F 40.0 20.0 Watermelon)
            ]

----------
-- Main --
----------

main = do

   fruit <- return $ initFruit

   mapM (\f@(F{fType=t}) -> putStrLn ("Volume -> " ++ show t ++ 
                                    " = " ++ show (volume f))) fruit

Main.hs:12:16: Not in scope: type constructor or class `Banana'
Main.hs:15:16: Not in scope: type constructor or class `Watermelon'