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'
