Question

I am a C# developer. Coming from OO side of the world, I start with thinking in terms of interfaces, classes and type hierarchies. Because of lack of OO in Haskell, sometimes I find myself stuck and I cannot think of a way to model certain problems with Haskell.

How to model, in Haskell, real world situations involving class hierarchies such as the one shown here: http://www.braindelay.com/danielbray/endangered-object-oriented-programming/isHierarchy-4.gif

Answer 1

Let’s assume the following operations: Humans can speak, Dogs can bark, and all members of a species can mate with members of the same species if they have opposite gender. I would define this in haskell like this:

data Gender = Male | Female deriving Eq

class Species s where
    gender :: s -> Gender

-- Returns true if s1 and s2 can conceive offspring
matable :: Species a => a -> a -> Bool
matable s1 s2 = gender s1 /= gender s2

data Human = Man | Woman
data Canine = Dog | Bitch

instance Species Human where
    gender Man = Male
    gender Woman = Female

instance Species Canine where
    gender Dog = Male
    gender Bitch = Female

bark Dog = "woof"
bark Bitch = "wow"

speak Man s = "The man says " ++ s
speak Woman s = "The woman says " ++ s

Now the operation matable has type Species s => s -> s -> Bool, bark has type Canine -> String and speak has type Human -> String -> String.

I don’t know whether this helps, but given the rather abstract nature of the question, that’s the best I could come up with.

Edit: In response to Daniel’s comment:

A simple hierarchy for collections could look like this (ignoring already existing classes like Foldable and Functor):

class Foldable f where
    fold :: (a -> b -> a) -> a -> f b -> a

class Foldable m => Collection m where
    cmap :: (a -> b) -> m a -> m b
    cfilter :: (a -> Bool) -> m a -> m a

class Indexable i where
    atIndex :: i a -> Int -> a

instance Foldable [] where
    fold = foldl

instance Collection [] where
    cmap = map
    cfilter = filter

instance Indexable [] where
    atIndex = (!!)

sumOfEvenElements :: (Integral a, Collection c) => c a -> a
sumOfEvenElements c = fold (+) 0 (cfilter even c)

Now sumOfEvenElements takes any kind of collection of integrals and returns the sum of all even elements of that collection.