Newbie question: How to model class hierarchies in Haskell?

Question

Hi, I am a C# developer and these days I am trying to learn Haskell from the online book Real World Haskell. From what I have learnt so far, I am quite impressed with the language. However coming from OO side of the world, I always 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.

So my question is: 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

Thanks.

PS: Sorry for my poor English. I am not a native English speaker.

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 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.

Answer 2

First of all: Standard OO design is not going to work nicely in Haskell. You can fight the language and try to make something similar, but it will be an exercise in frustration. So step one is look for Haskell-style solutions to your problem instead of looking for ways to write an OOP-style solution in Haskell.

But that's easier said than done! Where to even start?

So, let's disassemble the gritty details of what OOP does for us, and think about how those might look in Haskell.

• Objects: Roughly speaking, an object is the combination of some data with methods operating on that data. In Haskell, data is normally structured using algebraic data types; methods can be thought of as functions taking the object's data as an initial, implicit argument.
• Encapsulation: However, the ability to inspect an object's data is usually limited to its own methods. In Haskell, there are various ways to hide a piece of data, two examples are:
  • Define the data type in a separate module that doesn't export the type's constructors. Only functions in that module can inspect or create values of that type. This is somewhat comparable to protected or internal members.
  • Use partial application. Consider the function map with its arguments flipped. If you apply it to a list of Ints, you'll get a function of type (Int -> b) -> [b]. The list you gave it is still "there", in a sense, but nothing else can use it except through the function. This is comparable to private members, and the original function that's being partially applied is comparable to an OOP-style constructor.
• "Ad-hoc" polymorphism: Often, in OO programming we only care that something implements a method; when we call it, the specific method called is determined based on the actual type. Haskell provides type classes for compile-time function overloading, which are in many ways more flexible than what's found in OOP languages.
• Code reuse: Honestly, my opinion is that code reuse via inheritance was and is a mistake. Mix-ins as found in something like Ruby strike me as a better OO solution. At any rate, in any functional language, the standard approach is to factor out common behavior using higher-order functions, then specialize the general-purpose form. A classic example here are fold functions, which generalize almost all iterative loops, list transformations, and linearly recursive functions.
• Interfaces: Depending on how you're using an interface, there are different options:
  • To decouple implementation: Polymorphic functions with type class constraints are what you want here. For example, the function sort has type (Ord a) => [a] -> [a]; it's completely decoupled from the details of the type you give it other than it must be a list of some type implementing Ord.
  • Working with multiple types with a shared interface: For this you need either a language extension for existential types, or to keep it simple, use some variation on partial application as above--instead of values and functions you can apply to them, apply the functions ahead of time and work with the results.
• Subtyping, a.k.a. the "is-a" relationship: This is where you're mostly out of luck. But--speaking from experience, having been a professional C# developer for years--cases where you really need subtyping aren't terribly common. Instead, think about the above, and what behavior you're trying to capture with the subtyping relationship.

You might also find this blog post helpful; it gives a quick summary of what you'd use in Haskell to solve the same problems that some standard Design Patterns are often used for in OOP.

As a final addendum, as a C# programmer, you might find it interesting to research the connections between it and Haskell. Quite a few people responsible for C# are also Haskell programmers, and some recent additions to C# were heavily influenced by Haskell. Most notable is probably the monadic structure underlying LINQ, with IEnumerable being essentially the list monad.