Question

I have a question about Haskell polymorphism.

As I’ve learned, there are two types of polymorphism:

1. Parametric: where you do not specify the input type.

   Example:

    functionName :: [a] -> a
   

2. Overloading: as imperative programming, i.e. passing different arguments to the same function.

My problem is: how does Haskell handle overloading?

Answer 1

Overloading in Haskell is done using type classes. For example, let’s say you want to overload a function foo that returns an Int:

class Fooable a where
    foo :: a -> Int

instance Fooable Int where
    foo = id

instance Fooable Bool where
    foo _ = 42

However, they are more powerful than the overloading mechanisms found in most languages. For example, you can overload on the return type:

class Barable a where
    bar :: Int -> a

instance Barable Int where
    bar x = x + 3

instance Barable Bool where
    bar x = x < 10

For more examples, have a look at the predefined type classes in Haskell.