What is polymorphism? 

Parametric polymorphism 

Nick says:

In this function head, it takes a list of A’s, and returns an A. And it doesn’t matter what the A is: It could be Ints, Strings, Oranges, Cars, whatever. Any A would work, and the function is defined for every A that there can be.

scala> def head[A](xs: List[A]): A = xs(0)
head: [A](xs: List[A])A

scala> head(1 :: 2 :: Nil)
res0: Int = 1

scala> case class Car(make: String)
defined class Car

scala> head(Car("Civic") :: Car("CR-V") :: Nil)
res1: Car = Car(Civic)

Haskell wiki says:

Parametric polymorphism refers to when the type of a value contains one or more (unconstrained) type variables, so that the value may adopt any type that results from substituting those variables with concrete types.

Subtype polymorphism 

Let’s think of a function plus that can add two values of type A:

scala> def plus[A](a1: A, a2: A): A = ???
plus: [A](a1: A, a2: A)A

Depending on the type A, we need to provide different definition for what it means to add them. One way to achieve this is through subtyping.

scala> trait Plus[A] {
         def plus(a2: A): A
       }
defined trait Plus

scala> def plus[A <: Plus[A]](a1: A, a2: A): A = a1.plus(a2)
plus: [A <: Plus[A]](a1: A, a2: A)A

We can at least provide different definitions of plus for A. But, this is not flexible since trait Plus needs to be mixed in at the time of defining the datatype. So it can’t work for Int and String.

Ad-hoc polymorphism 

The third approach in Scala is to provide an implicit conversion or implicit parameters for the trait.

scala> trait Plus[A] {
         def plus(a1: A, a2: A): A
       }
defined trait Plus

scala> def plus[A: Plus](a1: A, a2: A): A = implicitly[Plus[A]].plus(a1, a2)
plus: [A](a1: A, a2: A)(implicit evidence$1: Plus[A])A

This is truely ad-hoc in the sense that

1. we can provide separate function definitions for different types of A
2. we can provide function definitions to types (like Int) without access to its source code
3. the function definitions can be enabled or disabled in different scopes

The last point makes Scala’s ad-hoc polymorphism more powerful than that of Haskell. More on this topic can be found at Debasish Ghosh @debasishg’s Scala Implicits : Type Classes Here I Come.

Let’s look into plus function in more detail.