haskell - Scala: How to define "generic" function parameters?

Question

I am trying to learn Scala now, with a little bit of experience in Haskell. One thing that stood out as odd to me is that all function parameters in Scala must be annotated with a type - something that Haskell does not require. Why is this? To try to put it as a more concrete example: an add function is written like this:

def add(x:Double, y:Double) = x + y

But, this only works for doubles(well, ints work too because of the implicit type conversion). But what if you want to define your own type that defines its own + operator. How would you write an add function which works for any type that defines a + operator?

Answer 1

Haskell uses Hindley-Milner type inference algorithm whereas Scala, in order to support Object Oriented side of things, had to forgo using it for now.

In order to write an add function for all applicable types easily, you will need to use Scala 2.8.0:

Welcome to Scala version 2.8.0.r18189-b20090702020221 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_15).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import Numeric._
import Numeric._

scala> def add[A](x: A, y: A)(implicit numeric: Numeric[A]): A = 
     | numeric.plus(x, y)
add: [A](x: A,y: A)(implicit numeric: Numeric[A])A

scala> add(1, 2)
res0: Int = 3

scala> add(1.1, 2.2)
res1: Double = 3.3000000000000003