A Tour of Scala: Currying

Methods may define multiple parameter lists. When a method is called with a fewer number of parameter lists, then this will yield a function taking the missing parameter lists as its arguments.

Here is an example:

object CurryTest extends Application {

  def filter(xs: List[Int], p: Int => Boolean): List[Int] =
    if (xs.isEmpty) xs
    else if (p(xs.head)) xs.head :: filter(xs.tail, p)
    else filter(xs.tail, p)

  def modN(n: Int)(x: Int) = ((x % n) == 0)

  val nums = List(1, 2, 3, 4, 5, 6, 7, 8)
  println(filter(nums, modN(2)))
  println(filter(nums, modN(3)))
}

Note that method modN is partially applied in the two filter calls; i.e. only its first argument is actually applied. The termmodN(2) yields a function of type Int => Boolean and is thus a possible candidate for the second argument of function filter.

Here's the output of the program above:

List(2,4,6,8)
List(3,6)