Multiple Parameter Lists (Currying)

Methods may have multiple parameter lists.

Example

Here is an example, as defined on the TraversableOnce trait in Scala’s collections API:

def foldLeft[B](z: B)(op: (B, A) => B): B

foldLeft applies a two-parameter function op to an initial value z and all elements of this collection, going left to right. Shown below is an example of its usage.

Starting with an initial value of 0, foldLeft here applies the function (m, n) => m + n to each element in the List and the previous accumulated value.

val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val res = numbers.foldLeft(0)((m, n) => m + n)
println(res) // 55

Use cases

Suggested use cases for multiple parameter lists include:

SINGLE FUNCTIONAL PARAMETER

In case of a single functional parameter, like op in the case of foldLeft above, multiple parameter lists allow a concise syntax to pass an anonymous function to the method. Without multiple parameter lists, the code would look like this:

numbers.foldLeft(0, (m: Int, n: Int) => m + n)

Note that the use of multiple parameter lists here also allows us to take advantage of Scala type inference to make the code more concise, like this:

numbers.foldLeft(0)(_ + _)

this would not be possible with only a single parameter list, as the Scala compiler would not be able to infer the parameter types of the function.

IMPLICIT PARAMETERS

To specify only certain parameters as implicit, they must be placed in their own implicit parameter list.

An example of this is:

def execute(arg: Int)(implicit ec: scala.concurrent.ExecutionContext) = ???

PARTIAL APPLICATION

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. This is formally known as partial application.

For example:

val numbers = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val numberFunc = numbers.foldLeft(List[Int]()) _

val squares = numberFunc((xs, x) => xs :+ x*x)
println(squares) // List(1, 4, 9, 16, 25, 36, 49, 64, 81, 100)

val cubes = numberFunc((xs, x) => xs :+ x*x*x)
println(cubes)  // List(1, 8, 27, 64, 125, 216, 343, 512, 729, 1000)