Scala Cookbook

5.4. Using Parameter Names When Calling a Method

Problem

You prefer a coding style where you specify the method parameter names when calling a method.

Solution

The general syntax for calling a method with named parameters is this:

methodName(param1=value1, param2=value2, ...)

This is demonstrated in the following example.

Given this definition of a Pizza class:

class Pizza {
  var crustSize = 12
  var crustType = "Thin"
  def update(crustSize: Int, crustType: String) {
    this.crustSize = crustSize
    this.crustType = crustType
  }
  override def toString = {
    "A %d inch %s crust pizza.".format(crustSize, crustType)
  }
}

you can create a Pizza:

val p = new Pizza

You can then update the Pizza, specifying the field names and corresponding values when you call the update method:

p.update(crustSize = 16, crustType = "Thick")

This approach has the added benefit that you can place the fields in any order:

p.update(crustType = "Pan", crustSize = 14)

Discussion

You can confirm that this example works by running it in the Scala REPL:

scala> val p = new Pizza
p: Pizza = A 12 inch Thin crust pizza.

scala> p.updatePizza(crustSize = 16, crustType = "Thick")

scala> println(p)
A 16 inch Thick crust pizza.

scala> p.updatePizza(crustType = "Pan", crustSize = 14)

scala> println(p)
A 14 inch Pan crust pizza.

The ability to use named parameters when calling a method is available in other languages, including Objective-C. Although this approach is more verbose, it can also be more readable.

This technique is especially useful when several ...