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Scala Higher Order Function


Using methods as function values

The Scala compiler will automatically convert methods into function values for the purpose of passing them into higher-order functions.

object MyObject {
 def mapMethod(input: Int): String = {
Seq(1, 2, 3).map(MyObject.mapMethod) // Seq("1", "2", "3")

In the example above, MyObject.mapMethod is not a function call, but instead is passed to map as a value. Indeed, map requires a function value passed to it, as can be seen in it's signature. The signature for the map of a List[A] (a list of objects of type A) is:

def map[B](f: (A) ⇒ B): List[B]

The f: (A) => B part indicates that the parameter to this method call is some function that takes an object of type A and returns an object of type B. A and B are arbitrarily defined. Returning to the first example, we can see that mapMethod takes an Int (which corresponds to A) and returns a String (which corresponds to B). Thus mapMethod is a valid function value to pass to map. We could rewrite the same code like this:

Seq(1, 2, 3).map(x:Int => int.toString)

This inlines the function value, which may add clarity for simple functions.

High Order Functions(Function as Parameter)

A higher-order function, as opposed to a first-order function, can have one of three forms:

• One or more of its parameters is a function, and it returns some value.

• It returns a function, but none of its parameters is a function.

•Both of the above: One or more of its parameters is a function, and it returns a function.

object HOF {
 def main(args: Array[String]) {
 val list =
 val fullNameList= => getFullName(n._1, n._2))
 def getFullName(firstName: String, lastName: String): String = firstName + "." +

Here the map function takes a getFullName(n._1,n._2) function as a parameter. This is called HOF (Higher order function).

Arguments lazy evaluation

Scala supports lazy evaluation for function arguments using notation: def func(arg: => String). Such function argument might take regular String object or a higher order function with String return type. In second case, function argument would be evaluated on value access.

Please see the example:

def calculateData: String = {
 print("Calculating expensive data! ")
 "some expensive data"
def dumbMediator(preconditions: Boolean, data: String): Option[String] = {
 print("Applying mediator")
 preconditions match {
 case true => Some(data)
 case false => None
def smartMediator(preconditions: Boolean, data: => String): Option[String] = {
 print("Applying mediator")
 preconditions match {
 case true => Some(data)
 case false => None
smartMediator(preconditions = false, calculateData)
dumbMediator(preconditions = false, calculateData)


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