Friday, April 5, 2013

Higher-Order Functions in Scala

The functions that most of us are familiar with take some type(s) as parameters and return some type as a result. These are called first order functions. Higher-order functions, however, are functions that take other functions as parameters and/or return a function as a result. In other words, higher-order functions act on other functions.

To demonstrate with a simple example, let's look at how we might define a first order function that takes two integer values and returns the sum of both values squared.

def sumOfSquares(a: Int, b: Int): Int = {
   a * a + b * b

Next, let's look at how we could refactor this to use a higher-order function. Here is a function that takes 3 parameters:
  • a function that takes an Int and returns an Int
  • an Int named a
  • an Int named b

def sumOfTwoOperations(f: Int => Int, a: Int, b: Int): Int = {
   f(a) + f(b)

Next, let's call the sumOfTwoOperations function, passing a "squared" function as the first parameter.

def squared(x: Int): Int = x * x

val result = sumOfTwoOperations(squared, 2, 5)   // result = 29

The beauty of higher-order functions is that now we can define another operation, such as "cubed", and pass that to the sumOfTwoOperations.

def cubed(x: Int): Int = x * x * x

val result = sumOfTwoOperations(cubed, 2, 5)   // result = 133

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