Scala - High Order Function (Function as parameter or return object)

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Higher order functions are functions that take a function as a parameter or return functions.

Functions in functional programming are higher order functions since following actions can be performed with them.

Functions can be passed within functions as arguments.
Functions can be created within functions just as objects can be created in functions
Functions can be returned from functions

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Example

// sum() returns a function that takes two integers and returns an integer  
def sum(f: Int => Int): (Int, Int) => Int = {  
  def sumf(a: Int, b: Int): Int = {...}  
  sumf  
} 

// same as above. Its type is (Int => Int) => (Int, Int) => Int  
def sum(f: Int => Int)(a: Int, b: Int): Int = { ... } 

// Called like this
sum((x: Int) => x * x * x)          // Anonymous function, i.e. does not have a name  
sum(x => x * x * x)                 // Same anonymous function with type inferred

def cube(x: Int) = x * x * x  
sum(x => x * x * x)(1, 10) // sum of cubes from 1 to 10
sum(cube)(1, 10)           // same as above

Function as parameter

val numbers = List(1,2,3,4,5,6,7,8,9,10)
val total = numbers.foldLeft(0){(a,b) => a+b }

where the unction “foldLeft” is provided by Scala library on collections.