Scala Higher order functions & Currying

Higher order functions

These are functions that take a function as a parameter or return functions.

// 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      

Currying

Converting a function with multiple arguments into a function with a single argument that returns another function.

def f(a: Int, b: Int): Int // uncurried version (type is (Int, Int) => Int)
def f(a: Int)(b: Int): Int // curried version (type is Int => Int => Int)