Partially Applied Functions

A partially applied function is a function that you do not apply any or all the arguments, creating another function. This partially applied function doesn't apply any arguments. 

def sum(a: Int, b: Int, c: Int) = a + b + c
val sum3 = sum _
sum3(1, 9, 7) should be(res0)
sum(4, 5, 6) should be(res1)

Partially applied functions can replace any number of arguments: 

def sum(a: Int, b: Int, c: Int) = a + b + c
val sumC = sum(1, 10, _: Int)
sumC(4) should be(res0)
sum(4, 5, 6) should be(res1)

Currying is a technique to transform a function with multiple parameters into multiple functions which each take one parameter: 

def multiply(x: Int, y: Int) = x * y
(multiply _).isInstanceOf[Function2[_, _, _]] should be(res0)
val multiplyCurried = (multiply _).curried
multiply(4, 5) should be(res1)
multiplyCurried(3)(2) should be(res2)
val multiplyCurriedFour = multiplyCurried(4)
multiplyCurriedFour(2) should be(res3)
multiplyCurriedFour(4) should be(res4)

Currying allows you to create specialized versions of generalized functions: 

def customFilter(f: Int => Boolean)(xs: List[Int]) =
  xs filter f
def onlyEven(x: Int) = x % 2 == 0
val xs = List(12, 11, 5, 20, 3, 13, 2)
customFilter(onlyEven)(xs) should be(res0)

val onlyEvenFilter = customFilter(onlyEven) _
onlyEvenFilter(xs) should be(res1)