Higher Order Functions

A higher order function is a function or method that:

Takes other functions as parameters
Has a result as a function

A method that takes a function

def process(x:Int, y:Int, f:(Int, Int) => Int) = f(x,y)

process: (x: Int, y: Int, f: (Int, Int) => Int)Int

The above example has three arguments: An Int, another Int, and a higher order function that takes two Int and returns an Int

To run the above we can run:

process(4, 6, (x, y) => x * y)

res5: Int = 24

process(4, 6, _ * _)

res6: Int = 24

A function that takes a function

Of course a function can also take a function

val process = (x:Int, y:Int, f:(Int, Int) => Int) => f(x, y)

process: (Int, Int, (Int, Int) => Int) => Int = <function3>

The above example is a function that has three arguments: An Int, another Int, and a higher order function that takes two Int and returns an Int

To run the above we can run:

process(4, 6, (x, y) => x * y)

res7: Int = 24

process(4, 6, _ * _)

res8: Int = 24

A function that returns a function

A function, or method can return a function
This is also a higher order function

val process = (x:Int, y:Int) => (z:Int) => x + y + z
val f2 = process(10, 10)
f2(3)

process: (Int, Int) => Int => Int = <function2>
f2: Int => Int = <function1>
res9: Int = 23

A method returning a function, note the def

def process(x:Int, y:Int) = (z:Int) => x + y + z
val f2 = process(10, 10)
f2(3) //23

process: (x: Int, y: Int)Int => Int
f2: Int => Int = <function1>
res10: Int = 23

Running both a def and a function as if they are they’re the same semantics is no accident. Though mechanically different after a while the difference between the two becomes blurry.