functional programming

// Functional Programming

def incAll(list: List[Int]): List[Int] = {
    if(list.isEmpty) List.empty[Int]
    else list.head+1 :: incAll(list.tail)
}

def incAllBy10(list: List[Int]): List[Int] = {
    if(list.isEmpty) List.empty[Int]
    else list.head+10 :: incAll(list.tail)
}

// Or you can make a more adaptable version by adding a parameter
def incAll(byNum:Int, list:List[Int]): List[Int] = {
    if(list.isEmpty) List.empty[Int]
    else list.head+byNum :: incAll(byNum, list.tail)
}
def incAllBy10(list) = incAll(10, list)

// In this one the operator is different too
def doubleNums(list) =
    if(list.isEmpty) List.empty[Int]
    else list.head*2 :: doubleNums(list.tail)

// ok so let's do like an interpreter or some shit
def map(f: Int -> Int, list: List[Int]): List[Int] = {
    if(list.isEmpty) List.empty[Int]
    else f(list.head) :: map(f, list.tail)
}
def incAll(list) = map(x -> x+1, list)
def doubleNums(list) = map(x -> x*2, list)
def absNums(list) = map(x -> x.abs, list)
// this is called a lambda or anonymous function
// x -> x+1 is a function that takes x and returns x+1

/*
Functional Programming - taking functions seriously
- Functions as first-class entities

- we can put an int into a variable
- we can pass an int into a function
- we can return an int from a function
- we can store ints in data structures like list array etc
*/

// going back to the map() function

def capitalize(list: List[String]): List[String] = map(str -> str.toUpperCase(), list)

// we can add map[A] to account for type parameter
def map[A](f: A -> A, list: List[A]): List[A] = {
    if(list.isEmpty) List.empty[Int]
    else f(list.head) :: map(f, list.tail)
}

def convert(list: List[String]): List[Int] = map(str -> str.toInt, list)

// can also use
_ + 1
// instead of
x -> x+1

// also this
(x,y) -> y + x*2