vvviiimmm

// A simple sum type that has two options
sealed trait Option[A]

// A value of some type 'A' (product type with a single field)
case class Some[A](value: A) extends Option[A]

// Type constructor that represents absence of a value
case class None[A]() extends Option[A]

// Instead of throwing an exception or crashing the function will

vvviiimmm

object MyProgram extends App {
  unsafeRun(             //       <-- This guy takes your pure program 
                         //           and actually runs it
    printResult(         // --
      doSomethingElse(   //   | 
        doThat(          //   |
          doThis()       //   | This is your 'pure' part, nothing happened yet
        )                //   |
      )                  //   |
    )                    // --

vvviiimmm

object MyProgram extends App {
  doThis()
  doThat()
  doSomethingElse()
  printResult()
}

vvviiimmm

-- Add 1 to every item in the list
map (+1) [1,2,3,4,5] -- [2,3,4,5,6]

-- Sum all the values
foldl' (+) 0 [1,2,3,4,5] -- 15

-- Multiply each number by 2, convert to string and 
-- produce a total string appending the results together
foldl' (\acc n -> acc ++ show (n * 2)) "" [1,2,3,4,5] -- "246810"

vvviiimmm

// Pass a function to transform every item in the list
List(1,2,3,4,5).map(n => n + 1) // List(2,3,4,5,6)

// Or shorter syntax
List(1,2,3,4,5).map(_ * 2) // List(2,4,6,8,10)

// Fold example. Sum all the values in a list
List(1,2,3,4,5).fold(0)((l, r) => l + r) // 15

// Multiply each number by 2, convert to string and 

vvviiimmm

-- Similar to Scala version, List is just a sum type with two options
-- [] is a type constructor for an empty list
mySum :: [Int] -> Int
mySum [] = 0
mySum (x : xs) = x + (mySum xs)

-- We will step through the list building a sum expression until we hit
-- the empty list case which returns 0. That will close the loop and
-- the built up expression will be summed up

vvviiimmm

// 'List' is defined as a sum type so we can use pattern matching.
// There are two type constructors we need to check: one for empty list and
// one for head and tail:
def sum(lst: List[Int]): Int = lst match {
  case Nil => 0 // Nil is a type constructor for an empty list
  case x :: xs => x + sum(xs) // x :: xs looks weird but it's actually just
                              // a product type named '::' and can be rewritten as
                              // case ::(x, xs) => ...
                              // Scala allows infix operators for type constructors
                              // so it's possible to say 'case x :: xs'

vvviiimmm

isPositive :: Int -> Bool
isPositive number = undefined -- compiles but throws exception when called 

-- Or
isPositive number = error "meh" -- same

vvviiimmm

// Just an example, don't do this at home
def isPositive(number: Int): Boolean = {
  if (number == 0)
    throw new Exception("meh") // Partial (non total)
  else {
    println("Calling isPositive!") // Impure (writing to std out is a side effect)

    if (System.currentTimeMillis() % 2 == 0) // Non deterministic
      number > 0
    else

vvviiimmm

data Shape
  = Circle { radius :: Double }
  | Rectangle { width :: Double
              , height :: Double }

-- In haskell different options of a sum type can
-- be handled with different 'functions'
area :: Shape -> Double
area (Circle r) = pi * r * r
area (Rectangle w h) = w * h -- width and height are captured in 'w' and 'h'