foreach and the functional style

Introduction to Scala

David Venturi

Curriculum Manager, DataCamp

Scala is functional

Scala is functional

Functions are first-class values
Operations of a program should map input values to output values rather than change data in place

Scala is a imperative/functional hybrid

Scala usually is functional but can also be imperative sometimes

Scala is functional:

Functions are first-class values
Operations of a program should map input values to output values rather than change data in place

Scala nudges us towards the functional style

Polar bear nudging a panda bear

The word "functional"

"imperative" according to Oxford Dictionary

Imperative (English):

Definition: Of the nature of or expressing a command.

¹ https://www.thefreedictionary.com/imperative

The imperative style

Scala usually is functional but can also be imperative sometimes

Scala can be imperative:

One command at a time
Iterate with loops
Mutate shared state (e.g., mutating variables out of scope)
Examples: C, Java, Python

The imperative style

// Define counter variable
var i = 0

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// Loop through hands and see if each busts
while (i < hands.length) {
  println(bust(hands(i)))
  i = i + 1
}

The imperative style

// Define counter variable
var i = 0

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// Loop through hands and see if each busts
while (i < hands.length) {
  println(bust(hands(i)))
  i = i + 1
}

¹ http://bit.ly/state_wikipedia

The functional style

Scala usually is functional but can also be imperative sometimes

Scala is functional:

Functions are first-class values

The functional style

Functions are first-class values

// Define counter variable
var i = 0

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// Loop through hands and see if each busts
while(i < hands.length) {
  println(bust(hands(i)))
  i += 1
}

The functional style

Functions are first-class values

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// See if each hand busts
hands.foreach(INSERT FUNCTION HERE)

Scala fuses OOP and FP -> Scala is scalable

Functions are first-class values

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// See if each hand busts
hands.foreach(INSERT FUNCTION HERE)

From Chapter 1:

Scala combines object-oriented and functional programming

Modify the bust function

// Define a function to determine if hand busts
def bust(hand: Int) = {
  println(hand > 21)
}

The functional style

Functions are first-class values

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// See if each hand busts
hands.foreach(INSERT FUNCTION HERE)

The functional style

Functions are first-class values

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// See if each hand busts
hands.foreach(bust)

false
true
false

What is a side effect?

Scala usually is functional but can also be imperative sometimes

Scala is functional:

Functions are first-class values
Operations of a program should map input values to output values rather than change data in place

Side effect: code that modifies some variable outside of its local scope

¹ http://bit.ly/side_effect_wikipedia

What is a side effect?

def bust(hand: Int) = {
  println(hand > 21)
}

scala> val myHand = bust(22)

true
myHand: Unit = ()

What is a side effect?

// Define counter variable
var i = 0


// Initialize array with each player's hand
var hands = Array(17, 24, 21)


// Loop through hands and see if each busts
while (i < hands.length) {
  println(bust(hands(i)))
  i = i + 1
}

The spectrum of functional style

Less functional than before

// Define a function to determine if hand busts
def bust(hand: Int) = {
  println(hand > 21)
}

More functional than before

// Initialize array with each player's hand
var hands = Array(17, 24, 21)

// See if each hand busts
hands.foreach(bust)

¹ http://bit.ly/array_foreach_documentation

Signs of style

Imperative

var
Side effects
Unit

Functional

val
No side effects
Non-Unit value types
- Int
- Boolean
- Double

¹ http://bit.ly/scala_unit_documentation

Let's practice!

Introduction to Scala