Giving up the Object-Oriented Ghost

@morganlaco

github.com/mlaco

hello@morganlaco.com

Ghosts

Ghosts

• Objects
• Mutability
• elsif
• Loops

What is functional programming?

Functional vs Imperative

Lambda Calculus

Lambda Calculus

0 ≡ λf.λx.x
1 ≡ λf.λx.f x
2 ≡ λf.λx.f (f x)
3 ≡ λf.λx.f (f (f x))

Lambda Calculus

2 ≡ λf.λx.f (f x)

fn(f) {
  return fn(x) {
    return f(f(x))
}}

Lambda Calculus

2 ≡ λf.λx.f (f x)

fn(f) {
  return fn(x) {
    return f(f(x))
}}

Ghost 1: Objects

The Fours Awakens

github.com/mlaco/ConnectFour

class ConnectFour::Game
  def initialize
    @board = ConnectFour::Board.new
    # other instance variables
  end
  
  def play
    while continue_game
      get_move
      handle_move
    end
    
    display_result
  end
  # ...
end

ConnectFour::Game.new.play

defmodule ConnectFour do
  def start(_type, _args) do
    ConnectFour.initial_state
    |> do_turn
  end
  
  def do_turn(state) do
    ConnectFour.IO.display_board(state[:board])
    
    case get_new_state() do
      # ...
      new_state ->
        if ConnectFour.WinChecker.check(new_state[:board]) do
          ConnectFour.IO.do_win(new_state)
        else  
          do_turn(new_state)
        end
    end
  end

ConnectFour.start

Ghost 2: elsif

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

0

1

-1

Key

#

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

h

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

h

t

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

h

t

Pattern Matching

Pattern Matching

[h|t] = col_list

  def check(remaining, []) do
    [next_row | other_rows] = remaining
    check(other_rows, next_row)
  end
  
  def check([], row) do
    check_row(row)
  end

  def check(remaining, row) do
    if check_row(row) do
      true
    else
      [next_row | other_rows] = remaining
      check(other_rows, next_row)
    end
  end

  def check(remaining, row) do
    if Enum.empty?(row) do
      [next_row | other_rows] = remaining
      check(other_rows, next_row)
    else
      if !Enum.empty?(remaining) do
        check_row(row)
      else
        if check_row(row) do
          true
        else
          [next_row | other_rows] = remaining
          check(other_rows, next_row)
        end
      end
    end
  end

  def find_top(col, n\\0) do
    col_list = Tuple.to_list(col)
    [h|t] = col_list
    
    cond do
      !Enum.any?(t) && h != 0 ->
        {:error}
      h == 0 ->
        {:ok, n}
      true ->
        List.to_tuple(t) |> find_top(n+1)
    end
  end

Ghost n: Mutability

What the fuzz?

immutability

before

after

foo

foo

foo = 4
foo = 5

4

5

immutability

before

after

foo

foo

foo = 4
foo = 5

4

5

4

foo

Why variables can be re-bound

Benefits of Immutability

FP in other languages

http://www.lispcast.com/imperative-mindset

Ghost n: Loops

Loops

(1..100).each do |i|
  do_something(i)
end

No Loops - why tho?

Recursion

  def fibonacci(n) do
    cond do
      n == 0
        0
      n == 1
        1
      true ->
        fibonacci(n, 0, 1)
    end
  end

  def fibonacci(n, a, b) do
    if n <= 0 do
      a
    else
      fibonacci(n - 1, b, a + b)
    end
  end

Recursion

def self.check(grid)
  grid.each_with_index do |column, column_index|
    n = 1
    last = 0
    column.each_with_index do |cell, row_index|
      if cell == last && cell != 0
        n += 1
      else
        n = 1
      end
      last = cell
      
      return true if n == 4
    end
  end

  def check_horizontal(board) do
    Transpose.transpose(board)
    |> check([])
  end

  def check(remaining, []) do
    [next_row | other_rows] = remaining
    check(other_rows, next_row)
  end
  
  def check([], row) do
    check_row(row)
  end

  def check(remaining, row) do
    if check_row(row) do
      true
    else
      [next_row | other_rows] = remaining
      check(other_rows, next_row)
    end
  end

  def check([next_row | other_rows], []) do
    check(other_rows, next_row)
  end
  
  # Equivalent to:
  def check(remaining, []) do
    [next_row | other_rows] = remaining
    check(other_rows, next_row)
  end

Tail Recursion

tail recursion

how does it work?

fibonacci(n)

fibonacci(n)

fibonacci(n)

fibonacci(n)

<process>

def fibonacci(n) do
  cond do
    n == 0
      0
    n == 1
      1
    true ->
      fibonacci(n, 0, 1)
  end
end

def fibonacci(n, a, b) do
  if n <= 0 do
    a
  else
    fibonacci(n - 1, b, a + b)
  end
end

tail recursion

how does it work?

fibonacci(n)

fibonacci(n)

fibonacci(n)

fibonacci(n)

<process>

def fibonacci(n) do
  cond do
    n == 0
      0
    n == 1
      1
    true ->
      fibonacci(n, 0, 1)
  end
end

def fibonacci(n, a, b) do
  if n <= 0 do
    a
  else
    fibonacci(n - 1, b, a + b)
  end
end

Loops

Enum.reduce( (1..100), \
  fn(x, acc) -> x + acc end )
  # => 5050

Enum.map( (1..5), \
  fn(x) -> 2 * x end )
  # => [2, 4, 6, 8, 10]

Enum.filter( (1..5), \
  fn(x) -> rem(x, 2) == 0 end )
  # => [2, 4]

What I told you was true...

Ghosts Review

resources

1. From Imperative to Functional and Back-Monads are for Functional Languages
2. From Imperative to Functional: How to Make the Leap
3. A Practical Introduction to Functional Programming
4. StackOverflow: What are the core concepts in functional programming?
5. Elixir Getting Started Guide
6. Destroy All Ifs
7. 16 Months of Functional Programming
8. Learn You Haskell
9. Is object immutability in functional programming inherently performance intensive?
10. How to Switch from the Imperative Mindset
11. Functional Programming Techniques With Ruby: Part I
12. From Imperative to Functional Programming (for Absolute Beginners)
13. Transposing a Matrix: Thinking Recursively in Elixir
14. Don’t Be Scared Of Functional Programming
15. Myth of the Day: Functional Programmers Don't Use Loops

THANK YOU!!!