How would you write this code using Functional programming - oop

Question

I am learning Object Oriented and Functional programming. I found this very good and brief example (if you know better example, please suggest) on youtube explaining the OOP in 10 minutes, yay!
https://www.youtube.com/watch?v=4c4nP7GLL1c
Now, I would like to learn how to write this code in a Functional way - with immutability, recursion no loops etc.
DEMO.PHP
<?php
Include_once(“player.php”);
$p1 = new player();
$p1->name = “Brutus”;
$p1->type = “Warrior”;
$p1->hitpoints = 100;
$p2 = new player();
$p2->name = “Mysterio”;
$p2->type = “Sorcerer”;
$p2->hitpoints = 100;
$p1->meleeAttack($p2);
$p2->meleeAttack($p1);
Echo “<h1> Fight Results</h1>”;
Echo “<h2>.$p1->name.” now has “.$p1->hitpoints.” hit points</h2>”;
Echo “<h2>.$p2->name.” now has “.$p2->hitpoints.” hit points</h2>”;
?>
PLAYER.PHP
<?php
Class player {
public $name;
public $type;
public $hitpoints;
public function meleeAttack($target){
$current_hp = $target->hitpoints; // hp is for hitpoints
$damage = rand(10,20);
if($this->type == “Warrior”) { // condition logic
$damage += 10;
}
$target->hitpoints = $current_hp - $damage;
}
}
?>
Cheers,
P.S. I know, PHP is not a language of choice for functional programming, no need to remind it :)

Answer 1

I'll explain how to do functional programming using Haskell because I don't know PHP:
import System.Random (randomRIO)
-- Data Declarations
data Class = Warrior | Sorcerer deriving Eq
data Player = Player { name :: String
, archetype :: Class
, hitpoints :: Int
}
-- Example Data
p1 :: Player
p1 = Player "Brutus" Warrior 100
p2 :: Player
p2 = Player "Mysterio" Sorcerer 100
-- Code Section
meleeAttack :: Player -> Player -> IO Player
source `meleeAttack` target = do
damage <- randomRIO (10, 20)
let bonus = if archetype source == Warrior then 10 else 0
let newHP = hitpoints target - damage - bonus
return target { hitpoints = newHP }
-- Main Function
main :: IO ()
main = do
p2 <- p1 `meleeAttack` p2
p1 <- p2 `meleeAttack` p1
putStrLn "Fight Results"
putStrLn (name p1 ++ " now has " ++ show (hitpoints p1) ++ " hitpoints")
putStrLn (name p2 ++ " now has " ++ show (hitpoints p2) ++ " hitpoints")
Alright, so one of the biggest differences between object oriented programming and functional programming is separation of code and data:
In object oriented programming code and data are not kept separate. Just look at your own Player class. It has three public fields and a public method that operates on those fields.
In functional programming code and data are kept separate. Our data declarations are at the beginning of the program and the functions operating on that data come afterwards.
Another big difference is mutability. In object oriented programming, objects are usually mutable. Hence, methods usually modify an existing object. In functional programming mutation is considered bad. Hence, functions usually never modify anything. Instead, they return an updated copy of whatever data they were manipulating.
For example, consider the meleeAttack function. It takes two inputs, source and target, which are both Player data. The output is also Player data. It's the updated target. The original target is left unmodified.
Anyway, if you want to learn functional programming then you should read Learn You A Haskell.