Benefits of Referential Transparency in Functional Programming

An idiomatic Scala/LWJGL implementation of Space Invaders wouldn’t look that much like a Haskell/OpenGL implementation. Writing a Haskell implementation might be a better exercise in my opinion. But if you want to stick with Scala, here are some ideas for how to write it in functional style.

Try to use immutable objects only. You could have a Game object which holds a Player, a Set[Invader] (be sure to use immutable.Set), etc. Give Player an update(state: Game): Player (it could also take depressedKeys: Set[Int], etc), and give the other classes similar methods.

For randomness, scala.util.Random isn’t immutable like Haskell’s System.Random, but you could make your own immmutable generator. This one is inefficient but it demonstrates the idea.

case class ImmutablePRNG(val seed: Long) extends Immutable {
    lazy val nextLong: (Long, ImmutableRNG) =
        (seed, ImmutablePRNG(new Random(seed).nextLong()))
    ...
}

For keyboard/mouse input and rendering, there’s no way around calling impure functions. They’re impure in Haskell too, they’re just encapsulated in IO etc. so that your actual function objects are technically pure (they don’t read or write state themselves, they describe routines which do, and the runtime system executes those routines).

Just don’t put I/O code in your immutable objects like Game, Player and Invader. You can give Player a render method, but it should look like

render(state: Game, buffer: Image): Image

Unfortunately this doesn’t fit well with LWJGL since it’s so state-based, but you could build your own abstractions on top of it. You could have an ImmutableCanvas class that holds an AWT Canvas, and its blit (and other methods) could clone the underlying Canvas, pass it to Display.setParent, then perform the rendering and return the new Canvas (in your immutable wrapper).

Update: Here is some Java code showing how I would go about this. (I would have written almost the same code in Scala, except that an immutable set is built-in and a few for-each loops could be replaced with maps or folds.) I made a player which moves around and fires bullets, but I didn’t add enemies since the code was getting long already. I made just about everything copy-on-write – I think this is the most important concept.

import java.awt.*;
import java.awt.geom.*;
import java.awt.image.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;

import static java.awt.event.KeyEvent.*;

// An immutable wrapper around a Set. Doesn't implement Set or Collection
// because that would require quite a bit of code.
class ImmutableSet<T> implements Iterable<T> {
  final Set<T> backingSet;

  // Construct an empty set.
  ImmutableSet() {
    backingSet = new HashSet<T>();
  }

  // Copy constructor.
  ImmutableSet(ImmutableSet<T> src) {
    backingSet = new HashSet<T>(src.backingSet);
  }

  // Return a new set with an element added.
  ImmutableSet<T> plus(T elem) {
    ImmutableSet<T> copy = new ImmutableSet<T>(this);
    copy.backingSet.add(elem);
    return copy;
  }

  // Return a new set with an element removed.
  ImmutableSet<T> minus(T elem) {
    ImmutableSet<T> copy = new ImmutableSet<T>(this);
    copy.backingSet.remove(elem);
    return copy;
  }

  boolean contains(T elem) {
    return backingSet.contains(elem);
  }

  @Override public Iterator<T> iterator() {
    return backingSet.iterator();
  }
}

// An immutable, copy-on-write wrapper around BufferedImage.
class ImmutableImage {
  final BufferedImage backingImage;

  // Construct a blank image.
  ImmutableImage(int w, int h) {
    backingImage = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
  }

  // Copy constructor.
  ImmutableImage(ImmutableImage src) {
    backingImage = new BufferedImage(
        src.backingImage.getColorModel(),
        src.backingImage.copyData(null),
        false, null);
  }

  // Clear the image.
  ImmutableImage clear(Color c) {
    ImmutableImage copy = new ImmutableImage(this);
    Graphics g = copy.backingImage.getGraphics();
    g.setColor(c);
    g.fillRect(0, 0, backingImage.getWidth(), backingImage.getHeight());
    return copy;
  }

  // Draw a filled circle.
  ImmutableImage fillCircle(int x, int y, int r, Color c) {
    ImmutableImage copy = new ImmutableImage(this);
    Graphics g = copy.backingImage.getGraphics();
    g.setColor(c);
    g.fillOval(x - r, y - r, r * 2, r * 2);
    return copy;
  }
}

// An immutable, copy-on-write object describing the player.
class Player {
  final int x, y;
  final int ticksUntilFire;

  Player(int x, int y, int ticksUntilFire) {
    this.x = x;
    this.y = y;
    this.ticksUntilFire = ticksUntilFire;
  }

  // Construct a player at the starting position, ready to fire.
  Player() {
    this(SpaceInvaders.W / 2, SpaceInvaders.H - 50, 0);
  }

  // Update the game state (repeatedly called for each game tick).
  GameState update(GameState currentState) {
    // Update the player's position based on which keys are down.
    int newX = x;
    if (currentState.keyboard.isDown(VK_LEFT) || currentState.keyboard.isDown(VK_A))
      newX -= 2;
    if (currentState.keyboard.isDown(VK_RIGHT) || currentState.keyboard.isDown(VK_D))
      newX += 2;

    // Update the time until the player can fire.
    int newTicksUntilFire = ticksUntilFire;
    if (newTicksUntilFire > 0)
      --newTicksUntilFire;

    // Replace the old player with an updated player.
    Player newPlayer = new Player(newX, y, newTicksUntilFire);
    return currentState.setPlayer(newPlayer);
  }

  // Update the game state in response to a key press.
  GameState keyPressed(GameState currentState, int key) {
    if (key == VK_SPACE && ticksUntilFire == 0) {
      // Fire a bullet.
      Bullet b = new Bullet(x, y);
      ImmutableSet<Bullet> newBullets = currentState.bullets.plus(b);
      currentState = currentState.setBullets(newBullets);

      // Make the player wait 25 ticks before firing again.
      currentState = currentState.setPlayer(new Player(x, y, 25));
    }
    return currentState;
  }

  ImmutableImage render(ImmutableImage img) {
    return img.fillCircle(x, y, 20, Color.RED);
  }
}

// An immutable, copy-on-write object describing a bullet.
class Bullet {
  final int x, y;
  static final int radius = 5;

  Bullet(int x, int y) {
    this.x = x;
    this.y = y;
  }

  // Update the game state (repeatedly called for each game tick).
  GameState update(GameState currentState) {
    ImmutableSet<Bullet> bullets = currentState.bullets;
    bullets = bullets.minus(this);
    if (y + radius >= 0)
      // Add a copy of the bullet which has moved up the screen slightly.
      bullets = bullets.plus(new Bullet(x, y - 5));
    return currentState.setBullets(bullets);
  }

  ImmutableImage render(ImmutableImage img) {
    return img.fillCircle(x, y, radius, Color.BLACK);
  }
}

// An immutable, copy-on-write snapshot of the keyboard state at some time.
class KeyboardState {
  final ImmutableSet<Integer> depressedKeys;

  KeyboardState(ImmutableSet<Integer> depressedKeys) {
    this.depressedKeys = depressedKeys;
  }

  KeyboardState() {
    this(new ImmutableSet<Integer>());
  }

  GameState keyPressed(GameState currentState, int key) {
    return currentState.setKeyboard(new KeyboardState(depressedKeys.plus(key)));
  }

  GameState keyReleased(GameState currentState, int key) {
    return currentState.setKeyboard(new KeyboardState(depressedKeys.minus(key)));
  }

  boolean isDown(int key) {
    return depressedKeys.contains(key);
  }
}

// An immutable, copy-on-write description of the entire game state.
class GameState {
  final Player player;
  final ImmutableSet<Bullet> bullets;
  final KeyboardState keyboard;

  GameState(Player player, ImmutableSet<Bullet> bullets, KeyboardState keyboard) {
    this.player = player;
    this.bullets = bullets;
    this.keyboard = keyboard;
  }

  GameState() {
    this(new Player(), new ImmutableSet<Bullet>(), new KeyboardState());
  }

  GameState setPlayer(Player newPlayer) {
    return new GameState(newPlayer, bullets, keyboard);
  }

  GameState setBullets(ImmutableSet<Bullet> newBullets) {
    return new GameState(player, newBullets, keyboard);
  }

  GameState setKeyboard(KeyboardState newKeyboard) {
    return new GameState(player, bullets, newKeyboard);
  }

  // Update the game state (repeatedly called for each game tick).
  GameState update() {
    GameState current = this;
    current = current.player.update(current);
    for (Bullet b : current.bullets)
      current = b.update(current);
    return current;
  }

  // Update the game state in response to a key press.
  GameState keyPressed(int key) {
    GameState current = this;
    current = keyboard.keyPressed(current, key);
    current = player.keyPressed(current, key);
    return current;
  }

  // Update the game state in response to a key release.
  GameState keyReleased(int key) {
    GameState current = this;
    current = keyboard.keyReleased(current, key);
    return current;
  }

  ImmutableImage render() {
    ImmutableImage img = new ImmutableImage(SpaceInvaders.W, SpaceInvaders.H);
    img = img.clear(Color.BLUE);
    img = player.render(img);
    for (Bullet b : bullets)
      img = b.render(img);
    return img;
  }
}

public class SpaceInvaders {
  static final int W = 640, H = 480;

  static GameState currentState = new GameState();

  public static void main(String[] _) {
    JFrame frame = new JFrame() {{
      setSize(W, H);
      setTitle("Space Invaders");
      setContentPane(new JPanel() {
        @Override public void paintComponent(Graphics g) {
          BufferedImage img = SpaceInvaders.currentState.render().backingImage;
          ((Graphics2D) g).drawRenderedImage(img, new AffineTransform());
        }
      });
      addKeyListener(new KeyAdapter() {
        @Override public void keyPressed(KeyEvent e) {
          currentState = currentState.keyPressed(e.getKeyCode());
        }
        @Override public void keyReleased(KeyEvent e) {
          currentState = currentState.keyReleased(e.getKeyCode());
        }
      });
      setLocationByPlatform(true);
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setVisible(true);
    }};

    for (;;) {
      currentState = currentState.update();
      frame.repaint();
      try {
        Thread.sleep(20);
      } catch (InterruptedException e) {}
    }
  }
}

It is certainly possible to program in a purely functional style in an imperative language. In fact, if you look at books like Effective Java or Java Concurrency in Practice, much of the advice in those books basically boils down to "don't use mutable state", "don't use side-effects", etc.

However, it may not always be a pleasant experience, and you may not be able to do everything you want to do.

You mentioned C specifically as an example. Unless I'm missing something, the purely functional subset of C is not Turing-complete, because there is no way to express iteration. C does not have Proper Tail Calls, so, you will eventually overflow the stack when trying to express something like iterating over an infinite list. You will need to use a loop for iteration, but loops rely on mutable state and side-effects.

Of course, the standard Turing-tarpit argument applies … you can do functional programming C by implementing a Haskell interpreter in C and then program in Haskell. But that's not how I interpret the OP's question.