Posts tagged game programming

PezHack–Abstracting flow control with monads

:: fsharp, game programming, roguelike

It's been forever since I last posted, I worked quite a bit on PezHack and then stopped for a while. I'm back to it now. In this post I will describe a technique I used to greatly reduce the amount of code and abstract away some repetitive imperative code.

The Problem

PezHack is a turn based game. The screen does not re-render itself all the time like a real-time game, but only when something changes and at the end of a the player's turn. The agent-based approach I used to separate the various sub systems of the game allow me to provide isolation around the graphics processing. The graphics agent is totally responsible for knowing what it needs to draw and will draw it on demand when it receives the relevant message. It does not really know anything else about the game state at all except the visual data of the various tiles that are visible, some player data that allow it to draw the various data about the player, and any menus / other UI that it needs to draw. Other systems can send it messages to inform it of some new visual state it should care about.

Most actions that Pezi can perform require some form of additional input and conditional flow logic. For example, when you press the 'e' key to Eat, first a menu is displayed that shows the stuff in your inventory that is edible, if possible. If not it will display a message and not end the player's turn. Assuming the menu is displayed, the player then presses the key that relates to the item they wish to eat from the menu. If this key is invalid a relevant message is displayed, else the item in question is eaten. What then happens is dependent on the item, it might provide sustenance, or if it's a mushroom it could perform one of various kinds of effects, and then probably end the player's turn

This is a common pattern that appears everywhere, and at each stage the graphics need to be re-drawn to show the various messages, menus and so on. At each stage it might continue or it might not depending the player's input, and the returns differs at each stage (end the player turn, or not, for example). In an imperative language we would almost certainly model this control flow with a bunch of nested if/then/else statements which quickly gets ugly. Because I am using the agent based approach for the graphics, I would also need to post a request to the graphics agent each and every time I needed the changes to show on the screen, so suddenly the actions become a list of imperative statements peppered with common code to keep the screen updated.

PezHack–A Functional Roguelike

:: fsharp, game programming, roguelike

In my quest to learn the functional paradigm, one thing I have struggled with is game development. Assuming I mostly stick to the functional style of having little to no mutable state, how do you go about writing games? Games are pretty much ALL mutable state. Obviously, in a multi-paradigm language like F# you can have mutable state - and if used judiciously this can be very effective (not to mention offer some significant performance improvements). The blend of imperative and functional styles can indeed work well, I wrote a few small games with XNA and F# using this approach. However, I am still more interested for educational value to stick with a more pure functional approach. Along they way I have wrote a few small games such as a functional console based Tetris in about 300 lines, and a two player console based PONG clone (using the libtcod library as I will introduce in a bit) that uses the Windows Kinect as the input (was cool!) In these programs I would tend to have the game state formed with an F# record that is copied/modified on each game cycle, passing the new state back into the loop. This works well and both of these games used no mutable state at all. This approach soon falls down with something more ambitious though, you can't realistically propagate the entire game state through one loop.

The Roguelike

I decided to create something a lot more complex whilst attempting to stick with the functional guns. If you don't know what a roguelike is you can read all about them here and a great set of development resources with links to many on-going roguelike efforts here. I used to play these games a lot. Stemmed from D&D back in the days where the only computers were the terminal sort in colleges in universities (before I was alive!), a roguelike traditionally uses just ASCII characters as its graphics, leaving the rest to the player's imagination. If you haven't tried this before I highly recommend you try one, Nethack is probably the biggest most complicated one out there, but you can also play the original Rouge (where the roguelike genre name comes from) online here. A few things that make a roguelike;

  • Random procedurally generated dungeons - every time you play there is a new dungeon
  • Randomness in item drops - until things have been identified, you don't know what they are. The "clear potion" might be a healing potion in one game and a potion that causes severe hallucinations in the next
  • Peramadeath and hard difficulty. These games are hard. Expect to die lots, and when you die you have to start again. Often the objective isn't to finish the game, but just see how long you can survive
  • Roguelikes are usually turn-based affairs - although there is some variation with this
  • Complexity - these games are amazingly complex and deep. The developer(s) don't have to worry about impressive graphics engines, so they can focus a lot more on interesting game mechanics. You will be amazed at some of the stuff you can do in a game like Nethack.

Here's a picture from Nethack to illustrate how a typical roguelike looks :