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About a month ago I announced my then-latest programming project, Star Rover 2. It’s a space flying game written in Haskell, and while programming that was fun and I learnt a lot from the project, it was time to find something else to do. So I started another soccer project, again in Haskell (like in September 2008), but with a bit more Haskell experience under my belt.

And now, after almost a month, it’s time for the first release. The new soccer game is called freekick2, and it’s an open source 2D arcade style soccer game.

If you have cabal, you can install it with a simple cabal update && cabal install freekick2. Here’s a screen  shot from a menu:

The freekick2 distribution includes only a few fantasy teams, but you can use the data files from a Sensible Soccer game for some additional realism.

One lesson I learnt when doing Star Rover 2 is that I should avoid over-engineering like plague, as that was probably one reason why I couldn’t get the first Freekick versions in a releaseable state.

Freekick2 now consists of about 3200 lines of Haskell code, around 1000 more than Star Rover 2, and it’s also a bit fancier, graphics-wise. With Star Rover 2 I used monad transformers to handle the main loop and state changes, which worked pretty well, and I’m using the same technique with freekick2. I still haven’t figured out the whole FRP thing, but game programming in Haskell seems to work out pretty well without.

Things I’ve learnt while programming freekick2:

* Texture mapping with OpenGL in Haskell. To load the textures you need to play around with pointers a bit, which are not really that usual in Haskell. But you can do it, and with the help of some great small libraries like pngload it’s relatively simple in the end. Adding dynamic recoloring of the textures after loading them (to support multiple kit colors) was fun as well.

* Parsing binary data in Haskell. Over a year ago, when I was writing Freekick in C++, I wanted to deserialize the SWOS data files (the format’s known; many thanks to the ones who figured the format out) and I thought parsing binary data sounds like a job for C or C++. The result from back then is about 850 lines long and took a few hours to write. Granted, it also generates some XML. Now I wrote a program to do the same task in Haskell; the result has 250 lines and was written in less than two hours. The Haskell version also includes code for writing SWOS data files – a feature I added just because it was so easy to do. Later, for the freekick-internal data structures, I let BinaryDerive derive the serialization/deserialization code for me. Needless to say, I won’t be doing binary data parsing in C anymore if I get to choose.

* Template Haskell. The state in freekick2 changes pretty often, and is managed in a State monad. I needed a lot of functions of type (FieldType -> FieldType) -> State -> State (like already with Star Rover 2), usually one for each field in a record, and so I let TH generate them all for me. Bulat’s tutorials (links to them can be found here) were a life saver.

* Implementing a GUI with colors, textures and dynamic content, with a tree-like hierarchy for screens.

* Use of the existential quantification language extension for heterogeneous collections. I’ve spent some time with object oriented design in the past, which sometimes leads to object-oriented designs in Haskell, so that something like existential types are needed. In this concrete case, I created the class Sprite, with both the player and the ball implementing it. The easiest way to draw all sprites sorted by depth is then to just put all sprites in one list and sort it. Without existential quantification, I would’ve had to write code to sort the players and then call the draw method of the ball somewhere between the draw methods of the players.

So, programming in Haskell is more fun than ever, and the productivity boost given by Haskell is starting to show more and more.