Is a functional language a good choice for a Fligh

Question

I have been doing object-oriented programming for a few years now, and I have not done much functional programming. I have an interest in flight simulators, and am curious about the functional programming aspect of Lisp. Flight simulators or any other real world simulator makes sense to me in an object-oriented paradigm.

Here are my questions:

Is object oriented the best way to represent a real world simulation domain?

I know that Common Lisp has CLOS (OO for lisp), but my question is really about writing a flight simulator in a functional language. So if you were going to write it in Lisp, would you choose to use CLOS or write it in a functional manner?

Does anyone have any thoughts on coding a flight simulator in lisp or any functional language?

UPDATE 11/8/12 - A similar SO question for those interested -> How does functional programming apply to simulations?

Answer 1

If you look at the game or simulator industry you find a lot of C++ plus maybe some added scripting component. There can also be tools written in other languages for scenery design or related tasks. But there is only very little Lisp used in that domain. You need to be a good hacker to get the necessary performance out of Lisp and to be able to access or write the low-level code. How do you get this knowhow? Try, fail, learn, try, fail less, learn, ... There is nothing but writing code and experimenting with it. Lisp is really useful for good software engineers or those that have the potential to be a good software engineer.

One of the main obstacles is the garbage collector. Either you have a very simple one (then you have a performance problem with random pauses) or you have a sophisticated one (then you have a problem getting it working right). Only few garbage collectors exist that would be suitable - most Lisp implementations have good GC implementations, but still those are not tuned for real-time or near real-time use. Exceptions do exist. With C++ you can forget the GC, because there usually is none.

The other alternative to automatic memory management with a garbage collector is to use no GC and manage memory 'manually'. This is used by some (even commercial) Lisp applications that need to support some real-time response (for example process control expert systems).

The nearest thing that was developed in that area was the Crash Bandicoot (and also later games) game for the Playstation I (later games were for the Playstation II) from Naughty Dog. Since they have been bought by Sony, they switched to C++ for the Playstation III. Their development environment was written in Allegro Common Lisp and it included a compiler for a Scheme (a Lisp dialect) variant. On the development system the code gets compiled and then downloaded to the Playstation during development. They had their own 3d engine (very impressive, always got excellent reviews from game magazines), incremental level loading, complex behaviour control for lots of different actors, etc. So the Playstation was really executing the Scheme code, but memory management was not done via GC (afaik). They had to develop all the technology on their own - nobody was offering Lisp-based tools - but they could, because their were excellent software developers. Since then I haven't heard of a similar project. Note that this was not just Lisp for scripting - it was Lisp all the way down.

One the Scheme side there is also a new interesting implementation called Ypsilon Scheme. It is developed for a pinball game - this could be the base for other games, too.

On the Common Lisp side, there have been Lisp applications talking to flight simulators and controlling aspects of them. There are some game libraries that are based on SDL. There are interfaces to OpenGL. There is also something like the 'Open Agent Engine'. There are also some 3d graphics applications written in Common Lisp - even some complex ones. But in the area of flight simulation there is very little prior art.

On the topic of CLOS vs. Functional Programming. Probably one would use neither. If you need to squeeze all possible performance out of a system, then CLOS already has some overheads that one might want to avoid.