I believe I have got the first Scheme application past Apple review into the iTunes App Store. It is yet another Reversi clone, called Reverso. It is a combination of 90% Scheme and 10% Objective-C, written with Gambit-C Scheme. James Long has already shown how to compile Gambit-C for the iPhone, and I started from there. My Scheme code is compiled to C by Gambit and later by GCC to produce native ARM code, bundled in a static library, which is ok with the iPhone SDK license agreement. The Objective-C then calls the library as a pure C library. The Scheme code deals with position evaluation, alpha-beta pruning, transposition tables, move legality, different strategies and so on. The Objective-C code deals with sound, animations, GUI, user preferences, basically everything that calls the iPhone OS API. Reversi was chosen because I like strategy games and it is much more algorithmic than artistic, and I am no artist.

The performance of the code is excellent. I used some Gambit-specific declarations, only fixnum arithmetic, pre-allocated a large heap, and called the garbage-collector every time the user needed to think. The search is not memory intensive, but the transposition tables are. I did not write a specific hash function but relied on Gambit-C’s table type. The boards used during search were retrieved from a pool (the newest Gambit-C has made subu8vector-move! a public API), so they did not put pressure on the garbage-collector. In the end it was a very successful experiment. Developing with Scheme is orders of magnitude more productive than with most other languages. Gambit-C is also one of the best Scheme compilers out there, and made my job a lot easier.

Update: There was a bug in the application that caused it to play poorly. The weak AI was not Gambit’s or Scheme’s fault, but mine. I already sent an updated version to Apple that will play much better. Besides fixing the bug, I am now using Zobrist’s hashing instead of Gambit’s own hash function, that performs poorly for game positions. It is still written in Scheme, though. Now I am waiting for the approval of the update, and to hear further feedback from my users.