Executing prolog code on an iPhone

Half a year later, I would just like to provide some insight on this. I ended up writing a server with an interface to prolog in Java, accepting prolog calls through TCP. It works almost exactly like the live prolog interpreter SWI-prolog (among others) provides, and mostly works quite well. However, it is far from an optimal solution, as you can't call functions from inside prolog. You lose the possibility of having two-way communication.

If I were to start all over again, I would certainly have tried harder to compile one of the pure C implementations for iOS. I gave it a quick go, but my lack of experience stopped me from even removing all of the errors I got. Judging by the fact that you cannot have prolog running as a background process on a unmodified version of iOS as well, some major rewriting would have to be done. Because of this, one might just have to write a new implementation (perhaps inspired by some of the more lightweight ones out there) from scratch to get the perfect solution.

Having faced similar difficulties calling prolog code, albeit in a different situation, I'd recommend checking out the castor c++ library. This allows you to write logic paradigm code in native c++ without needing to extend the language at all. As castor is a header only library it is easy to compile wherever c++ is available.
Castor website: http://www.mpprogramming.com/cpp/default.aspx