[djcjr]

IMO, an interpreter is the best way to interface between two systems, to which you feed a string or a file of your "language". This is the most flexible, and can even be separated into different versions, without ever breaking existing clients, so long as specifying the version is possible in your "language".

Thus, I think what makes API's so difficult is that they roughly approximate an interpreter, and in a particularly inflexible way.