A Proposal for a Foreign Language Interface to Prolog1

We show how the use of coroutining can lead to a simple but complete foreign language interface between Prolog and imperative programming languages such as C, Pascal or Basic. We show that both explicit and implicit coroutining must be supported. Explicit coroutining is achieved using the freeze/2 predicate. Implementation of the interface is straight-forward. The system has been used in an expert-system. Its user-interface was developed using Visual Basic. The rule set was implemented in Prolog. 1.0 Multi-paradigm programming and orthogonality Multi-paradigm programming[7,9] is a useful technique for the development of software. One approach to multi-paradigm programming is to select programming languages with non-overlapping features and gain the product of their capabilities by appropriate combination of their use. When a software system is developed using more than one programming language, there must be a mechanism that allows the languages to communicate with each other. One way this might be done is to use message-passing, either synchronous or asynchronous (see, for example [8]). In this method, programs communicate through the use of message-passing primitives such as send and receive. Encapsulation is obtained because the software designer must define the structure of the messages. That is, the message formats serve as a kind of signature for access to the data or control objects that can be transmitted. Furthermore, message-passing systems can be run as a single or multiple execution thread on one or several processors. A more mundane mechanism for communication between units is to use function or procedure calls. This is less flexible than message passing, but is most likely to be portable. Almost all programming languages allow procedure calls to other programming languages. Most commercial Prolog systems come with a facility that allows C functions to be called from Prolog. Some allow Prolog predicates to be called from other programming languages (see, for example, [11] and [2]). For the purposes of this paper, we are primarily concerned with foreign language access to Prolog, rather than Prolog access to foreign languages. The reason for this choice is that the application that motivated the development of the foreign-language nterface was an expert system whose front-end was developed using Visual Basic. In Visual Basic (and in similar visual systems like Borland’s Delphi), the top-level