Quintus Prolog

Introduction

Quintus Prolog provides tools making it possible to call Prolog predicates from foreign languages. This is useful for a number of reasons.

In the simplest case, a foreign function may want to get at information that is contained in the Prolog database. It might be inconvenient or unnatural for the foreign function to return to Prolog, Prolog to query its database and reinvoke foreign code.
Foreign functions might also want to make use of Prolog's inferencing capabilities, again without having to contort algorithms.
When Quintus Prolog is embedded in another application, that application invokes Prolog by calling it from foreign functions. This aspect of calling Prolog from foreign functions will be handled fully in the section on embedding Prolog (see fli-emb).

To take advantage of Quintus Prolog's ability to be called from foreign functions, you must first know how to do either of the following:

load foreign code into Quintus Prolog; see fli-p2f
embed Quintus Prolog in another application; see fli-emb

Currently, only functions written in C can call Prolog predicates directly; this restriction may be lifted in future releases of Quintus Prolog. However, other languages such as Pascal and FORTRAN can call Prolog by virtue of their ability to call C. foreign code!) calls Prolog. The FORTRAN and Pascal stuff will then help users call the C functions. It just wasn't making sense writing all of this in terms of generic "foreign languages", because I knew full well I'd have to eventually do something different for FORTRAN and Pascal. While this should be more clear for release 3, it may as a result require more rewriting in case we ever do make Prolog directly callable from FORTRAN and Pascal.}

The interface between C and Prolog supports direct exchange of Prolog's atomic data types (atoms, integers or database reference). The data is automatically converted between Prolog's internal representation and the internal representation of the foreign language. The interface also supports passing any Prolog term from C and returning any Prolog term to C.