11.8.6 Compatibility of the Foreign Interface

The term reference mechanism was first used by Quintus Prolog version 3. SICStus Prolog version 3 is strongly based on the Quintus interface. The described SWI-Prolog interface is similar to using the Quintus or SICStus interfaces, defining all foreign-predicate arguments of type +term. SWI-Prolog explicitly uses type functor_t, while Quintus and SICStus use <name> and <arity>. As the names of the functions differ from Prolog to Prolog, a simple macro layer dealing with the names can also deal with this detail. For example:

#define QP_put_functor(t, n, a) \
        PL_put_functor(t, PL_new_functor(n, a))

The PL_unify_*() functions are lacking from the Quintus and SICStus interface. They can easily be emulated, or the put/unify approach should be used to write compatible code.

The PL_open_foreign_frame()/PL_close_foreign_frame() combination is lacking from both other Prologs. SICStus has PL_new_term_refs(0), followed by PL_reset_term_refs(), that allows for discarding term references.

The Prolog interface for the graphical user interface package XPCE shares about 90% of the code using a simple macro layer to deal with different naming and calling conventions of the interfaces.