1.3.2 Adding numbers

This example shows arithmetic using the C++ interface, including unification, type-checking and conversion. The predicate add/3 adds the two first arguments and unifies the last with the result.

PREDICATE(add, 3)
{ return A3 = (long)A1 + (long)A2;
}

Casting a PlTerm to a long performs a PL_get_long() and throws a C++ exception if the Prolog argument is not a Prolog integer or float that can be converted without loss to a long. The = operator of PlTerm is defined to perform unification and returns TRUE or FALSE depending on the result.

?- add(1, 2, X).

X = 3.
?- add(a, 2, X).
[ERROR: Type error: `integer' expected, found `a']
   Exception: (  7) add(a, 2, _G197) ?