This simple example shows the basic definition of the predicate hello/1
and how a Prolog argument is converted to C-data:
PREDICATE(hello, 1)
{ cout << "Hello " << A1.as_string() << endl;
return true;
}
The arguments to PREDICATE()
are the name and arity of the predicate. The macros A<n>
provide access to the predicate arguments by position and are of the
type PlTerm. The C or C++ string for a PlTerm
can be extracted using as_string(),
or as_wstring() methods;26The
C-string values can be extracted from std::string by using c_str(),
but you must be careful to not return a pointer to a local/stack value,
so this isn't recommende. and similar access methods
provide an easy type-conversion for most Prolog data-types, using the
output of write/1
otherwise:
?- hello(world).
Hello world
Yes
?- hello(X)
Hello _G170
X = _G170
