1.2 A simple example

Here is the “simple example” in the Foreign Language Interface, rewritten in C++. As before, it is compiled by

  swipl-ld -o calc -goal true calc.cpp calc.pl

The Prolog file is simple:

calc(String) :-
    term_string(Expr, String),
    A is Expr,
    writeln(A).

The C part of the application parses the command line options, initialises the Prolog engine, locates the calc/1 predicate and calls it. The code is in figure 1.

#include <string>
#include <SWI-cpp2.h>

int
main(int argc, char **argv) {

  PlEngine e(argv[0]);

  // combine all the arguments in a single string
  std::string expression;
  for (int n = 1; n < argc; n++) {
    if (n != 1) {
      expression.append(" ");
    }
    expression.append(argv[n]);
  }

  // Lookup calc/1 and make the arguments and call

  PlPredicate pred("calc", 1, "user");
  PlTerm_string h0(expression);
  PlQuery q(pred, PlTermv(h0), PL_Q_NORMAL);

  return PlWrap<int>(q.next_solution()) ? 0 : 1;
}