6.5.5 Reading a goal from a string

A simple way to call arbitrary Prolog code is to use SP_read_from_string() (see Creating Prolog Terms) to create an argument to call/1. It is a good idea to always explicitly specify the module context when using call/1 or other meta-predicates from C.

This example calls a compound goal (without error checking):

     SP_pred_ref call_pred = SP_predicate("call", 1, "prolog");
     SP_term_ref x = SP_new_term_ref();
     SP_term_ref goal = SP_new_term_ref();
     SP_term_ref vals[] = {x, 0 /* zero termination */};
     long len;
     
     SP_put_variable(x);
     /* The X=_ is a trick to ensure that X is the first variable
        in the depth-first order and thus corresponds to vals[0] (x).
        There are no entries in vals for _,L1,L2.
     */
     SP_read_from_string(goal,
        "user:(X=_, length([0,1,2],L1), length([3,4],L2), X is L1+L2).", vals);
     
     SP_query(call_pred, goal);
     SP_get_integer(x, &len);
     /* here len is 5 */