1.14.2 Non-deterministic predicates

Non-deterministic predicates are defined using PREDICATE_NONDET(plname, cname, arity) or NAMED_PREDICATE_NONDET(plname, cname, arity).

A non-deterministic predicate returns a “context” , which is passed to a subsequent retry. Typically, this context is allocated on the first call to the predicate and freed when the predicate either fails or does its last successful return (the context is nullptr on the first call). To simplify this, a template helper function PlControl::context_unique_ptr<ContextType>() provides a “smart pointer” that frees the context on normal return or an exception; when used with PL_retry_address(), the context's std:unique_ptr<ContextType>::release() is used to pass the context to Prolog for the next retry, and to prevent the context from being freed. If the predicate is called with PL_PRUNE, the normal return true will implicitly free the context.

The skeleton for a typical non-deterministic predicate is as follows. The test for PL_PRUNED is done first to avoid an unneeded PlFrame and also to ensure that A1, A2, etc. aren't used when they have the value PlTerm::null.^{31This code could be structured as a switch statement, but typically the PL_FIRST_CALL case falls through to the PL_REDO case.} There are a number of examples of non-deterministic predicates in the test code test_cpp.cpp.

struct PredContext { ... }; // The "context" for retries

PREDICATE_NONDET(pred, <arity>)
{ // "ctxt" must be acquired so that the destructor deletes it
  auto ctxt = handle.context_unique_ptr<PredContext>();
  const auto control  = handle.foreign_control();
  if ( control == PL_PRUNED )
    return true;

  // Can use A1, A2, etc. after we know control != PL_PRUNED

  if ( ... ) // deterministic result
  { assert(control == PL_FIRST_CALL);
    if ( ... )
      return true;  // Success (and no more solutions)
    else
      return fase;
  }

  if ( control = PL_FIRST_CALL )
  { ctxt.reset(new PredContext(...));
    ...
  } else
  { assert(control == PL_REDO);
  }

  PlFrame fr;
  for ( ; ctxt->valid(...) ; ctxt->next() )
  { if ( ... unify a result ... )
    { ctxt->next();
      if ( ctxt->valid(...) )
        PL_retry_addresss(ctxt.release()); // Succeed with a choice point
      else
        return true; // deterministic success
    }
    fr.rewind();
  }

  return false;
}