2.14 The class PlQuery (version 2)

This class encapsulates the call-backs onto Prolog.

PlQuery :: PlQuery(const char *name, const PlTermv &av)

Create a query where name defines the name of the predicate and av the argument vector. The arity is deduced from av. The predicate is located in the Prolog module user.

PlQuery :: PlQuery(const char *module, const char *name, const PlTermv &av)

Same, but performs the predicate lookup in the indicated module.

int PlQuery::next_solution()

Provide the next solution to the query. Yields true if successful and false if there are no (more) solutions. Prolog exceptions are mapped to C++ exceptions.

void PlQuery::cut()()

Discards the query, but does not delete an of the data created by the query. If there is any pending Prolog exception, it is mapped to a C++ exception and thrown. The call to PlQuery::cut() is done implicitly by PlQuery’s destructor.

Below is an example listing the currently defined Prolog modules to the terminal.

PREDICATE(list_modules, 0)
{ PlTermv av(1);

  PlQuery q("current_module", av);
  while( q.next_solution() )
    cout << av[0].as_string() << endl;

  return true;
}

In addition to the above, the following functions have been defined.

int PlCall(const char *predicate, const PlTermv &av)

Creates a PlQuery from the arguments generates the first next_solution() and destroys the query. Returns the result of next_solution() or an exception.

int PlCall(const char *module, const char *predicate, const PlTermv &av)

Same, locating the predicate in the named module.

int PlCall(const wchar_t *goal)

int PlCall(const char *goal)

Translates goal into a term and calls this term as the other PlCall() variations. Especially suitable for simple goals such as making Prolog load a file.

2.14.1 The class PlFrame (version 2)

The class PlFrame provides an interface to discard unused term-references as well as rewinding unifications (data-backtracking). Reclaiming unused term-references is automatically performed after a call to a C++-defined predicate has finished and returns control to Prolog. In this scenario PlFrame is rarely of any use. This class comes into play if the toplevel program is defined in C++ and calls Prolog multiple times. Setting up arguments to a query requires term-references and using PlFrame is the only way to reclaim them.

PlFrame :: PlFrame()

Creating an instance of this class marks all term-references created afterwards to be valid only in the scope of this instance.

~ PlFrame()

Reclaims all term-references created after constructing the instance.

void PlFrame::rewind()

Discards all term-references and global-stack data created as well as undoing all unifications after the instance was created.

A typical use for PlFrame is the definition of C++ functions that call Prolog and may be called repeatedly from C++. Consider the definition of assertWord(), adding a fact to word/1:

void
assertWord(const char *word)
{ PlFrame fr;
  PlTermv av(1);

  av[0] = PlCompound("word", PlTermv(word));
  PlQuery q("assert", av);
  PlCheck(q.next_solution());
}

This example shows the most sensible use of PlFrame if it is used in the context of a foreign predicate. The predicate's thruth-value is the same as for the Prolog unification (=/2), but has no side effects. In Prolog one would use double negation to achieve this.

PREDICATE(can_unify, 2)
{ PlFrame fr;

  int rval = (A1=A2);
  fr.rewind();
  return rval;
}

PlRewindOnFail(f) is a convenience function that does a frame rewind if unification fails. Here is an example, where name_to_term contains a map from names to terms (which are made global by using the PL_record() function):

static const std::map<const std::string, record_t> name_to_term =
    { {"a", PlTerm(...).record()}, ...};

bool lookup_term(const std::string name, PlTerm result)
{ const auto it = name_to_term.find(name);
  if ( it == name_to_term.cend() )
    return false;

  PlTerm t = PlTerm_recorded(it->second);
  return PlRewindOnFail([result,t]() -> bool { return result.unify_term(t); });
}