2.16 Exceptions (version 2)

Prolog exceptions are mapped to C++ exceptions using the subclass PlException of PlTerm to represent the Prolog exception term. All type-conversion functions of the interface raise Prolog-compliant exceptions, providing decent error-handling support at no extra work for the programmer.

For some commonly used exceptions, subclasses of PlException have been created to exploit both their constructors for easy creation of these exceptions as well as selective trapping in C++. Currently, these are PlTypeEror and PlDomainError, PlTermvDomainError, PlInstantiationError, PlExistenceError, PermissionError, PlResourceError, and PlException_qid.

To throw an exception, create an instance of PlException and use throw. This is intercepted by the PREDICATE macro and turned into a Prolog exception. See section 2.18.2.

  char *data = "users";

  throw PlException(PlCompound("no_database", PlTerm(data)));

2.16.1 The class PlException (version 2)

This subclass of PlTerm is used to represent exceptions. Currently defined methods are:

PlException :: PlException(const PlTerm &)

Create an exception from a general Prolog term. This provides the interface for throwing any Prolog terms as an exception.

std::string as_string()

The exception is translated into a message as produced by print_message/2. The character data is stored in a ring. Example:

  ...;
  try
  { PlCall("consult(load)");
  } catch ( PlException& ex )
  { cerr << ex.as_string() << endl;
  }

int plThrow()

Used in the PREDICATE() wrapper to pass the exception to Prolog. See PL_raise_exeption().

2.16.2 The class PlTypeError (version 2)

A type error expresses that a term does not satisfy the expected basic Prolog type.

PlTypeError :: PlTypeError(const char *expected, const PlTerm &actual)

Creates an ISO standard Prolog error term expressing the expected type and actual term that does not satisfy this type.

2.16.3 The class PlDomainError (version 2)

A domain error expresses that a term satisfies the basic Prolog type expected, but is unacceptable to the restricted domain expected by some operation. For example, the standard Prolog open/3 call expect an io_mode (read, write, append, ...). If an integer is provided, this is a type error, if an atom other than one of the defined io-modes is provided it is a domain error.

PlDomainError :: PlDomainError(const char *expected, const PlTerm &actual)

Creates an ISO standard Prolog error term expressing a the expected domain and the actual term found.