## B.5 Adding context to errors: prolog_exception_hook

The hook prolog_exception_hook/4 has been introduced in SWI-Prolog 5.6.5 to provide dedicated exception handling facilities for application frameworks, for example non-interactive server applications that wish to provide extensive context for exceptions for offline debugging.

prolog_exception_hook(+ExceptionIn, -ExceptionOut, +Frame, +CatcherFrame)
  This hook predicate, if defined in the module `user`, is between raising an exception and handling it. It is intended to allow a program adding additional context to an exception to simplify diagnosing the problem. ExceptionIn is the exception term as raised by throw/1 or one of the built-in predicates. The output argument ExceptionOut describes the exception that is actually raised. Frame is the innermost frame. See prolog_frame_attribute/3 and the library `library(prolog_stack)` for getting information from this. CatcherFrame is a reference to the frame calling the matching catch/3, `none` if the exception is not caught or `'C'` if the exception is caught in C calling Prolog using the flag `PL_Q_CATCH_EXCEPTION`.

  The hook is run in `nodebug' mode. If it succeeds, ExceptionOut is considered the current exception. If it fails, ExceptionIn is used for further processing. The hook is never called recursively. The hook is not allowed to modify ExceptionOut in such a way that it no longer unifies with the catching frame.

  Typically, prolog_exception_hook/4 is used to fill the second argument of `error(Formal, Context)` exceptions. Formal is defined by the ISO standard, while SWI-Prolog defines Context as a term `context(Location, Message)`. Location is bound to a term <name>/<arity> by the kernel. This hook can be used to add more information on the calling context, such as a full stack trace.

  Applications that use exceptions as part of normal processing must do a quick test of the environment before starting expensive gathering information on the state of the program.

  The hook can call trace/0 to enter trace mode immediately. For example, imagine an application performing an unwanted division by zero while all other errors are expected and handled. We can force the debugger using the hook definition below. Run the program in debug mode (see debug/0) to preserve as much as possible of the error context.

      user:prolog_exception_hook(
               error(evaluation_error(zero_divisor), _),
               _, _, _) :-
              trace, fail.