Starting with SWI-Prolog 9.3.13, SWI-Prolog introduces a new class of
reserved exceptions. An exception of the shape unwind(Term)
is handled special by catch/3
and friends. Rather than simply calling the
Recover goal (third argument), catch/3
acts as if called as
catch(Goal, Ball, call_cleanup(once(Recover), throw(Ball)))
The above implies that cleanup that may be required on exceptions
should use call_cleanup/2
(or one of its variations) or should perform the cleanup in the Recover
goal. For example, the following does
not properly cleanup after an exception:
( catch(Goal, Error, true)
-> ( var(Error)
-> <no exception>
; <cleanup> % NOT called on throw(unwind(...))
...
This implies that, unless the user ensures Recover does
not terminate, the exception will unwind the entire Prolog stack. While
foreign code that catches these exceptions should propagate them to the
parent environment, we cannot enforce this behaviour.
Currently, this mechanism defines these values for Term:
- abort
- Abort the current Prolog thread. This exception is raised by abort/0.
Previous versions raised
'$aborted' using the same
unwind semantics.
- halt(Status)
- In the standard setup, if
unwind(halt(Status))
is raised in the main thread, this system halts with exit
code
Status
- thread_exit(ExitValue)
- This exception is raised by thread_exit/1.
See this predicate for details.
