Prolog Language Error handling and exceptions Cleaning up after exceptions


Example

The predicate setup_call_cleanup/3, which is currently being considered for inclusion in the Prolog ISO standard and provided by an increasing number of implementations, lets us ensure that resources are correctly freed after an exception is thrown.

A typical invocation is:

setup_call_cleanup(open(File, Mode, Stream), process_file(File), close(Stream))

Note that an exception or interrupt may even occur immediately after open/3 is called in this case. For this reason, the Setup phase is performed atomically. In Prolog systems that only provide call_cleanup/2, this is much harder to express.