4.10 Exception handling
All Application Manual Name SummaryHelp

Availability:built-in
Source[ISO]catch(:Goal, +Catcher, :Recover)
Behaves as call/1 if no exception is raised when executing Goal. If an exception is raised using throw/1 while Goal executes, and the Goal is the innermost goal for which Catcher unifies with the argument of throw/1, all choice points generated by Goal are cut, the system backtracks to the start of catch/3 while preserving the thrown exception term, and Recover is called as in call/1.

As of version 9.3.13, constraints (attributed variables) in Catcher are respected. If evaluating the constraint raises an exception, the most urgent exception is preserved (see section 4.10.2) and searching for a matching catch/3 call is continued. If both exceptions are equally urgent, the exception raised by the constraint evaluation is preserved.

The overhead of calling a goal through catch/3 is comparable to call/1. Recovery from an exception is much slower, especially if the exception term is large due to the copying thereof or is decorated with a stack trace using, e.g., the library library(prolog_stack) based on the prolog_exception_hook/4 hook predicate to rewrite exceptions.