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noneif the exception is not caught and with a fully qualified (e.g., Module:Name/Arity) predicate indicator of the predicate that called catch/3 if the exception is caught.
The exception is of the form
error(Formal, ImplDef) and this
hook succeeds, ImplDef is unified to a term
context(prolog_stack(StackData), Message). This context
information is used by the message printing system to print a
human readable representation of the stack when the exception
For example, using a clause
stack_guard(none) prints contexts
for uncaught exceptions only. Using a clause
prints a full stack-trace for any error exception if the
exception is given to print_message/2. See also
library(http/http_error), which limits printing of exceptions to
exceptions in user-code called from the HTTP server library.
Details of the exception decoration is controlled by two Prolog flags:
'C'or the predicate indicator of the guard, the predicate calling catch/3. The exception must be of compatible with the shape
error(Formal, context(Stack, Msg)). The default is to catch
none, uncaught exceptions.
'C'implies that the callback from C will handle the exception.