4.15.6 Legacy Predicates

Exception handling for Prolog was originally introduced in Quintus Prolog, and later inherited by SICStus Prolog, with an API that predated the ISO standard. This API is still supported but should be regarded as legacy, and consists of the two predicates raise_exception/1 and on_exception/3:

on_exception(?Template, :ProtectedGoal, :Handler)

Equivalent to catch(:ProtectedGoal, ?Template, :Handler). Any exception term matching Template is caught and handled. See mpg-ref-on_exception.

raise_exception(+ExceptionTerm)

If ExceptionTerm matches one of the SICStus error terms listed in ref-ere-err, then the corresponding error term error(ISO_Error, SICStus_Error) is constructed and thrown. Otherwise, ExceptionTerm is thrown as is.

Prior to release 4.3, throw/1 and raise_exception/3 used to be equivalent and throw their argument as is, whereas catch/3 and on_exception/3 both used to attempt to recognize and expand SICStus error terms into error/2 terms. Unless a forged SICStus error term is thrown by throw/1, the net behavior is unchanged. See mpg-ref-raise_exception.