Traditionally,
Prolog systems used the immediate update view: new clauses
became visible to predicates backtracking over dynamic predicates
immediately, and retracted clauses became invisible immediately.
Starting with SWI-Prolog 3.3.0 we adhere to the logical update
view, where backtrackable predicates that enter the definition of a
predicate will not see any changes (either caused by assert/1
or
retract/1)
to the predicate. This view is the ISO standard, the most commonly used
and the most‘safe’.92For
example, using the immediate update view, no call to a dynamic predicate
is deterministic. Logical updates are realised by keeping
reference counts on predicates and generation information on
clauses. Each change to the database causes an increment of the
generation of the database. Each goal is tagged with the generation in
which it was started. Each clause is flagged with the generation it was
created in as well as the generation it was erased from. Only clauses
with a‘created’ ...‘erased’interval that
encloses the generation of the current goal are considered visible.
