Dynamic Database State Restoration: Predicate Order Preservation

Answer to the Core Question

ISO Prolog does not provide a dedicated predicate or option that forces the restored database to retain the exact clause order of the original state. The standard only specifies that assert/1 inserts a clause at the *beginning* of the predicate’s clause list and assertz/1 inserts at the *end*. Consequently, a simple read/1 loop that uses assert/1 will reverse the original order, while using assertz/1 will preserve it only if the clauses are read in the same sequence they were originally asserted.

Recommended Verification Approach

When you need to confirm that the restored set is logically equivalent to the original, follow these steps:

1. Dump the original database: Use listing/1 or clause/2 to write each clause to a file in the order they appear. A typical dump loop looks like:

dump_predicate(Pred) :-
    clause(Pred, Body),
    write_term((Pred :- Body), [quoted(true)]),
    nl, fail.

1. Restore with order‑preserving asserts: Read the file line‑by‑line and use assertz/1 to insert each clause at the end, ensuring the same sequence:

restore_predicate :-
    open('dump.pl', read, Stream),
    read_term(Stream, Clause, []),
    ( Clause == end_of_file -> close(Stream) ;
      assertz(Clause),
      restore_predicate
    ).

1. Verify equivalence: After restoration, run listing/1 again and compare the output to the original dump. A simple diff on the two dump files is usually sufficient. If they match exactly, the order is preserved.

Additional Checks

• Use clause/2 with an index argument (if supported) to iterate in the stored order and double‑check the sequence.
• If your Prolog supports current_predicate/1 and current_clause/3, you can enumerate clauses and compare their positions.

Missing Diagnostic Detail

Which Prolog implementation are you using? Some systems expose assertz/1 or provide additional predicates for ordering guarantees. Knowing the exact engine will help tailor the restoration script to that environment.