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'.83For 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.
The indexing capabilities of SWI-Prolog are described in section 2.18. Summarizing, SWI-Prolog creates indexes for any applicable argument, pairs of arguments and indexes on the arguments of compound terms when applicable. Extended JIT indexing is not widely supported amoung Prolog implementations. Programs that aim at portability should consider using term_hash/2 and term_hash/4 to design their database such that indexing on constant or functor (name/arity reference) on the first argument is sufficient. In some cases, using the predicates below to add one or more additional columns (arguments) to a database predicate may improve performance. The overall design of code using these predicates is given below. Note that as term_hash/2 leaves the hash unbound if Term is not ground. This causes the lookup to be fast if Term is ground and correct (but slow) otherwise.
:- dynamic x/2. assert_x(Term) :- term_hash(Term, Hash), assertz(x(Hash, Term)). x(Term) :- term_hash(Term, Hash), x(Hash, Term).
- [det]term_hash(+Term, -HashKey)
- If Term is a ground term (see ground/1), HashKey
is unified with a positive integer value that may be used as a hash key
to the value. If Term is not ground, the predicate leaves HashKey
an unbound variable. Hash keys are in the range 0 ... 16,777,215,
the maximal integer that can be stored efficiently on both 32 and 64 bit
This predicate may be used to build hash tables as well as to exploit argument indexing to find complex terms more quickly.
The hash key does not rely on temporary information like addresses of atoms and may be assumed constant over different invocations and versions of SWI-Prolog.84Last change: version 5.10.4 Hashes differ between big and little endian machines. The term_hash/2 predicate is cycle-safe.bugAll arguments that (indirectly) lead to a cycle have the same hash key.
- [det]term_hash(+Term, +Depth, +Range, -HashKey)
- As term_hash/2,
but only considers Term to the specified
Depth. The top-level term has depth 1, its arguments have
depth 2, etc. That is, Depth = 0 hashes nothing; Depth
= 1 hashes atomic values or the functor and arity of a compound
term, not its arguments; Depth = 2 also indexes
the immediate arguments, etc.
HashKey is in the range [0 ...Range-1]. Range must be in the range [1 ... 2147483647]
- [det]variant_sha1(+Term, -SHA1)
- Compute a SHA1-hash from Term. The hash is represented as a
40-byte hexadecimal atom. Unlike term_hash/2
and friends, this predicate produces a hash key for non-ground terms.
The hash is invariant over variable-renaming (see =@=/2)
and constants over different invocations of Prolog.bugThe
hash depends on word order (big/little-endian) and the wordsize (32/64
This predicate raises an exception when trying to compute the hash on a cyclic term or attributed term. Attributed terms are not handled because subsumes_chk/2 is not considered well defined for attributed terms. Cyclic terms are not supported because this would require establishing a canonical cycle. That is, given A=[a|A] and B=[a,a|B], A and B should produce the same hash. This is not (yet) implemented.
This hash was developed for lookup of solutions to a goal stored in a table. By using a cryptographic hash, heuristic algorithms can often ignore the possibility of hash collisions and thus avoid storing the goal term itself as well as testing using =@=/2.
- [det]variant_hash(+Term, -HashKey)
- Similar to variant_sha1/2, but using a non-cryptographic hash and produces an integer result like term_hash/2. This version does deal with attributed variables, processing them as normal variables. This hash is primarily intended to speedup finding variant terms in a set of terms. bugAs variant_sha1/2, cyclic terms result in an exception.