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Fast/concurrent update of facts

Author(s): Daniel Cabeza, Manuel Carro.

Version: 1.10#6 (2004/8/7, 21:46:39 CEST)

Version of last change: 1.7#95 (2001/5/2, 12:18:6 CEST)

Prolog implementations traditionally implement the concept of dynamic predicates: predicates which can be inspected or modified at run-time, adding or deleting individual clauses. The power of this feature comes at a cost: as new clause bodies can be arbitrarily added to the program, new predicate calls can arise which are not 'visible' at compile-time, thus complicating global analysis and optimization of the code. But it is the case that most of the time what the programmer wants is simply to store data, with the purpose of sharing it between search branches, predicates, or even execution threads. In Ciao the concept of data predicate serves this purpose: a data predicate is a predicate composed exclusively by facts, which can be inspected, and dynamically added or deleted, at run-time. Using data predicates instead of normal dynamic predicates brings benefits in terms of speed, but above all makes the code much easier to analyze automatically and thus allows better optimization.

Also, a special kind of data predicates exists, concurrent predicates, which can be used to communicate/synchronize among different execution threads (see section Low-level concurrency/multithreading primitives).

Data predicates must be declared through a data/1 declaration. Concurrent data predicates must be declared through a concurrent/1 declaration.

Usage and interface (data_facts)

Documentation on exports (data_facts)

PREDICATE: asserta_fact/1:

asserta_fact(Fact)

Fact is added to the corresponding data predicate. The fact becomes the first clause of the predicate concerned.

Meta-predicate with arguments: asserta_fact(fact).

PREDICATE: asserta_fact/2:

asserta_fact(Fact, Ref)

Same as asserta_fact/1, instantiating Ref to a unique identifier of the asserted fact.

Meta-predicate with arguments: asserta_fact(fact,?).

PREDICATE: assertz_fact/1:

assertz_fact(Fact)

Fact is added to the corresponding data predicate. The fact becomes the last clause of the predicate concerned.

Meta-predicate with arguments: assertz_fact(fact).

PREDICATE: assertz_fact/2:

assertz_fact(Fact, Ref)

Same as assertz_fact/1, instantiating Ref to a unique identifier of the asserted fact.

Meta-predicate with arguments: assertz_fact(fact,?).

PREDICATE: current_fact/1:

current_fact(Fact)

Gives on backtracking all the facts defined as data or concurrent which unify with Fact. It is faster than calling the predicate explicitly, which do invoke the meta-interpreter. If the Fact has been defined as concurrent and has not been closed, current_fact/1 will wait (instead of failing) for more clauses to appear after the last clause of Fact is returned.

Meta-predicate with arguments: current_fact(fact).

PREDICATE: current_fact/2:

current_fact(Fact, Ref)

Fact is a fact of a data predicate and Ref is its reference identifying it uniquely.

Meta-predicate with arguments: current_fact(fact,?).

Usage 1: current_fact(+callable, -reference)

Usage 2: current_fact(?callable, +reference)

PREDICATE: retract_fact/1:

retract_fact(Fact)

Unifies Fact with the first matching fact of a data predicate, and then erases it. On backtracking successively unifies with and erases new matching facts. If Fact is declared as concurrent and is non- closed, retract_fact/1 will wait for more clauses or for the closing of the predicate after the last matching clause has been removed.

Meta-predicate with arguments: retract_fact(fact).

PREDICATE: retractall_fact/1:

retractall_fact(Fact)

Erase all the facts of a data predicate unifying with Fact. Even if all facts are removed, the predicate continues to exist.

Meta-predicate with arguments: retractall_fact(fact).

PREDICATE: current_fact_nb/1:

current_fact_nb(Fact)

Behaves as current_fact/1 but a fact is never waited on even if it is concurrent and non-closed.

Meta-predicate with arguments: current_fact_nb(fact).

PREDICATE: retract_fact_nb/1:

retract_fact_nb(Fact)

Behaves as retract_fact/1, but never waits on a fact, even if it has been declared as concurrent and is non- closed.

Meta-predicate with arguments: retract_fact_nb(fact).

PREDICATE: close_predicate/1:

close_predicate(Pred)

Changes the behavior of the predicate Pred if it has been declared as a concurrent predicate: calls to this predicate will fail (instead of wait) if no more clauses of Pred are available.

Meta-predicate with arguments: close_predicate(fact).

PREDICATE: open_predicate/1:

open_predicate(Pred)

Reverts the behavior of concurrent predicate Pred to waiting instead of failing if no more clauses of Pred are available.

Meta-predicate with arguments: open_predicate(fact).

PREDICATE: set_fact/1:

set_fact(Fact)

Sets Fact as the unique fact of the corresponding data predicate.

Meta-predicate with arguments: set_fact(fact).

PREDICATE: erase/1:

erase(Ref)

Deletes the clause referenced by Ref.

Documentation on internals (data_facts)

DECLARATION: data/1:

Usage: :- data Predicates.

DECLARATION: concurrent/1:

Usage: :- concurrent Predicates.

REGTYPE: reference/1:

Usage: reference(R)


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