9.3.3 Threads and dynamic predicates

Besides queues (section 9.3.1) threads can share and exchange data using dynamic predicates. The multithreaded version knows about two types of dynamic predicates. By default, a predicate declared dynamic (see dynamic/1) is shared by all threads. Each thread may assert, retract and run the dynamic predicate. Synchronisation inside Prolog guarantees the consistency of the predicate. Updates are logical: visible clauses are not affected by assert/retract after a query started on the predicate. In many cases primitives from section 9.4 should be used to ensure that application invariants on the predicate are maintained.

Besides shared predicates, dynamic predicates can be declared with the thread_local/1 directive. Such predicates share their attributes, but the clause list is different in each thread.

thread_local +Functor/+Arity, ...

This directive is related to the dynamic/1 directive. It tells the system that the predicate may be modified using assert/1, retract/1, etc., during execution of the program. Unlike normal shared dynamic data, however, each thread has its own clause list for the predicate. As a thread starts, this clause list is empty. If there are still clauses when the thread terminates, these are automatically reclaimed by the system (see also volatile/1). The thread_local property implies the properties dynamic and volatile.

Thread-local dynamic predicates are intended for maintaining thread-specific state or intermediate results of a computation.

It is not recommended to put clauses for a thread-local predicate into a file, as in the example below, because the clause is only visible from the thread that loaded the source file. All other threads start with an empty clause list.

:- thread_local
        foo/1.

foo(gnat).

DISCLAIMER Whether or not this declaration is appropriate in the sense of the proper mechanism to reach the goal is still debated. If you have strong feelings in favour or against, please share them in the SWI-Prolog mailing list.