Atomic Counters in Prolog

Our primary mechanism for managing state in Prolog is through logical variables and backtracking. However, we can simulate atomic counters using Prolog’s dynamic predicates. Here’s an example of how we might implement atomic counters in Prolog:

:- module(atomic_counters, [run/0]).
:- use_module(library(thread)).

:- dynamic counter/1.

increment :-
    retract(counter(X)),
    Y is X + 1,
    asserta(counter(Y)),
    !.
increment :-
    asserta(counter(1)).

worker :-
    forall(between(1, 1000, _),
           increment).

run :-
    thread_count(50, Workers),
    maplist(thread_create(worker), Workers),
    maplist(thread_join, Workers),
    counter(Result),
    format('ops: ~w~n', [Result]).

:- initialization(run).

In this Prolog implementation:

1. We use a dynamic predicate counter/1 to represent our counter.

2. The increment/0 predicate atomically increments the counter. It first tries to retract the current counter value, increment it, and assert the new value. If the counter doesn’t exist yet, it initializes it to 1.

3. The worker/0 predicate simulates a goroutine, incrementing the counter 1000 times.

4. In the run/0 predicate:

   • We create 50 worker threads using thread_count/2 and maplist/3 with thread_create/2.
   • We wait for all threads to finish using maplist/2 with thread_join/1.
   • Finally, we retrieve and print the final counter value.

5. The :- initialization(run). directive ensures that run/0 is called when the program is loaded.

To run this program, save it as atomic_counters.pl and use:

$ swipl atomic_counters.pl
ops: 50000

We expect to get exactly 50,000 operations. The use of retract/1 and asserta/1 in the increment/0 predicate ensures that updates to the counter are atomic, preventing race conditions between threads.

Note that while this example demonstrates the concept of atomic counters in Prolog, it’s not as efficient as native atomic operations in languages like Go. For high-performance concurrent programming in Prolog, you might want to consider using built-in predicates or foreign language interfaces that provide true atomic operations.