Mutexes in Prolog

:- use_module(library(thread)).
:- use_module(library(aggregate)).

% Container holds a dictionary of counters; since we want to
% update it concurrently from multiple threads, we
% add a mutex to synchronize access.
:- dynamic container/1.

container(container(Mutex, Counters)) :-
    mutex_create(Mutex),
    dict_create(Counters, counters, [a-0, b-0]).

% Increment a named counter in the container
increment(Container, Name) :-
    Container = container(Mutex, Counters),
    with_mutex(Mutex, (
        get_dict(Name, Counters, Value),
        NewValue is Value + 1,
        put_dict(Name, Counters, NewValue, _)
    )).

% This predicate increments a named counter in a loop
do_increment(Container, Name, N) :-
    forall(between(1, N, _), increment(Container, Name)).

% Main predicate to run the concurrent increments
main :-
    container(Container),
    concurrent(3, [
        do_increment(Container, a, 10000),
        do_increment(Container, a, 10000),
        do_increment(Container, b, 10000)
    ], []),
    Container = container(_, Counters),
    writeln(Counters).

% Run the program
:- main.

This Prolog code demonstrates the use of mutexes to safely access shared data across multiple threads. Here’s a breakdown of the code:

1. We use the thread and aggregate libraries for concurrency and aggregation operations.

2. We define a container/1 predicate that holds a mutex and a dictionary of counters. The mutex is created using mutex_create/1, and the dictionary is initialized with dict_create/3.

3. The increment/2 predicate safely increments a named counter in the container. It uses with_mutex/2 to ensure thread-safe access to the shared dictionary.

4. do_increment/3 is a helper predicate that calls increment/2 in a loop for a specified number of times.

5. The main/0 predicate sets up the container and runs three concurrent threads using concurrent/3. Each thread calls do_increment/3 with different parameters.

6. After all threads complete, we print the final state of the counters.

To run the program, save it to a file (e.g., mutexes.pl) and execute it with a Prolog interpreter that supports multi-threading, such as SWI-Prolog:

$ swipl mutexes.pl
counters{a:20000, b:10000}

The output shows that the counters have been updated as expected, with a incremented 20000 times (10000 from each of the two threads incrementing a) and b incremented 10000 times.

This example demonstrates how to use mutexes in Prolog to safely manage shared state across multiple threads, which is conceptually similar to using mutexes in other languages for concurrent programming.