Channel Synchronization in Prolog

Our example demonstrates how to synchronize execution across multiple threads in Prolog. We’ll use a simple predicate to simulate work and another to wait for its completion.

:- use_module(library(threads)).

% This is the predicate we'll run in a separate thread.
% The Done variable will be used to notify the main thread
% that this predicate's work is done.
worker(Done) :-
    write('working...'),
    sleep(1),
    writeln('done'),
    Done = true.

% Main predicate to demonstrate thread synchronization
main :-
    % Create a thread to run the worker predicate
    thread_create(worker(Done), ThreadId, []),
    
    % Wait for the worker thread to finish
    thread_join(ThreadId, _),
    
    % Check if the work is done
    (Done == true ->
        writeln('Worker thread finished successfully.')
    ;   writeln('Worker thread did not finish as expected.')
    ).

% Run the main predicate
:- main.

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

$ swipl -s thread_sync.pl
working...done
Worker thread finished successfully.

In this Prolog version:

1. We use the threads library for multi-threading support.

2. The worker/1 predicate simulates work by writing a message, sleeping for a second, and then signaling completion by unifying the Done variable with true.

3. In the main/0 predicate, we create a new thread to run the worker/1 predicate using thread_create/3.

4. We use thread_join/2 to wait for the worker thread to finish. This is equivalent to the channel synchronization in the original example.

5. Finally, we check if the work was completed successfully by examining the Done variable.

Note that Prolog’s approach to concurrency is different from imperative languages. Instead of channels, we use logical variables for synchronization. The thread_join/2 predicate serves a similar purpose to the channel receive operation in the original example, allowing us to wait for the thread to complete before proceeding.

If you removed the thread_join/2 line from this program, the main thread might exit before the worker thread even started or completed its work, similar to the behavior described in the original example.