Closing Channels in Prolog

In Prolog, we don’t have the concept of channels or goroutines as in concurrent programming languages. However, we can demonstrate a similar concept using Prolog’s built-in predicates for managing concurrent processes. We’ll use threads and message queues to simulate the behavior of channels.

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

% Main predicate
main :-
    message_queue_create(Jobs),
    message_queue_create(Done),
    
    % Start the worker thread
    thread_create(worker(Jobs, Done), _, []),
    
    % Send jobs
    forall(between(1, 3, J),
           (   thread_send_message(Jobs, J),
               format('sent job ~w~n', [J])
           )),
    
    % Close the jobs queue
    thread_send_message(Jobs, end_of_jobs),
    format('sent all jobs~n', []),
    
    % Wait for the worker to finish
    thread_get_message(Done, _),
    
    % Try to receive more jobs
    (   thread_get_message(Jobs, _, [timeout(0)])
    ->  format('received more jobs: true~n', [])
    ;   format('received more jobs: false~n', [])
    ).

% Worker predicate
worker(Jobs, Done) :-
    repeat,
    thread_get_message(Jobs, Job),
    (   Job == end_of_jobs
    ->  format('received all jobs~n', []),
        thread_send_message(Done, finished),
        !
    ;   format('received job ~w~n', [Job]),
        fail
    ).

In this Prolog example:

1. We use message_queue_create/1 to create message queues that simulate channels.

2. The worker/2 predicate simulates the worker goroutine. It repeatedly receives messages from the Jobs queue until it receives an end_of_jobs message.

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

4. We send jobs to the worker using thread_send_message/2, which is similar to sending values on a channel.

5. After sending all jobs, we send an end_of_jobs message to signal that no more jobs will be sent.

6. We wait for the worker to finish by receiving a message on the Done queue.

7. Finally, we attempt to receive more jobs with a timeout of 0, which simulates checking a closed channel.

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

$ swipl -q -t main -s closing_channels.pl
sent job 1
received job 1
sent job 2
received job 2
sent job 3
received job 3
sent all jobs
received all jobs
received more jobs: false

This Prolog implementation demonstrates a similar concept to closing channels, using message queues and threads to manage concurrent processes and communication between them.