Channels in Prolog

% Channels are the pipes that connect concurrent processes. You can send
% values into channels from one process and receive those values into
% another process.

% Create a new channel
create_channel(Channel) :-
    message_queue_create(Channel).

% Send a message to the channel
send_message(Channel, Message) :-
    thread_send_message(Channel, Message).

% Receive a message from the channel
receive_message(Channel, Message) :-
    thread_get_message(Channel, Message).

% Main predicate
main :-
    % Create a new channel
    create_channel(Messages),

    % Send a value into the channel from a new thread
    thread_create(send_message(Messages, ping), _),

    % Receive the value from the channel and print it
    receive_message(Messages, Msg),
    writeln(Msg).

% Run the program
:- main.

In Prolog, we don’t have built-in channels like in some other languages, but we can simulate them using message queues and threads. Here’s how the concepts translate:

1. We use message_queue_create/1 to create a new message queue, which acts as our channel.

2. The send_message/2 predicate uses thread_send_message/2 to send a message to the channel.

3. The receive_message/2 predicate uses thread_get_message/2 to receive a message from the channel.

4. In the main/0 predicate, we create a new channel, start a new thread to send a message, and then receive and print the message in the main thread.

To run the program, you would typically save this code in a file (e.g., channels.pl) and then run it using a Prolog interpreter:

$ swipl -s channels.pl
ping

By default, sending and receiving messages block until both the sender and receiver are ready. This property allowed us to wait at the end of our program for the “ping” message without having to use any other synchronization.

Note that Prolog’s concurrency model is different from many imperative languages. It uses threads and message passing for concurrent execution, which aligns well with Prolog’s logical programming paradigm.