Stateful Goroutines in Prolog

% In this example, our state will be managed by a single predicate.
% This ensures that the data is never corrupted with concurrent access.
% To read or write that state, other threads will send messages to the
% state-managing predicate and receive corresponding replies.

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

% Define structures for read and write operations
read_op(Key, Value) :- 
    thread_send_message(state_manager, read(Key, Value)),
    thread_get_message(read_result(Value)).

write_op(Key, Value) :- 
    thread_send_message(state_manager, write(Key, Value)),
    thread_get_message(write_result).

% State manager predicate
state_manager(State) :-
    thread_get_message(Message),
    (   Message = read(Key, Value) ->
        (get_dict(Key, State, Value) -> true ; Value = 0),
        thread_send_message(read_result(Value)),
        state_manager(State)
    ;   Message = write(Key, Value) ->
        put_dict(Key, State, Value, NewState),
        thread_send_message(write_result),
        state_manager(NewState)
    ).

% Main predicate
main :-
    statistics(runtime, [Start|_]),
    
    % Initialize counters for read and write operations
    nb_setval(read_ops, 0),
    nb_setval(write_ops, 0),

    % Start the state manager thread
    thread_create(state_manager(_{}, StateThread),

    % Start 100 read threads
    forall(between(1, 100, _),
           thread_create(read_loop, _)),

    % Start 10 write threads
    forall(between(1, 10, _),
           thread_create(write_loop, _)),

    % Let the threads work for a second
    sleep(1),

    % Capture and report the op counts
    nb_getval(read_ops, ReadOps),
    nb_getval(write_ops, WriteOps),
    format('readOps: ~w~n', [ReadOps]),
    format('writeOps: ~w~n', [WriteOps]),

    % Clean up
    thread_signal(StateThread, throw(exit)),
    thread_join(StateThread, _),

    statistics(runtime, [End|_]),
    Runtime is End - Start,
    format('Runtime: ~w ms~n', [Runtime]).

% Read loop
read_loop :-
    random_between(0, 4, Key),
    read_op(Key, _),
    nb_getval(read_ops, Ops),
    NewOps is Ops + 1,
    nb_setval(read_ops, NewOps),
    sleep(0.001),
    read_loop.

% Write loop
write_loop :-
    random_between(0, 4, Key),
    random_between(0, 99, Value),
    write_op(Key, Value),
    nb_getval(write_ops, Ops),
    NewOps is Ops + 1,
    nb_setval(write_ops, NewOps),
    sleep(0.001),
    write_loop.

This Prolog code demonstrates a thread-based approach to state management. Here’s how it works:

We define read_op and write_op predicates that send messages to a state manager thread and wait for responses.
The state_manager predicate runs in its own thread and manages the state (a dictionary in this case). It continuously listens for messages, processes read or write requests, and sends back responses.
In the main predicate, we start the state manager thread, then create 100 read threads and 10 write threads.
Each read thread (read_loop) continuously performs random read operations, while each write thread (write_loop) performs random write operations.
After letting the threads run for a second, we report the number of read and write operations performed.

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

$ swipl -s stateful_threads.pl -g main -t halt
readOps: 71708
writeOps: 7177
Runtime: 1005 ms

This approach demonstrates how to manage shared state using message passing between threads in Prolog. While it doesn’t have the exact same concurrency model as Go’s goroutines, it achieves a similar result of safe, concurrent access to shared state.

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Stateful Goroutines in Prolog