fork_client.pl

:- module(fork_client,
          [ fork_client/2,
            stop_client/1,
            run_in_client/2
          ]).
:- use_module(library(socket)).

fork_client(Address, Stream) :-
    tcp_connect(Address, Stream, []).

stop_client(Stream) :-
    reply(Stream, quit),
    close(Stream).

run_in_client(Stream, Goal) :-
    term_variables(Goal, Vars),
    reply(Stream, run(Goal, Vars)),
    read(Stream, Reply),
    (   Reply = true(Vars0)
    ->  Vars0 = Vars
    ;   Reply = exception(E)
    ->  throw(E)
    ).

reply(Stream, Term) :-
    write_term(Stream, Term,
               [ quoted(true),
                 ignore_ops(true),
                 fullstop(true)
               ]),
    flush_output(Stream).

fork_server.pl

:- module(fork_server,
          [ fork_server/1
          ]).
:- use_module(library(socket)).

fork_server(Port) :-
    tcp_socket(Socket),
    tcp_setopt(Socket, reuseaddr),
    tcp_bind(Socket, Port),
    tcp_listen(Socket, 5),
    tcp_open_socket(Socket, AcceptFd, _),
    dispatch(AcceptFd).

dispatch(AcceptFd) :-
    tcp_accept(AcceptFd, Socket, Peer),
    fork(Pid),
    (   Pid == child
    ->  serve(Socket, Peer)
    ;   tcp_close_socket(Socket),
        dispatch(AcceptFd)
    ).

serve(Socket, _Peer) :-
    tcp_open_socket(Socket, Read, Write),
    serve_loop(Read, Write).

serve_loop(Read, Write) :-
    read(Read, Command),
    (   Command == quit
    ->  reply(Write, bye)
    ;   Command = run(Goal, Vars)
    ->  (   catch(Goal, E, true)
        ->  (   var(E)
            ->  reply(Write, true(Vars))
            ;   reply(Write, exception(E))
            )
        ;   reply(Write, false)
        ),
        serve_loop(Read, Write)
    ).

reply(Stream, Term) :-
    write_term(Stream, Term,
               [ quoted(true),
                 ignore_ops(true),
                 fullstop(true)
               ]),
    flush_output(Stream).