Signals in Prolog

Here’s the translation of the Go code to Prolog, formatted in Markdown suitable for Hugo:

Our program demonstrates how to handle signals in Prolog. Here’s the full source code:

:- use_module(library(unix)).

main :-
    writeln('awaiting signal'),
    catch(
        wait_for_signal,
        Signal,
        (format('~nReceived signal: ~w~n', [Signal]),
         writeln('exiting'))
    ).

wait_for_signal :-
    on_signal(int, _, sig_handler),
    on_signal(term, _, sig_handler),
    thread_get_message(exit).

sig_handler(Signal) :-
    thread_send_message(exit),
    throw(Signal).

In Prolog, we use the library(unix) module to handle Unix signals. Here’s how the program works:

1. We define the main/0 predicate as the entry point of our program.

2. Inside main/0, we use catch/3 to handle any signals that might be thrown.

3. The wait_for_signal/0 predicate sets up signal handlers for SIGINT and SIGTERM using on_signal/3.

4. We use thread_get_message/1 to block the main thread until a message is received.

5. The sig_handler/1 predicate is called when a signal is received. It sends an ’exit’ message and throws the signal.

To run this program:

1. Save the code in a file named signals.pl.
2. Run it using a Prolog interpreter that supports the library(unix) module, such as SWI-Prolog:

$ swipl signals.pl
?- main.
awaiting signal

When we run this program, it will block waiting for a signal. By typing ctrl-C (which the terminal shows as ^C), we can send a SIGINT signal, causing the program to print the received signal and then exit:

awaiting signal
^C
Received signal: int
exiting

This example demonstrates how to handle signals in Prolog, allowing for graceful shutdown or specific actions in response to system signals.