Timeouts in Prolog

Timeouts are important for programs that connect to external resources or that otherwise need to bound execution time. Implementing timeouts in Prolog can be achieved using built-in predicates and library modules.

:- use_module(library(process)).
:- use_module(library(system)).

main :-
    % For our example, suppose we're executing an external
    % call that returns its result after 2 seconds.
    process_create(path(sleep), ['2'], [process(P1)]),

    % Here we implement a timeout using the wait_for_process/3 predicate.
    % It waits for the process to finish or for a timeout to occur.
    (   wait_for_process(P1, exit(_), [timeout(1000)])
    ->  writeln('result 1')
    ;   writeln('timeout 1'),
        process_kill(P1)
    ),

    % If we allow a longer timeout of 3 seconds, then the process
    % will complete and we'll print the result.
    process_create(path(sleep), ['2'], [process(P2)]),
    (   wait_for_process(P2, exit(_), [timeout(3000)])
    ->  writeln('result 2')
    ;   writeln('timeout 2'),
        process_kill(P2)
    ).

:- main.

In this Prolog implementation:

1. We use the process library to create external processes that simulate long-running operations.

2. The process_create/3 predicate is used to start a process that sleeps for 2 seconds, simulating a time-consuming task.

3. We use wait_for_process/3 to wait for the process to finish or for a timeout to occur. The timeout is specified in milliseconds.

4. If the process finishes before the timeout, we print the result. Otherwise, we print a timeout message and kill the process.

5. We demonstrate two scenarios: one with a 1-second timeout (which will fail) and another with a 3-second timeout (which will succeed).

Running this program shows the first operation timing out and the second succeeding:

$ swipl -q -t main timeouts.pl
timeout 1
result 2

This Prolog implementation provides similar functionality to the original Go code, adapting the concepts of goroutines and channels to Prolog’s process handling and timeout mechanisms.