Context in Prolog

Here’s the translation of the Go code to Prolog, along with explanations in Markdown format suitable for Hugo:

Our example demonstrates how to handle HTTP requests with timeouts and cancellation in Prolog. We’ll use the library(http/thread_httpd) for creating an HTTP server and library(http/http_dispatch) for handling requests.

:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_headers)).

% Define the handler for the '/hello' route
:- http_handler('/hello', hello_handler, []).

hello_handler(Request) :-
    format('Content-type: text/plain~n~n'),
    print_message(informational, 'server: hello handler started'),
    (   sleep(10),
        format('hello~n')
    ;   catch(
            (   http_read_data(Request, _, [timeout(5)]),
                throw(http_reply(timeout))
            ),
            Error,
            (   print_message(warning, 'server: ~w', [Error]),
                throw(http_reply(server_error(Error)))
            )
        )
    ),
    print_message(informational, 'server: hello handler ended').

% Start the server
start_server(Port) :-
    http_server(http_dispatch, [port(Port)]).

% Main predicate to start the server
main :-
    start_server(8090),
    print_message(informational, 'Server started on port 8090').

In this Prolog version:

1. We use library(http/thread_httpd) and library(http/http_dispatch) to set up an HTTP server.

2. The hello_handler predicate is defined to handle requests to the ‘/hello’ route.

3. Inside hello_handler, we simulate work by using sleep(10). This will wait for 10 seconds before responding.

4. We use a catch/throw mechanism to handle timeouts. If the client disconnects before the 10 seconds are up, an error will be thrown and caught.

5. The start_server predicate initializes the HTTP server on the specified port.

6. The main predicate starts the server on port 8090.

To run the server:

$ swipl -s server.pl -g main
Server started on port 8090

To test the server, you can use curl in another terminal:

$ curl localhost:8090/hello

If you interrupt the curl command before 10 seconds, you’ll see an error message from the server indicating that the operation was cancelled.

Note that Prolog doesn’t have built-in concepts directly equivalent to Go’s contexts or goroutines. Instead, we’re using Prolog’s built-in concurrency model and the HTTP library’s timeout mechanism to achieve similar functionality.