Http Server in Prolog

Here’s the translation of the HTTP Server example from Go to Prolog, formatted in Markdown suitable for Hugo:

Our HTTP server example demonstrates how to create a basic server using Prolog. We’ll use the library(http/thread_httpd) and library(http/http_dispatch) libraries to handle HTTP requests.

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

% Define our handlers
:- http_handler('/hello', hello_handler, []).
:- http_handler('/headers', headers_handler, []).

% Handler for the /hello route
hello_handler(_Request) :-
    format('Content-type: text/plain~n~n'),
    format('hello~n').

% Handler for the /headers route
headers_handler(Request) :-
    format('Content-type: text/plain~n~n'),
    http_headers(Request, Headers),
    print_headers(Headers).

% Helper predicate to print headers
print_headers([]).
print_headers([Name=Value|Rest]) :-
    format('~w: ~w~n', [Name, Value]),
    print_headers(Rest).

% Main predicate to start the server
start_server(Port) :-
    http_server(http_dispatch, [port(Port)]).

% Entry point
:- start_server(8090).

In Prolog, we define our HTTP handlers using the http_handler/3 predicate. The first argument is the path, the second is the handler predicate, and the third is a list of options (which we leave empty in this example).

The hello_handler/1 predicate simply writes “hello” to the response. The headers_handler/1 predicate retrieves the headers from the request and prints them.

The start_server/1 predicate starts the HTTP server on the specified port. We use the http_server/2 predicate, passing http_dispatch as the handler and specifying the port.

To run the server, you would typically save this code in a file (e.g., http_server.pl) and then consult it in your Prolog interpreter:

?- [http_server].

This will start the server on port 8090. You can then access it using curl or a web browser:

$ curl localhost:8090/hello
hello

$ curl localhost:8090/headers
User-Agent: curl/7.68.0
Accept: */*

Note that Prolog’s approach to HTTP servers is quite different from imperative languages. It uses declarative definitions of routes and handlers, and the execution flow is managed by the Prolog runtime and the HTTP libraries.

This example provides a basic structure for an HTTP server in Prolog. You can expand on this by adding more routes, implementing more complex logic in your handlers, or integrating with other Prolog libraries for additional functionality.