Providing HTTP Services with Prolog

Introduction

Continuing from the previous article, where we attempted to write a Hello World program in Prolog, we learned about the basic structure and execution of Prolog programs. In this article, we will build a simple HTTP server based on hello_world.pl.

Starting the HTTP Server

Using the built-in library of SWI-Prolog, it is easy to start an HTTP server. You just need to call the function http_server.

:- initialization(main, main).

:- use_module(library(http/http_server), [http_server/1]).

main(_) :-
    http_server([port(8082)]),
    sleep(100000000).

Save the above code to a file http_server.pl and run it to start the HTTP server.

Providing HTTP Services with Prolog — Starting the HTTP server

Since no interfaces are defined in the source file, any request will return a 404 response.

Using Modules

The use_module command mentioned above is a directive used to import functions from a module. For example, :- use_module(library(http/http_server), [http_server/1]). means importing a function http_server with one parameter from the module http/http_server into the current module, allowing it to be called directly.

Providing Interfaces

As mentioned earlier, since the current application has not registered any routing rules, any request will return a 404 response. Next, I will add a very simple interface to this HTTP server that returns the text Hello, world!. To register an interface, we need to use the function http_handler from the module http/http_dispatch. According to its documentation^[1], it requires three parameters:

Interface path Path, such as a path represented by an atom /api/shorten;
The function object responsible for implementing the interface business logic Closure;
A list describing the interface properties Options, such as the request method for the interface.

An interface that returns the Hello World string might look like this:

:- initialization(main, main).

:- use_module(library(http/http_dispatch), [http_handler/3]).
:- use_module(library(http/http_server), [http_server/1]).

hello(_Request) :-
    format('Content-Type: text/html~n~n'),
    format('Hello, world!').

main(_) :-
    http_handler('/api/shorten', hello, [methods([get])]),
    http_server([port(8082)]),
    sleep(100000000).

The test results after starting the service in the old way are as follows:

Interface Path

As the first parameter of http_handler, '/api/shorten' looks very much like a string familiar in other mainstream languages; however, in Prolog, it is an atom (atom). Although the content inside the quotes looks the same, atoms and strings are not equal, as shown in the following image:

Interface Output

From the documentation^[2] of SWI-Prolog, we know that the second parameter of http_handler must output at least Content-Type and the body of the data. Furthermore, although the implementation of hello seems to output the response content to standard output (in a style reminiscent of ancient CGI scripts), the destination of the format output has already been replaced by the framework. We can replace it with the following code to better understand this:

:- initialization(main, main).

:- use_module(library(http/http_dispatch), [http_handler/3]).
:- use_module(library(http/http_server), [http_server/1]).

hello(_Request) :-
    current_output(Stream),
    format(Stream, 'Content-Type: text/html~n~n', []),
    format(Stream, 'Hello, world!', []).

main(_) :-
    http_handler('/api/shorten', hello, [methods([get])]),
    http_server([port(8082)]),
    sleep(100000000).

The request results will be the same.

References[1]

Documentation: https://www.swi-prolog.org/pldoc/man?predicate=http_handler/3

[2]

Documentation: https://www.swi-prolog.org/pldoc/man?section=html-body