Below is the source from our first web-server. It serves a single page, named /hello_world, which returns a simple plain-text message. Note the three elements of the page:

1. Declare a handler, binding an HTTP path to a predicate. The notation root(hello_world) uses an alias-mechanism similar to absolute_file_name/3 and allows for moving parts of the server locations easily. See http_absolute_location/3. We could also have used '/hello_world'.
2. The predicate server(?Port) starts the server. It simply creates a number of Prolog threads and then returns to the toplevel, so you can (re-)load code, debug, etc.
3. The implementation of /hello_world. The single argument provides the request details, which we ignore for now. Our task is to write a CGI-Document: a number of name: value -pair lines, followed by two newlines, followed by the document content, The only obligatory header line is the Content-type: <mime-type> header. Printing can be done using any Prolog printing predicate, but the format-family is the most useful. See format/2.

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

:- http_handler(root(hello_world), say_hi, []).		% (1)

server(Port) :-						% (2)
        http_server(http_dispatch, [port(Port)]).

say_hi(_Request) :-					% (3)
        format('Content-type: text/plain~n~n'),
        format('Hello World!~n').

To run this server, put the above code in a file, load the file into Prolog and run the goal below. 5000 is the port-number. This can be any number between 1001 and 65000 (depending on your OS). Now direct your browser to http://localhost:5000/hello_world and enjoy your first Prolog-based web-server!

?- server(5000).

Where is Apache, IIS, Tomcat, ...?

Oops, we do not need that. The full manual gives options for redirecting requests from Apache to our server, so that your server is reachable on the default port 80. Actually, the SWI-Prolog HTTP server libraries are very much like Tomcat, but now for Prolog. The SWI-Prolog website is handled by SWI-Prolog, redirected from an Apache server.

See also

- Source: hello_world.pl

- Next: Development support for web-servers