SWI-Prolog -- Creating an HTTP reply

3.1 Creating an HTTP reply

The handler (e.g., home_page/1 above) is called with the parsed request (see section 3.13) as argument and current_output set to a temporary buffer. Its task is closely related to the task of a CGI script; it must write a header declaring at least the Content-type field and a body. Below is a simple body writing the request as an HTML table.^{3Note
that writing an HTML reply this way is deprecated. In fact, the code is
subject to injection attacks as the HTTP request field values
are literally injected in the output while HTML reserved characters
should be properly escaped.}

reply(Request) :-
        format('Content-type: text/html~n~n', []),
        format('<html>~n', []),
        format('<table border=1>~n'),
        print_request(Request),
        format('~n</table>~n'),
        format('</html>~n', []).

print_request([]).
print_request([H|T]) :-
        H =.. [Name, Value],
        format('<tr><td>~w<td>~w~n', [Name, Value]),
        print_request(T).

The infrastructure recognises the header fields described below. Other header lines are passed verbatim to the client. Typical examples are Set-Cookie and authentication headers (see section 3.7).

Content-type: Type: This field is passed to the client and used by the infrastructure to determine the encoding to use for the stream. If type matches text/* or the type matches with UTF-8 (case insensitive), the server uses UTF-8 encoding. The user may force UTF-8 encoding for arbitrary content types by adding ; charset=UTF-8 to the end of the Content-type header.
Transfer-encoding: chunked: Causes the server to use chunked encoding if the client allows for it. See also section 5 and the chunked option in http_handler/3.
Connection: close: Causes the connection to be closed after the transfer. The default is to keep it open‘Keep-Alive’if possible.
Location: URL: This header may be combined with the Status header to force a redirect response to the given URL. The message body must be empty. Handling this header is primarily intended for compatibility with the CGI conventions. Prolog code should use http_redirect/3.
Status: Status: This header can be combined with Location, where Status must be one of 301 (moved), 302 (moved temporary, default) or 303 (see other). Using the status field also allows for formulating replies such as 201 (created).

Note that the handler may send any type of document instead of HTML. After the header has been written, the encoding of the current_output stream encoding is established as follows:

If the content type is text/* the stream is switched to UTF-8 encoding. If the content type does not provide attributes, ; charset=UTF-8 is added.
The content type contains UTF-8 the stream is switched to UTF-8 encoding.
http:mime_type_encoding/2 succeeds the returned encoding is used. The returned encoding must be valid for set_stream/2.
If the content type matches a list of known encodings, this is used. See mime_type_encoding/2 is http_header. The current list deals with JSON, Turtle and SPARQL.
Otherwise the stream uses octed (binary) encoding.

3.1.1 Returning special status codes

Besides returning a page by writing it to the current output stream, the server goal can raise an exception using throw/1 to generate special pages such as not_found, moved, etc. The defined exceptions are:

http_reply(+Reply, +HdrExtra, +Context): Return a result page using http_reply/3. See http_reply/3 for supported values for Reply and section 3.10 for providing a custom error page.
http_reply(+Reply, +HdrExtra): Return a result page using http_reply/3. Equivalent to http_reply(Reply, HdrExtra,[]).
http_reply(+Reply): Equivalent to http_reply(Reply, [],[]).
http(not_modified): Equivalent to http_reply(not_modified,[]). This exception is for backward compatibility and can be used by the server to indicate the referenced resource has not been modified since it was requested last time.

In addition, the normal "200 OK" reply status may be overruled by writing a CGI Status header prior to the remainder of the message. This is particularly useful for defining REST APIs. The following handler replies with a "201 Created" header:

handle_request(Request) :-
        process_data(Request, Id),      % application predicate
        format('Status: 201~n'),
        format('Content-type: text/plain~n~n'),
        format('Created object as ~q~n', [Id]).