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]).