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|Creating an HTTP reply|
The handler (e.g., home_page/1 above) is called with the
parsed request (see section 3.13) as
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
Set-Cookie and authentication headers (see section
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-8to the end of the
chunkedoption in http_handler/3.
Statusheader 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.
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:
text/*the stream is switched to UTF-8 encoding. If the content type does not provide attributes,
; charset=UTF-8is added.
UTF-8the stream is switched to UTF-8 encoding.
http_header. The current list deals with JSON, Turtle and SPARQL.
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
etc. The defined exceptions are:
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
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]).