How to process a POST request in SWI-Prolog? - html

Question

I have an HTML form like this one:
<form action="test" method="post">
<input name="first_name" type="text"/>
<input name="last_name" type="text" />
<input name="age" type="text" />
<input type="submit" value="Send"/>
</form>
How do I get the values of the input fields and print them on screen, just like in any other procedural programming language such as PHP, ASP or JSP?
I tried to solve the problem the following way:
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- http_handler(root(test), reply, []).
:- http_handler('test', reply, []).
server(Port) :-
http_server(http_dispatch, [port(Port)]).
reply(Request) :-
member(method(post), Request), !,
http_read_data(Request, Data, []),
format('application/x-www-form-urlencoded', []),
format(Data).
That brought me nothing more than an error with the 500 code (internal server error).

Answer 1

You should use the http/http_client library (:- use_module(library(http/http_client))).
Additionally, I'm not sure how having two handlers for test will work.
Finally, I think that format(Data) might not work, especially since it is expected to return an html document.
By the way, to retrieve the values of the fields you can do something like:
http_read_data(Request, [first_name=FN, last_name=LN, age=A|_], []).
I'm pretty new with the http prolog lib, I would suggest checking http://www.pathwayslms.com/swipltuts/html/

Answer 2

Essentially, you'll handle the request like normal, checking that the method(Method) term in the request is method(post).
http_read_data will read the request body.
the body will be encoded like an URI query string, so uri_query_components/2
will convert it to a list of Key=Value terms
?- uri_query_components('a=b&c=d%2Bw&n=VU%20Amsterdam', Q).
Q = [a=b, c='d+w', n='VU Amsterdam'].
For others looking for similar info - if your response is json, you can use read_json_dict to get the data as a dict.

Answer 3

I use library(http/http_parameters). With that, I can do
load_graph(Request) :-
http_parameters(Request,
[path(Path, [atom]),
aperture(Aperture, [integer])]),
where load_graph is the handler for the form
...
html(form([action(Ref)],
dl([dt('Root Path'), dd(input([name=path, type=text, value=Default])),
dt('Aperture'), dd(select([name=aperture], Aplist)),
dt('Go!'), dd(input([type=submit, value='Load!']))
]))).