[cl-json-devel] jsonp

[Henrik Hjelte]

On Tue, Apr 12, 2011 at 11:28 PM, Haris <fbogdanovic at xnet.hr> wrote:
> Hi.
>
> Can someone give me an example of using jsonp ?
> For example, I have a lisp function:
> (defun add (x y)
>    (+ x y))
> and I want to call it from javascript:
> add (2, 2);
> How do I do that ?

It depends on the web-server. Here are some steps:
1. You need to define a query parameter that specifies the name of the
callback javascript function. Popular choices are "callback" or
"jsoncallback"
2. You need to set the response-type to application/javascript since
it is not really json you return but a small javascript function call.
3. You need to get the x and y parameter, call add and generate a json-reponse.
If the actual example is the add function the json equivalent to 4 is
just 4 so you don't even need cl-json.
4. The json-reponse 4 should be made to look as a javascript function
call, wrapped in parenthesis, end with a semicolon and have the
callback function prepended.
If you call mywebserver?fn=add&x=2&y=2&callback=foo you should
generate this string as an answer: foo(4);
And again served as application/javascript.

For hunchentoot, an untested implementation, not optimized for
efficiency or safety:

(hunchentoot:define-easy-handler (jsonp-sample :uri "/add") (callback x y)
  (setf (hunchentoot:content-type*) "application/javascript")
  (let ((answer (+ x y)))
    (format nil "~a(~a);"
            callback
            (json:encode-json-to-string answer))))

I would start to test with an even easier version, with "alert" as a
predefined callback and no x and y and no json-encode to see if you
get hunchentoot running and get a response that works, then take it
from there.

Hope it helps,
/Henrik