[Rangarajan]

I am looking for examples that show how to retrieve the contents of a JSON object. I have seen examples that show how to create and populate a JSON object, but I need to understand how to retrieve the different elements from an existing object.

[Thomas Linder Puls]

A JSON object is represented as a map:

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interface jsonObject supports mapM{string, jsonValue}
    open core, json
...

You can read about maps (and other interesting stuff) in Collection Library.

You should also notice that a json::jsonValue is a functor domain with several functor alternatives:

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class json
    open core
 
domains
    jsonValue =
        n;
        f;
        t;
        r(real Number);
        s(string String);
        a(jsonValue* Array);
        o(jsonObject Object)
        [presenter(presenter_jsonValue)].
    % @short Represent the JSON values corresponding to the ECMA 404 standard.<br>
    % n - the JSON constant null<br>
    % f - the  JSON constant false<br>
    % t - the  JSON constant true<br>
    % r(Number) - a JSON number<br>
    % s(String) - a JSON string String<br>
    % a(Array) - a JSON array as a list<br>
    % o(Object) - a JSON object<br>
    % See also http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
    % @end
...

And with that in mind here is a little example that "fetches" stuff in a JSON object:

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implement main
    open core
 
constants
    json : string = @'
{
    "f_int" : 12,
    "f_object" :
        {
            "nested_1" : false,
            "nested_2" : [1, 5, { "A": "a" }, 7]
        },
    "f_string" : "This is a string"
}'.
 
clauses
    run() :-
        % Parse the json string as a jsonObject
        JSON = jsonObject::fromString(json),
        stdio::writef("JSON = %p\n", JSON),
        % get the "f_object" member
        F_Object_value = JSON:get("f_object"),
        % unpack the object from the json::jsonValue
        json::o(F_Object) == F_Object_value,
        stdio::writef("F_Object = %p\n", F_Object),
        % get and unpack the "f_object" member
        F_Object_2 = JSON:get_object("f_object"),
        stdio::writef("F_Object_2 = %p\n", F_Object_2),
        % get and unpack the "nested_2" array (from F_Object)
        Nested_2 = F_Object:get_array("nested_2"),
        stdio::writef("Nested_2 = %p\n", Nested_2).
 
end implement main

If you need clarification then don't hesitate to ask more.

[Rangarajan]

Thanks a lot!

[Rangarajan]

I could get back to this example only now. Thanks again.
Where can I get the complete documentation for reading and parsing JSON objects?
For example, how do I access the nth element of an array inside a JSON object?

Can you include a full independent example of working with JSON objects in your next update?

[Thomas Linder Puls]

JSON arrays are mapped into Prolog lists, so treatment of them follows normal list processing rules.

Looping an "N" from 0 to the length of the list and then obtaining the n'th element in the list is a very bad (in-efficient) approach to list handling. But if you insist there is an nth predicate in the list class:

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predicates
    nth : (positive Index, Elem* List) -> Elem Item.
    % @short Return `Item` of `List` at position `Index`. `Index` is zero based.
    % @end

However I suggest you read the Lists and Recursion tutorial first.

JSON is just a "transport" data type, and as such not a major feature of the language. It uses "normal" Visual Prolog "stuff" and should not need independent treatment. So I do not think there will be a more comprehensive example than the one already given here.

There are more relevant tutorials on the tutorials page.

[Rangarajan]

OK. I will go through the tutorials.

Thanks.