bug#24252: 25.1; json.el doesn't distinguish null and empty object

[Dmitry Gutov <dgutov@yandex.ru>]

Hi!

On 08/17/2016 05:54 PM, Yoichi Nakayama wrote:

> When json-pretty-print applied to "{}", it is
> unexpectedly converted to "null".
> This is caused by internal representations of null
> and empty object are the same:
>   (json-read-from-string "{}") ; => nil
>   (json-read-from-string "null") ; => nil

Why don't you bind json-null to whatever value you need? Then the 
results will be different:

ELISP> (let ((json-null 'NULL)) (json-read-from-string "null"))
NULL

> +(defvar json-empty-object :json-empty-object
> +  "Value to use when reading JSON `{}'.
> +If this has the same value as `json-null', you might not be able to tell
> +the difference between `{}' and `null'.  Consider let-binding this
> +around your call to `json-read' instead of `setq'ing it.")

This doesn't look like a good default value, at least. It's not one of 
the types that we parse JSON objects to (alist, plist, hash-table). This 
will break real code.

By the way, another option to distinguish nil and {} is to bind 
json-object-type to `hash-table'. An empty hash table is not nil.