When I wrote the parser it was to better understand the intricacies of what is valid JSON for use with Cytoscape.js (ref), but then I found JSONLint (ref).
Now that I know the JSON needed for Cytoscape.js, the JSON should not pass a simple JSONLint (syntax), but pass a more complex set of rules (semantics).
While JSON provides a syntactic framework for data interchange, unambiguous data interchange also requires agreement between producer and consumer on the semantics of a specific use of the JSON syntax. (Spec)
Specifically for Cytoscape.js this implies that there should be a JSON recognizer (semantics) for the elements, style and layout. (ref) But the JSON recognizer will not recognize the syntax of JSON, but the semantics of each JSON file.
SWI-Prolog can check the syntax of JSON, but AFAIK there is no predicate to check the semantics. Such a check would probably need something like a DTD. I did find JSON Schema but have not looked at it beyond finding it. I would not be surprised if Jan W. tells me how it is done, but it was not apparent to me when I searched. 
While I could ask, I find that taking the time to do the search and understand the predicates I found when searching makes it much easier to understand the responses from others when asking, e.g. Is there a way to go from HTML to the Prolog representation of the HTML? before asking I was researching quasiquotations. (ref) I just did not see the connection.
No, as noted it is about the difference between syntax versus semantics and the need for something like a DTD for the semantics.
For something similar see the ini files used with ODBC connections. ini file type specifies the syntax but not the semantics. (ref)