Forbid retrieval of non-fully-instantiated json arrays

Prolog/jsonparse.pl

@@ -196,13 +196,69 @@ kvplist_getitem(Key, [_ | KVPs], Value) :-
     kvplist_getitem(Key, KVPs, Value).
 
 
+% nonvar_recursive/1: nonvar_recursive(Var)
+%
+% Extension of nonvar/1 that also checks for nonvar in lists and objects.
+% Used in order to respect the interesting project rule:
+% 
+% ?- jsonaccess(jsonarray(_), [], _).
+% false.
+%
+% while still allowing, as later clarified on Moodle:
+%
+% ?- jsonaccess(jsonobj([("a", jsonarray([]))]), ["a"], A).
+% A = jsonarray([]).
+
+nonvar_recursive(Var) :- 
+    var(Var),
+    !,
+    fail.
+
+nonvar_recursive(jsonobj(KVPs)) :-
+    !,
+    nonvar(KVPs),
+    nonvar_recursive(KVPs).
+
+nonvar_recursive(jsonarray(Items)) :-
+    !,
+    nonvar(Items),
+    nonvar_recursive(Items).
+
+nonvar_recursive([H | T]) :-
+    !,
+    nonvar_recursive(H),
+    nonvar_recursive(T).
+
+nonvar_recursive(V) :-
+    !,
+    nonvar(V).
+
+
 % jsonaccess/3: jsonaccess(JsonObj, Fields, Result).
 %
 % Access a JSON object or array by a sequence of keys or indices.
 % Fields is either list of strings/ints, a string or an int
 
-% Empty JSON path, computation is complete
-jsonaccess(JsonObj, [], JsonObj) :- !.
+% Empty JSON path, computation is complete; return only valid JSON objects
+jsonaccess(jsonobj(KVPList), [], jsonobj(KVPList)) :-
+    !,
+    nonvar_recursive(KVPList).
+
+jsonaccess(jsonarray(Items), [], jsonarray(Items)) :- 
+    !,
+    nonvar_recursive(Items).
+
+jsonaccess(String, [], String) :-
+    string(String),
+    !.
+
+jsonaccess(Number, [], Number) :-
+    number(Number),
+    !.
+
+jsonaccess(true, [], true) :- !.
+jsonaccess(false, [], false) :- !.
+jsonaccess(null, [], null) :- !.
 
 % Handle accessing a JSON array by index
 jsonaccess(jsonarray(JArr), [F | Fs], Result) :-