json get prolog predicate

问题

I'm tryung to create this predicate in prolog:

The predicate json_get/3 can be defined as: json_get(JSON_obj, Fields, Result). which is true when Result is recoverable by following the chain of fields in Fields (a list) starting from JSON_obj. A field represented by N (with N a major number o equal to 0) corresponds to an index of a JSON array.

Please help me to understand to follow the chain of fields.

Thanks

edit1:

Of course, so json object looks like this '{"name" : "Aretha", "surname" : "Franklin"}'. if i call json_parse predicate to this object prolog show me this

json_obj([(”name”, ”Aretha”), (”surname”, ”Franklin”)]), we call this obj O.

with json_get i need to extract from O the name in this way, json_get(O, ["name"], R)

edit2:

with someone's help this is the predicate now:

json_get(json_obj(JSON_obj), Field, Result) :-
    memberchk((Field,Result), JSON_obj).

json_get(JSON_obj, Fields, Result) :-
    maplist(json_get(JSON_obj), Fields, Result).

so now the problem is nested list. For example with this input

json_parse('{"nome" : "Zaphod",
            "heads" : ["Head1", "Head2"]}', Z),
json_get(Z, ["heads", 1], R).

the output will should be R = "Head2" but the predicate doesn't extract the field and fail.

edit3:

this is the output of json_parse

json_obj([("nome", "Zaphod"),  ("heads", json_array(["Head1", "Head2"]))]).

回答1:

How about this

json_get(json_obj(Obj),[F|Fs],Res) :-
  member((F,R),Obj),
  json_get(R,Fs,Res).
json_get(json_array(Is),[N|Fs],Res) :-
  nth1(N,Is,R),
  json_get(R,Fs,Res).
json_get(Res,[],Res).

This produces Head1 not Head2 in your 2nd example. Please explain how that is supposed to work, if you did not just make a typo. (If it is zero-based you can just change nth1/3 to nth0/3.)

来源：https://stackoverflow.com/questions/48153619/json-get-prolog-predicate