Why marshaling can serialize circular referenced list and json can't?

Question

Here I have a circular referenced list

2.1.9 :082 > a = []
 => [] 
2.1.9 :083 > a.append(a)
 => [[...]] 

When trying to dump a as json, I get the error

a.to_json
ActiveSupport::JSON::Encoding::CircularReferenceError: object references itself

but when i try to Marshal them i get a valid string

2.1.9 :085 > Marshal.dump(a)
 => "\x04\b[\x06@\x00" 

I just tried to ensure that they properly dumped the value by loading them again

 b = Marshal.load("\x04\b[\x06@\x00")
 => [[...]] 

Here are some more validations to ensure they properly dumped the object to string

2.1.9 :088 > a.object_id
 => 70257482733700 
2.1.9 :089 > a.first.object_id
 => 70257482733700 
2.1.9 :090 > b.object_id
 => 70257501553000 
2.1.9 :091 > b.first.object_id
 => 70257501553000 
2.1.9 :092 > 

In my understanding both of them are converting an object to string and get the object back from the string. I also able to see that json don't have any construct to refer other part of the json which may be the reason why it can't support this kind of operation. But is it that difficult to introduce such construct in json to facilitate the current situation. I may be missing something more fundamental regarding marshaling and serialization please enlighten me.

Answer 1

In my understanding both of them are converting an object to string and get the object back from the string.

Yes. That is pretty much the definition of "serialization" or "marshaling".

I also able to see that json don't have any construct to refer other part of the json which may be the reason why it can't support this kind of operation.

Yes, that is the reason.

But is it that difficult to introduce such construct in json to facilitate the current situation.

You cannot introduce constructs in JSON. It was deliberately designed to have no version number, so that it can never, ever be changed.

Of course, this only means that we cannot add it now, but could Doug Crockford have added it from the beginning, back when he was designing JSON? Yes, of course. But he didn't. JSON was deliberately designed to be simple (bold emphasis mine):

JSON is not a document format. It is not a markup language. It is not even a general serialization format in that it does not have a direct representation for cyclical structures […]

See, for example, YAML, a superset of JSON, which has references and thus can represent cyclical data.