🧠 Logic: json_prolog array serialization issue

[amimart]

Issue description

When unifying a json array through a Prolog empty list to its string representation, it outputs to the string "[]" instead of an empty json array: [].

How to reproduce

This can easily reproduced with a Ask query using the CLI:

> okp4d query logic ask "json_prolog(X, [])." -ojson | jq -r '.answer.results[-1].substitutions[-1].term.name'
'"[]"'

We can observe the same results specifying the '[]' atom instead of an empty list:

> okp4d query logic ask "json_prolog(X, [])." -ojson | jq -r '.answer.results[-1].substitutions[-1].term.name'
'"[]"'

The reason under this behavior is related to the equality between an empty list and the '[]' atom at the interpreter level:

> okp4d query logic ask "[] = '[]'." -ojson | jq -r '.answer.success'
true

First Analysis

An issue has been opened on the interpreter: ichiban/prolog#296.

It seems this behavior is expected, the SWI-PROLOG implementation having its specific empty list type being not standard.

Should the Prolog interpreter in the logic module be aligned on the standard or the SWI-PROLOG implementation? And then, how do we address the JSON array marshaling?

[amimart]

@bdeneux @ccamel I propose to respect to Prolog standard and not head towards SWI-PROLOG "unconventional" empty list consideration.

We still need a solution though, amending the original specification.

A json(Attributes) predicate was introduced to manage JSON objects (e.g. {"b": "a", "a":"c"} => json([a-c,b-a])), may I suggest a similar predicate to handle JSON arrays, like json_array(Elements)? It would be consistent with the way we manage objects. Moreover, the original issue is only related to empty arrays so we can support both.

As an example, the following Prolog:

json([
    empty-json_array([]),
    filled-[with, something],
    alsoFilled-json_array([0, 1])
])

What are your thoughts?

Would represents:

{
    "empty": [],
    "filled": [
        "with",
        "something"
    ],
    "alsoFilled": [
        0,
        1
    ]
}

[bdeneux]

@amimart Ok that seems a good solution we could use to allow empty list in json. Maybe to make more consistency we should rename the json(Attributes) predicate to json_obj(Attributes) or json_object(Attributes).

We have another solution, is to create a specific atom to represent an empty list, like null (@(null)) or boolean value (@(true)), we could create the atom @([]). But this one is not common so I don't know what is the best solution...

[ccamel]

I have a deep feeling, whose origin eludes me, that the choice of using the functor @ with [] as argument would be the most appropriate for designating the special value that is the empty list.

[amimart]

I have a deep feeling, whose origin eludes me, that the choice of using the functor @ with [] as argument would be the most appropriate for designating the special value that is the empty list.

@ccamel Yeah it seems simple, just add an @ in the case of an empty array, I'm ok with that :)

[bot-anik]

🎉 This issue has been resolved in version 5.0.0 🎉

The release is available on GitHub release

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