🧠 Logic: 🧱 json_prolog, add predicate to convert json to prolog terms

[bdeneux]

📝 Purpose

In the logic module, we will need a set of new predicates to handle json conversion between json string to prolog term and vice versa.

🧪 Expected behavior

json_prolog(?Json, ?Term).

Where

• Json is the string representation of the json
• Term is an Atom that would be unified by the JSON representation as Prolog terms.

In addition, when passing Json and Term, this predicate return true if both result match.

The canonical representation for Term is:

Json	Prolog	Value
Object	Mapped to a term json(Attributes), where Attributes is a list of all JSON attributes mapped to a pair term -(Attribute, Value), where Attribute is an atom created from the JSON string and Value is the JSON value.	{ "foo": "bar", "foobar": "barfoo" } Result: json[-(foo, 'bar'), -(foobar, 'barfoo')]
Array	List of JSON values ⚠️ An empty array is represented with a @ functor : @([]). See #398 for more details.	["foo", "bar"] Result: ['foo', 'bar']
String	Atom	"foo" Result: 'foo'
Number	Number	1 Result: 1
Constant true and false	Atom @(true) and @(false)	true Result: @true
Constant null	Atom @(null)	null Result: @null

Note: converting a simple string, boolean, integer or null value are valid json and will be unified to an simple atom.

To ensure determinism on the chain, the result of the JSON conversion is canonicalized by sorting the JSON name attribute. Same behavior for the json representation on prolog : {"b": "a", "a":"c"} => json([a-c,b-a]) and json([b-a,a-c]) => {"a":"c", "b":"a"}.

🎯 Example

Given the following JSON

{
  "s": "string",
  "n": 1,
  "a": ["string1", "string2"],
  "o": {
    "b": true,
    "n": null
  }
}

json([
  -(s, 'string'),
  -(n, 1),
  -(a, ['string1', 'string2']),
  -(o, json([
    -(b, @true),
    -(n, @null),
  ])
])

✅ Acceptance Criteria

• The predicate is implemented according to the described behavior and examples
• The predicate passes all test cases
• Any related documentation is updated

🔗 References and linked predicate

• SWI-Prolog : https://www.swi-prolog.org/pldoc/man?section=json
• A compatible JSON predicate for Tau Prolog's library's implemented on ichiban/prolog : https://github.com/guregu/predicates/blob/master/taujson/json.go

[amimart]

This is excellent! Thanks @bdeneux 🥇

I think this is functionally speaking complete and pretty suitable.

We may just need to mention the edge case of a simple string being a valid JSON.

[ccamel]

That's brilliant @bdeneux 🤩!

[bdeneux]

Main specification updated since implementation of the json empty array fix : #398.