8.2 library(http/json_convert): Convert between JSON terms and Prolog application terms

To be done

- Ignore extra fields. Using a partial list of extra?
- Consider a sensible default for handling JSON null. Conversion to Prolog could translate @null into a variable if the desired type is not any. Conversion to JSON could map variables to null, though this may be unsafe. If the Prolog term is known to be non-ground and JSON @null is a sensible mapping, we can also use this simple snipit to deal with that fact.

        term_variables(Term, Vars),
        maplist(=(@null), Vars).

The idea behind this module is to provide a flexible high-level mapping between Prolog terms as you would like to see them in your application and the standard representation of a JSON object as a Prolog term. For example, an X-Y point may be represented in JSON as {"x":25, "y":50}. Represented in Prolog this becomes json([x=25,y=50]), but this is a pretty non-natural representation from the Prolog point of view.

This module allows for defining records (just like library(record)) that provide transparent two-way transformation between the two representations.

:- json_object
        point(x:integer, y:integer).

This declaration causes prolog_to_json/2 to translate the native Prolog representation into a JSON Term:

?- prolog_to_json(point(25,50), X).

X = json([x=25, y=50])

A json_object/1 declaration can define multiple objects separated by a comma (,), similar to the dynamic/1 directive. Optionally, a declaration can be qualified using a module. The conversion predicates prolog_to_json/2 and json_to_prolog/2 first try a conversion associated with the calling module. If not successful, they try conversions associated with the module user.

JSON objects have no type. This can be solved by adding an extra field to the JSON object, e.g. {"type":"point", "x":25, "y":50}. As Prolog records are typed by their functor we need some notation to handle this gracefully. This is achieved by adding +Fields to the declaration. I.e.

:- json_object
        point(x:integer, y:integer) + [type=point].

Using this declaration, the conversion becomes:

?- prolog_to_json(point(25,50), X).

X = json([x=25, y=50, type=point])

The predicate json_to_prolog/2 is often used after http_read_json/2 and prolog_to_json/2 before reply_json/1. For now we consider them separate predicates because the transformation may be too general, too slow or not needed for dedicated applications. Using a separate step also simplifies debugging this rather complicated process.

[multifile]current_json_object(Term, Module, Fields)

Multifile predicate computed from the json_object/1 declarations. Term is the most general Prolog term representing the object. Module is the module in which the object is defined and Fields is a list of f(Name, Type, Default, Var), ordered by Name. Var is the corresponding variable in Term.

json_object(+Declaration)

Declare a JSON object. The declaration takes the same format as using in record/1 from library(record). E.g.

?- json_object
      point(x:int, y:int, z:int=0).

The type arguments are either types as know to library(error) or functor names of other JSON objects. The constant any indicates an untyped argument. If this is a JSON term, it becomes subject to json_to_prolog/2. I.e., using the type list(any) causes the conversion to be executed on each element of the list.

If a field has a default, the default is used if the field is not specified in the JSON object. Extending the record type definition, types can be of the form (Type1|Type2). The type null means that the field may not be present.

Conversion of JSON to Prolog applies if all non-defaulted arguments can be found in the JSON object. If multiple rules match, the term with the highest arity gets preference.

[semidet]prolog_bool_to_json(+Prolog, -JSON)

JSON is the JSON boolean for Prolog. It is a flexible the Prolog notation for thruth-value, accepting one of true, on or 1 for @true and one of false, fail, off or 0 for @false.

Errors

instantiation_error if Prolog is unbound.

[det]prolog_to_json(:Term, -JSONObject)

Translate a Prolog application Term into a JSON object term. This transformation is based on :- json_object/1 declarations. If a json_object/1 declaration declares a field of type boolean, commonly used thruth-values in Prolog are converted to JSON booleans. Boolean translation accepts one of true, on, 1, @true, false, fail, off or 0, @false.

Errors

- type_error(json_term, X)
- instantiation_error

[det]json_to_prolog(+JSON, -Term)

Translate a JSON term into an application term. This transformation is based on :- json_object/1 declarations. An efficient transformation is non-trivial, but we rely on the assumption that, although the order of fields in JSON terms is irrelevant and can therefore vary a lot, practical applications will normally generate the JSON objects in a consistent order.

If a field in a json_object is declared of type boolean, @true and @false are translated to true or false, the most commonly used Prolog representation for truth-values.