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http_json.pl -- HTTP JSON Plugin module
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This module adds hooks to several parts of the HTTP libraries, making them JSON-aware. Notably:

Typically JSON is used by Prolog HTTP servers. This module supports two JSON representations: the classical representation and the new representation supported by the SWI-Prolog version 7 extended data types. Below is a skeleton for handling a JSON request, answering in JSON using the classical interface.

handle(Request) :-
      http_read_json(Request, JSONIn),
      json_to_prolog(JSONIn, PrologIn),
      <compute>(PrologIn, PrologOut),         % application body
      prolog_to_json(PrologOut, JSONOut),
      reply_json(JSONOut).

When using dicts, the conversion step is generally not needed and the code becomes:

handle(Request) :-
      http_read_json_dict(Request, DictIn),
      <compute>(DictIn, DictOut),
      reply_json(DictOut).

This module also integrates JSON support into the http client provided by http_client.pl. Posting a JSON query and processing the JSON reply (or any other reply understood by http_read_data/3) is as simple as below, where Term is a JSON term as described in json.pl and reply is of the same format if the server replies with JSON.

      ...,
      http_post(URL, json(Term), Reply, [])
See also
- JSON Requests are discussed in http://json.org/JSONRequest.html
- json.pl describes how JSON objects are represented in Prolog terms.
- json_convert.pl converts between more natural Prolog terms and json terms.
Source http_client:http_convert_data(+In, +Fields, -Data, +Options)[multifile]
Hook implementation that supports reading JSON documents. It processes the following option:
json_object(+As)
Where As is one of term or dict. If the value is dict, json_read_dict/3 is used.
Source is_json_content_type(+ContentType) is semidet
True if ContentType is a header value (either parsed or as atom/string) that denotes a JSON value.
Source json_type(?MediaType) is semidet[multifile]
True if MediaType is a JSON media type. http_json:json_type/1 is a multifile predicate and may be extended to facilitate non-conforming clients.
Arguments:
MediaType- is a term Type/SubType, where both Type and SubType are atoms.
Source http:post_data_hook(+Data, +Out:stream, +HdrExtra) is semidet[multifile]
Hook implementation that allows http_post_data/3 posting JSON objects using one of the forms below.
http_post(URL, json(Term), Reply, Options)
http_post(URL, json(Term, Options), Reply, Options)

If Options are passed, these are handed to json_write/3. In addition, this option is processed:

json_object(As)
If As is dict, json_write_dict/3 is used to write the output. This is default if json(Dict) is passed.
To be done
- avoid creation of intermediate data using chunked output.
Source http_read_json(+Request, -JSON) is det
Source http_read_json(+Request, -JSON, +Options) is det
Extract JSON data posted to this HTTP request. Options are passed to json_read/3. In addition, this option is processed:
json_object(+As)
One of term (default) to generate a classical Prolog term or dict to exploit the SWI-Prolog version 7 data type extensions. See json_read_dict/3.
Errors
- domain_error(mimetype, Found) if the mimetype is not known (see json_type/1).
- domain_error(method, Method) if the request method is not a POST, PUT or PATCH.
Source http_read_json_dict(+Request, -Dict) is det
Source http_read_json_dict(+Request, -Dict, +Options) is det
Similar to http_read_json/2,3, but by default uses the version 7 extended datatypes.
Source reply_json(+JSONTerm) is det
Source reply_json(+JSONTerm, +Options) is det
Formulate a JSON HTTP reply. See json_write/2 for details. The processed options are listed below. Remaining options are forwarded to json_write/3.
content_type(+Type)
The default Content-type is application/json; charset=UTF8. charset=UTF8 should not be required because JSON is defined to be UTF-8 encoded, but some clients insist on it.
status(+Code)
The default status is 200. REST API functions may use other values from the 2XX range, such as 201 (created).
json_object(+As)
One of term (classical json representation) or dict to use the new dict representation. If omitted and Term is a dict, dict is assumed. SWI-Prolog Version 7.
Source reply_json_dict(+JSONTerm) is det
Source reply_json_dict(+JSONTerm, +Options) is det
As reply_json/1 and reply_json/2, but assumes the new dict based data representation. Note that this is the default if the outer object is a dict. This predicate is needed to serialize a list of objects correctly and provides consistency with http_read_json_dict/2 and friends.